Bull Trout (Salvelinus confluentus) South Coast British Columbia population, Western Arctic population, Upper Yukon Watershed population, Saskatchewan-Nelson Rivers population, Pacific population: COSEWIC assessment and status report 2025

Official title: COSEWIC assessment and status report on the Bull Trout (Salvelinus confluentus) South Coast British Columbia population, Western Arctic population, Upper Yukon Watershed population, Saskatchewan-Nelson Rivers population, Pacific population in Canada

South Coast British Columbia population - Special Concern

Western Arctic population - Special Concern

Upper Yukon Watershed population - Data Deficient

Saskatchewan-Nelson Rivers population - Threatened

Pacific population - Special Concern

2025

COSEWIC assessment summary

Assessment summary – May 2025 - DU1

Common name: Bull Trout - DU1 - South Coast British Columbia population

Scientific name: Salvelinus confluentus

Status: Special Concern

Reason for designation: This large freshwater fish is found in five river systems in southwestern British Columbia. It exhibits considerable diversity in life history traits, including a unique anadromous form that migrates to the sea. Although abundance is unknown for all areas, the total number of mature individuals is likely not high. This slow-growing and late-maturing species thrives in cold, pristine waters, and many sub-populations require long unimpeded migratory routes that connect spawning and adult habitats. The species is particularly vulnerable to negative effects from competition with non-native Brook Trout, fishing mortality, increases in water temperature, drought, and pollution, including sedimentation from forestry. This distinctive Canadian population could become Threatened if these threats are neither reversed nor managed effectively.

Occurrence: British Columbia

Status history: Designated Special Concern in November 2012. Status re-examined and confirmed in May 2025.

Assessment summary – May 2025 - DU2

Common name: Bull Trout - DU2 - Western Arctic population

Scientific name: Salvelinus confluentus

Status: Special Concern

Reason for designation: This large freshwater fish is distributed throughout northeastern British Columbia, southern Yukon, southwestern Northwest Territories, and parts of Alberta. There is evidence of a decline in numbers and distribution for some areas, but quantitative estimates for the entire range are lacking. This slow-growing and late-maturing species thrives in cold, pristine waters, and many sub-populations require long unimpeded migratory routes that connect spawning and adult habitats. The species is particularly vulnerable to negative effects from dams and water management activities, increases in water temperature, drought, fishing mortality, and pollution. This distinctive Canadian population could become Threatened if these threats are neither reversed nor managed effectively.

Occurrence: British Columbia, Alberta, Northwest Territories, Yukon

Status history: Designated Special Concern in November 2012. Status re-examined and confirmed in May 2025.

Assessment summary – May 2025 - DU3

Common name: Bull Trout - DU3 – Upper Yukon Watershed population

Scientific name: Salvelinus confluentus

Status: Data Deficient

Reason for designation: This large freshwater fish is found in the upper Yukon River drainage in northern British Columbia and southern Yukon, but information on population sizes and trends is not available. This slow-growing and late-maturing species thrives in cold, pristine waters, and many sub-populations require long unimpeded migratory routes that connect spawning and adult habitats. The species is particularly vulnerable to habitat degradation and fishing mortality, but specific threats to this population are largely unknown.

Occurrence: British Columbia, Yukon

Status history: Species considered in November 2012 and placed in the Data Deficient category. Status re-examined and confirmed in May 2025.

Assessment summary – May 2025 - DU4

Common name: Bull Trout - DU4 - Saskatchewan-Nelson Rivers population

Scientific name: Salvelinus confluentus

Status: Threatened

Reason for designation: This large freshwater fish is broadly distributed east of the Rocky Mountains in southern Alberta. It is a slow-growing and late-maturing species that thrives in cold, pristine waters and often requires long, unimpeded migratory routes that connect spawning and adult habitats. Historical range contractions now limit the species to the foothills and east slopes of the Rocky Mountains, and there is evidence of continuing habitat deterioration and a decline in area of spawning habitat and number of mature individuals. The species is particularly vulnerable to negative effects from increases in water temperature, drought, fishing mortality, dams and water management activities, pollution, and competition and hybridization with non-native Brook Trout. If these threats cannot be mitigated, they could lead to this population becoming Endangered.

Occurrence: Alberta

Status history: Designated Threatened in November 2012. Status re-examined and confirmed in May 2025.

Assessment summary – May 2025 - DU5

Common name: Bull Trout - DU5 – Pacific population

Scientific name: Salvelinus confluentus

Status: Special Concern

Reason for designation: This large freshwater fish is broadly distributed throughout Pacific drainages in British Columbia. It is a slow-growing and late-maturing species that thrives in cold, pristine waters, and requires unimpeded migratory routes that connect spawning and adult habitats. Although never abundant, there are many dispersed sub-populations across this area. There is no evidence of overall decline in abundance of mature individuals or distribution, but the species is particularly vulnerable to negative effects from road development associated with forest harvesting and mining exploration, dams and water management activities, increases in water temperature, drought, fishing mortality, and pollution. The revised status reflects these increasing threats; if they are not reversed or managed effectively, this distinctive Canadian population could become Threatened.

Occurrence: British Columbia

Status history: Designated Not at Risk in November 2012. Status re-examined and designated Special Concern in May 2025.

COSEWIC executive summary

Bull Trout

Salvelinus confluentus

South Coast British Columbia population

Western Arctic population

Upper Yukon Watershed population

Saskatchewan-Nelson Rivers population

Pacific population

Wildlife species description and significance

Bull Trout (Salvelinus confluentus) is a large char that derives its name from its large head and jaws. They are olive-green to blue-grey in colour with white leading edges on pelvic fins as in other chars. Pale round spots on their flanks and back distinguish them from most other similar-looking salmonids. It is difficult to visually distinguish from Dolly Varden char, however, and detailed measurements or genetic analyses are required for accurate identification where their ranges overlap. Because of its very specific habitat requirements, Bull Trout is highly sensitive to habitat changes.

Bull Trout is a top predator in ecosystems and viewed as an indicator species of general ecosystem health. Based on genetic analysis, range disjunction and distribution across National Freshwater Biogeographic Zones, five designatable units (DUs) are recognized from two genetic lineages: Genetic Lineage 1 (South Coast British Columbia population) and Genetic Lineage 2 (Western Arctic, Upper Yukon Watershed, Saskatchewan-Nelson Rivers, and Pacific populations).

Aboriginal (Indigenous) knowledge

All species are significant and are interconnected and interrelated. Publicly available ATK is reported from the Nak’azdli Whut’en Territory in the Pacific DU and the Blackfoot Confederacy in the Saskatchewan-Nelson Rivers DU.

Distribution

Bull Trout is native to western Canada and the US Pacific Northwest. They range north from the Oregon-California border and northern Nevada through British Columbia and Alberta to southern Yukon and southwestern Northwest Territories. The largest portion of their range (about 80%) occurs in Canada. They are generally restricted to interior drainages but reach the Pacific Coast in southwest British Columbia and northwest Washington. They are concentrated west of the Continental Divide but do extend across it, being found in all of the major eastern slope drainages in Alberta. Their range has become restricted over the last century, particularly in the U.S.A. and Alberta, where populations have become more fragmented and isolated. British Columbia, Yukon and the Northwest Territories are the last remaining jurisdictions with wide distributions of Bull Trout.

Habitat

Bull Trout has specific life stage requirements including cold, clean, complex, and connected habitats. Structurally complex habitat provides cover for shelter and the right requirements for breeding and rearing young, while connected habitat allows this migratory species to move between the areas it needs to complete its life cycle.

Biology

Among Canadian freshwater fishes, Bull Trout exhibit late age-at-maturity and are long-lived. In less productive and cold habitats they may reproduce only every second year. Bull Trout is a predator that eats fish when available. The species exhibits considerable diversity in life history traits, including four types: a non-migratory stream resident form; a migratory fluvial form that occurs in flowing water: a migratory adfluvial form that matures in lakes; and an anadromous form that migrates to the sea. Life history forms are sympatric in some areas. All forms breed in headwater or tributary streams at higher elevation but habitat occupied at other times varies. The first three forms are common throughout the Canadian range, but the anadromous form is restricted to the south coast of British Columbia.

Population sizes and trends

As inhabitants of cold unproductive waters, Bull Trout populations tend to be small and broadly dispersed across watersheds. Population surveys are incomplete across much of their range, and the Alberta surveys are most complete. Total population size has only been estimated for the Saskatchewan-Nelson rivers DU at 68,000 adults (CI 56,000 to 84,000). Population sizes of some individual subpopulations have been estimated in the South Coast British Columbia, Pacific, and Western Arctic DUs, but large areas remain under sampled.

In recent decades, Bull Trout populations have experienced declines in abundance across parts of their range, particularly in the U.S.A. and Alberta. Substantial long-term (since the middle of the last century) declines have been enumerated within the Saskatchewan-Nelson Rivers DU and the Alberta portion of the Western Arctic DU using a combination of data, expert opinion, and anecdotal information. Three generation trends are uncertain within Alberta due to variation in sampling techniques, sampling sites and goals of sampling. Where data are available from other areas within Bull Trout distribution, trends are uncertain with a diversity of negative and positive trends in adult abundance, most of which are non-significant due to high sampling variability. The full range of life histories is also being lost from some populations with the migratory ecotype most at risk in downstream reaches, particularly in Alberta.

Threats

There are four key threats to Bull Trout which vary across their range in western and northern Canada: habitat loss, interactions with invasive species, climate change, and exploitation and their interactions. Habitat loss through degradation and fragmentation is a common result of forestry, hydroelectric, oil, gas and mining development, off-road vehicle use, agriculture, urbanization, and their associated road development may all contribute to this threat. Interactions with other species strongly influence the local distribution and abundance of Bull Trout. Habitat degradation may result in displacement, increasing vulnerability to competition, and hybridization through invasion by non-native species, such as Brook Trout. Angling mortality, either intentional or through release mortality can be a threat to this species that is vulnerable to overharvesting. Finally, climate change is a key threat, particularly in downstream environments that are susceptible to elevated temperature and hydrological changes.

Protection, status, and recovery activities

Bull Trout is listed under the Canadian Species at Risk Act in 2019 as follows: South Coast British Columbia population – Special Concern; Western Arctic population – Special Concern; Saskatchewan-Nelson Rivers population – Threatened. In addition, COSEWIC (2012) assessed the Pacific population as Not At Risk and the Yukon Watershed population as Data Deficient. Bull Trout is listed as Threatened under Alberta’s Wildlife Act as of 2014 and on the British Columbia Provincial Blue List (that is, Species of Special Concern) and is an Identified Wildlife Species at Risk under the Identified Wildlife Management Strategy. Bull Trout in national park waters are afforded protection under the Canada National Parks Act. Critical Habitat has been identified within the Saskatchewan-Nelson Rivers DU. In addition, Critical Habitat was identified and is legally afforded protection in Banff, Jasper, and Waterton Lakes National Parks of Canada as of 2021.

Technical summary: DU1 - South Coast British Columbia population

Salvelinus confluentus

Bull Trout (South Coast British Columbia population)

Omble à tête plate (Population de la côte sud de la Colombie-Britannique)

Range of occurrence in Canada: British Columbia

Demographic information

Generation time (usually average age of parents in the population)

10 years

Is there an [observed, estimated, inferred, or projected] continuing decline in number of mature individuals?

Unknown

No evidence of decline but surveys are incomplete, and there are contrasting trends across subpopulations

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 3 years [or 1 generation; whichever is longer up to a maximum of 100 years]

Not applicable

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 5 years [or 2 generations; whichever is longer up to a maximum of 100 years]

Not applicable

[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last 10 years [or 3 generations; whichever is longer]

Not applicable

[Projected, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations, up to a maximum of 100 years]

Unknown

Projection from Threats Calculator 10 to 70%, but insufficient data to reliably project, infer, or suspect population trends

[Observed, estimated, inferred, projected, or suspected] percent [reduction or increase] in total number of mature individuals over any period of 10 years [or 3 generations; whichever is longer, up to a maximum of 100 years], including both the past and future (up to a maximum of 100 years in future)

Unknown

Are the causes of the decline clearly reversible?

Not applicable

Are the causes of the decline clearly understood?

Not applicable

Are the causes of the decline clearly ceased?

Not applicable

Are there extreme fluctuations in number of mature individuals?

No

Extent and occupancy information

Estimated extent of occurrence (EOO)

21,841 km²

Figure 9; all observations

Index of area of occupancy (IAO), reported as 2x2 km grid value

648 km²

Figure 9; discrete, 2x2 grid cells summed over all observations

No information is available on the number of subpopulation spawning sites and therefore IAO of the most restrictive life stage is unknown but likely <500 km² (if approximately 32% of IAO based on all observations; see DU4)

Is the population “severely fragmented”, that is, is >50% of individuals or >50% of the total area “occupied” (as a proxy for number of individuals) in habitat patches that are both (a) smaller than required to support a viable subpopulation, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. No
2. No

Population not concentrated within one subpopulation and the majority of the subpopulations are likely large enough to be viable

Number of “locations” (use plausible range to reflect uncertainty if appropriate)

>10

Based on spatially patchy primary threat of invasive species

Is there an [observed, inferred, or projected] continuing decline in extent of occurrence?

Unknown

Is there an [observed, inferred, or projected] continuing decline in area of occupancy?

Unknown

Is there an [observed, inferred, or projected] continuing decline in number of subpopulations?

Unknown

Is there an [observed, inferred, or projected] continuing decline in number of “locations”?

Unknown

Is there an [observed, inferred, or projected] continuing decline in [area, extent and/or quality] of habitat?

Yes

Observed, inferred, and projected decline in extent and quality of habitat through hydroelectric development, urbanization, agriculture, and transport systems

Are there extreme fluctuations in number of subpopulations?

No

Are there extreme fluctuations in number of “locations”?

No

Are there extreme fluctuations in extent of occurrence?

No

Are there extreme fluctuations in index of area of occupancy?

No

Number of mature individuals (by subpopulation)

There are five core areas identified in this DU (see Appendix 1), but the area is not fully sampled, so estimates are considered minima

Total: 4,800 to 6,500

Likely a substantial underestimate as not all areas are sampled

Quantitative analysis

Is the probability of extinction in the wild at least 20% within 20 years [or 5 generations], or 10% within 100 years

Unknown

Analysis not conducted

Threats

Was a threats calculator completed for this species?

Yes, see Appendix 2

Overall assigned threat impact: High

Key threats were identified and ranked as:

8 Invasive and other problematic species and genes – High-Medium

11 Climate change and severe weather – Medium

5 Biological resource use – Medium-Low

9 Pollution – Medium-Low

4 Transportation and service corridors – Medium-Low

7 Natural system modifications – Medium-Low

Unknown threats were:

3 Energy production and mining

6 Human intrusions and disturbance

10 Geological events

What limiting factors are relevant?

• Low reproduction rate
• Very specific habitat requirements

Rescue effect (from outside Canada)

Status of outside population(s) most likely to provide immigrants to Canada.

Declining

The southern extension of this DU in Washington State is listed as “Species of Greatest Conservation Need” and Critically Imperilled S1

Is immigration known or possible?

Unknown

Would immigrants be adapted to survive in Canada?

Unknown

Is there sufficient habitat for immigrants in Canada?

Yes

Are conditions deteriorating in Canada?

Yes

Are conditions for the source (that is, outside) population deteriorating?

Yes

Is the Canadian population considered to be a sink?

No

Is rescue from outside Canada likely, such that it could lead to a change in status?

No

Wildlife species with sensitive occurrence data (general caution for consideration)

Could release of certain occurrence data result in increased harm to the Wildlife Species or its habitat?

No

Current status

Year of previous assessment: 2012

COSEWIC status historyDesignated Special Concern in November 2012. Status re-examined and confirmed in May 2025.

Status and reasons for designation

Status: Special Concern

Alpha-numeric codes: n/a

Reason for change in status: n/a

Reasons for designation: This large freshwater fish is found in five river systems in southwestern British Columbia. It exhibits considerable diversity in life history traits, including a unique anadromous form that migrates to the sea. Although abundance is unknown for all areas, the total number of mature individuals is likely not high. This slow-growing and late-maturing species thrives in cold, pristine waters, and many sub-populations require long unimpeded migratory routes that connect spawning and adult habitats. The species is particularly vulnerable to negative effects from competition with non-native Brook Trout, fishing mortality, increases in water temperature, drought, and pollution, including sedimentation from forestry. This distinctive Canadian population could become Threatened if these threats are neither reversed nor managed effectively.

Applicability of criteria

A: Decline in total number of mature individuals

Not applicable.

Insufficient data to reliably infer, project, or suspect population trends.

B: Small range and decline or fluctuation

Not applicable.

IAO based on all observations (648 km²) is below the threshold for Threatened (and IAO of the most restrictive life stage (spawning) is likely below the threshold for Endangered), and although the population is experiencing a continuing decline in extent and quality of habitat, it is not severely fragmented, occurs at >10 locations, and does not experience extreme fluctuations.

C: Small and declining number of mature individuals

Not applicable.

Estimate of mature individuals (4,800 to 6,500) is below the threshold for Threatened, but this is likely a substantial underestimate, and there is insufficient data to estimate or project continuing decline.

D: Very small or restricted population

Not applicable.

Estimate of mature individuals (4,800 to 6,500) exceeds thresholds, and population is not vulnerable to rapid and substantial decline.

E: Quantitative analysis

Not applicable.

Analysis not conducted.

Reasons for Special Concern

(b) the Wildlife Species may become Threatened if factors suspected of negatively influencing the persistence of the Wildlife Species are neither reversed nor managed with demonstrable effectiveness

Technical summary: DU2 - Western Arctic population

Salvelinus confluentus

Bull Trout (Western Arctic population)

Omble à tête plate (Population de l'ouest de l'Arctique)

Range of occurrence in Canada: British Columbia, Alberta, Northwest Territories, Yukon

Demographic information

Generation time (usually average age of parents in the population)

10 years

Is there an [observed, estimated, inferred, or projected] continuing decline in number of mature individuals?

Yes

Extirpations observed on East Slopes of Alberta; majority of the decline likely happened prior to the most recent 3 generations (Figure 7), but projections imply continuing decline

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 3 years [or 1 generation; whichever is longer up to a maximum of 100 years]

Unknown

Declines likely in the Alberta portion as projected by the cumulative effects analyses but % uncertain in other parts of the range

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 5 years [or 2 generations; whichever is longer up to a maximum of 100 years]

Unknown

Declines likely in the Alberta portion as projected by the cumulative effects analyses but % uncertain in other parts of the range

[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last 10 years [or 3 generations; whichever is longer]

Unknown

Extirpations observed on East Slopes of Alberta, although majority of the decline likely happened prior to the most recent 3 generations

[Projected, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations, up to a maximum of 100 years]

Unknown

1. Projection from Threats Calculator 10 to 70%, but insufficient data to reliably project, infer, or suspect population trends
2. Cumulative-effects models for the Alberta portion of DU project 0 to 86.6% reduction (Table 4), but unknown across the entire DU

[Observed, estimated, inferred, projected, or suspected] percent [reduction or increase] in total number of mature individuals over any period of 10 years [or 3 generations; whichever is longer, up to a maximum of 100 years], including both the past and future (up to a maximum of 100 years in future)

Unknown

Are the causes of the decline clearly reversible?

No

Are the causes of the decline clearly understood?

No

Are the causes of the decline clearly ceased?

No

Are there extreme fluctuations in number of mature individuals?

No

Extent and occupancy information

Estimated extent of occurrence (EOO)

542,950 to 606,655 km²

542,950 (2012 to 2023 observations, likely an underestimate) – 606,655 km² (all observations, likely an overestimate based on some presumed extirpations in East Slope Alberta portion of DU) (Figure 10)

Index of area of occupancy (IAO), reported as 2x2 km grid value

9,792 to 11,136 km²

9,792 (2012 to 2023 observations) to 11,136 km² (all observations) (Figure 10); discrete, 2x2 grid cells. No information is available on the number of subpopulation spawning sites and therefore IAO of the most restrictive life stage is unknown, but unlikely <2,000 km²

Is the population “severely fragmented”, that is, is >50% of individuals or >50% of the total area “occupied” (as a proxy for number of individuals) in habitat patches that are both (a) smaller than required to support a viable subpopulation, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. No
2. No

Population not concentrated within one subpopulation and the majority of the subpopulations are likely large enough to be viable

Number of “locations” (use plausible range to reflect uncertainty if appropriate)

>10

Based on spatial patchiness of natural systems modifications

Is there an [observed, inferred, or projected] continuing decline in extent of occurrence?

Yes

Extirpations observed on East Slopes of Alberta; majority of the decline likely happened prior to the most recent 3 generations (Figure 7), but projections imply continuing decline

Is there an [observed, inferred, or projected] continuing decline in area of occupancy?

Yes

Extirpations observed on East Slopes of Alberta; majority of the decline likely happened prior to the most recent 3 generations (Figure 7), but projections imply continuing decline

Is there an [observed, inferred, or projected] continuing decline in number of subpopulations?

Yes

Extirpations observed on East Slopes of Alberta; majority of the decline likely happened prior to the most recent 3 generations (Figure 7), but projections imply continuing decline

Is there an [observed, inferred, or projected] continuing decline in number of “locations”?

No

Is there an [observed, inferred, or projected] continuing decline in [area, extent and/or quality] of habitat?

Yes

Observed, inferred, and projected decline in habitat extent and quality due to hydroelectric development, oil and gas development, transport systems, forest harvesting, and mining activity, particularly in the Alberta and BC components of the DU

Are there extreme fluctuations in number of subpopulations?

No

Are there extreme fluctuations in number of “locations”?

No

Are there extreme fluctuations in extent of occurrence?

No

Are there extreme fluctuations in index of area of occupancy?

No

Number of mature individuals (by subpopulation)

British Columbia (excluding National Parks) with data in 9 of 17 core areas, but even these are substantially under sampled (see Appendix 1)

>8,410

Minimum estimate

Alberta Range (excluding National Parks) includes 32 HUC8 areas

11,127 to 59,556

Jasper NP

3,000 to 4,000

Likely range based on expert opinion

Banff NP

Unknown

Yukon

Unknown

NWT

Unknown

Total

Likely >100,000

Quantitative analysis

Is the probability of extinction in the wild at least 20% within 20 years [or 5 generations], or 10% within 100 years]

Unknown

Analysis not conducted

Threats

Was a threats calculator completed for this species?

Yes

Overall assigned threat impact: High

Key threats were identified and ranked as:

7 Natural system modifications - High-Medium

11 Climate change and severe weather - High-Medium

5 Biological resource use - Medium

9 Pollution - Medium

4 Transportation and service corridors - Medium-Low

3 Energy production and mining - Low

6 Human intrusions and disturbance - Low

8 Invasive and other problematic species and genes – Low

Unknown threats were:

10 Geological events

What limiting factors are relevant?

• Low reproductive rate
• Very specific habitat requirements

Rescue effect (from outside Canada)

Status of outside population(s) most likely to provide immigrants to Canada.

Not applicable

This DU is a Canadian endemic

Is immigration known or possible?

Not applicable

Would immigrants be adapted to survive in Canada?

Not applicable

Is there sufficient habitat for immigrants in Canada?

Not applicable

Are conditions deteriorating in Canada?

Not applicable

Are conditions for the source (that is, outside) population deteriorating?

Not applicable

Is the Canadian population considered to be a sink?

Not applicable

Is rescue from outside Canada likely, such that it could lead to a change in status?

Not applicable

Wildlife species with sensitive occurrence data (general caution for consideration)

Could release of certain occurrence data result in increased harm to the Wildlife Species or its habitat?

No

Current status

Year of previous assessment

2012

COSEWIC status history

Designated Special Concern in November 2012. Status re-examined and confirmed in May 2025.

Status and reasons for designation

Status: Special Concern

Alpha-numeric codes: n/a

Reason for change in status: n/a

Reasons for designation: This large freshwater fish is distributed throughout northeastern British Columbia, southern Yukon, southwestern Northwest Territories, and parts of Alberta. There is evidence of a decline in numbers and distribution for some areas, but quantitative estimates for the entire range are lacking. This slow-growing and late-maturing species thrives in cold, pristine waters, and many sub-populations require long unimpeded migratory routes that connect spawning and adult habitats. The species is particularly vulnerable to negative effects from dams and water management activities, increases in water temperature, drought, fishing mortality, and pollution. This distinctive Canadian population could become Threatened if these threats are neither reversed nor managed effectively.

Applicability of criteria

A: Decline in total number of mature individuals

Not applicable.

Insufficient data to reliably infer, project, or suspect population trends. The results of the cumulative-effects models for the Alberta portion of this DU range widely in projected declines, and there is uncertainty when extrapolated to the entire population.

B: Small range and decline or fluctuation

Not applicable.

EOO (542,950 to 606,655 km²) and IAO (9,792 to 11,136 km²) exceed thresholds.

C: Small and declining number of mature individuals

Not applicable.

Estimate of mature individuals (>100,000) exceeds thresholds.

D: Very small or restricted population

Not applicable.

Estimate of mature individuals (>100,000) exceeds thresholds, and population is not vulnerable to rapid and substantial decline.

E: Quantitative analysis

Not applicable.

Analysis not conducted.

Reasons for Special Concern

(b) the Wildlife Species may become Threatened if factors suspected of negatively influencing the persistence of the Wildlife Species are neither reversed nor managed with demonstrable effectiveness

Technical summary: DU3 – Upper Yukon Watershed population

Salvelinus confluentus

Bull Trout (Upper Yukon Watershed population)

Omble à tête plate (Population de la partie supérieure du bassin versant du fleuve Yukon)

Range of occurrence in Canada: British Columbia, Yukon

Demographic information

Generation time (usually average age of parents in the population)

10 years

Is there an [observed, estimated, inferred, or projected] continuing decline in number of mature individuals?

Unknown

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 3 years [or 1 generation; whichever is longer up to a maximum of 100 years]

Unknown

Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 5 years [or 2 generations; whichever is longer up to a maximum of 100 years]

Unknown

[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last 10 years [or 3 generations; whichever is longer]

Unknown

[Projected, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations, up to a maximum of 100 years]

Unknown

[Observed, estimated, inferred, projected, or suspected] percent [reduction or increase] in total number of mature individuals over any period of 10 years [or 3 generations; whichever is longer, up to a maximum of 100 years], including both the past and future (up to a maximum of 100 years in future)

Unknown

Are the causes of the decline clearly reversible?

Unknown

Are the causes of the decline clearly understood?

Unknown

Are the causes of the decline clearly ceased?

Unknown

Are there extreme fluctuations in number of mature individuals?

Unknown

Extent and occupancy information

Estimated extent of occurrence (EOO)

5,885 km²

Figure 11; all observations but substantially undersampled

Index of area of occupancy (IAO), reported as 2x2 km grid value

120 km²

Figure 11; discrete, 2x2 grid cells summed over all observations but undersampled

No information is available on the number of subpopulation spawning sites and therefore IAO of the most restrictive life stage is unknown

Is the population “severely fragmented”, that is, is >50% of individuals or >50% of the total area “occupied” (as a proxy for number of individuals) in habitat patches that are both (a) smaller than required to support a viable subpopulation, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. Unknown
2. Unknown

Number of “locations” (use plausible range to reflect uncertainty if appropriate)

Unknown

Is there an [observed, inferred, or projected] continuing decline in extent of occurrence?

Unknown

Is there an [observed, inferred, or projected] continuing decline in area of occupancy?

Unknown

Is there an [observed, inferred, or projected] continuing decline in number of subpopulations?

Unknown

Is there an [observed, inferred, or projected] continuing decline in number of “locations”?

Unknown

Is there an [observed, inferred, or projected] continuing decline in [area, extent and/or quality] of habitat?

Unknown

Are there extreme fluctuations in number of subpopulations?

Unknown

Are there extreme fluctuations in number of “locations”?

Unknown

Are there extreme fluctuations in extent of occurrence?

Unknown

Are there extreme fluctuations in index of area of occupancy?

Unknown

Number of mature individuals (by subpopulation)

Total

Unknown

Quantitative analysis

Is the probability of extinction in the wild at least 20% within 20 years [or 5 generations], or 10% within 100 years]

Unknown

Analysis not conducted

Threats

Was a threats calculator completed for this species?

Yes

Overall assigned threat impact: Unknown

Key threats were identified as:

3 Energy production and mining – unknown

4 Transportation and service corridors - unknown

5 Biological resource use – unknown

9 Pollution – unknown

10 Geological events – unknown

11 Climate change and severe weather – unknown

What limiting factors are relevant?

• Low reproductive rate
• Very specific habitat requirements

Rescue effect (from outside Canada)

Status of outside population(s) most likely to provide immigrants to Canada.

Not applicable

This DU is a Canadian endemic

Is immigration known or possible?

Not applicable

Would immigrants be adapted to survive in Canada?

Not applicable

Is there sufficient habitat for immigrants in Canada?

Not applicable

Are conditions deteriorating in Canada?

Not applicable

Are conditions for the source (that is, outside) population deteriorating?

Not applicable

Is the Canadian population considered to be a sink?

Not applicable

Is rescue from outside Canada likely, such that it could lead to a change in status?

Not applicable

Wildlife species with sensitive occurrence data (general caution for consideration)

Could release of certain occurrence data result in increased harm to the Wildlife Species or its habitat?

No

Current status

Year of previous assessment

2012

COSEWIC status history

Species considered in November 2012 and placed in the Data Deficient category. Status re-examined and confirmed in May 2025.

Status and reasons for designation

Status: Data Deficient

Alpha-numeric codes: n/a

Reason for change in status: n/a

Reasons for designation: This large freshwater fish is found in the upper Yukon River drainage in northern British Columbia and southern Yukon, but information on population sizes and trends is not available. This slow-growing and late-maturing species thrives in cold, pristine waters, and many sub-populations require long unimpeded migratory routes that connect spawning and adult habitats. The species is particularly vulnerable to habitat degradation and fishing mortality, but specific threats to this population are largely unknown.

Applicability of criteria

A: Decline in total number of mature individuals

Not applicable.

Insufficient data to reliably infer, project, or suspect population trends.

B: Small range and decline or fluctuation

Not applicable.

EOO (5,885 km²) below threshold for Threatened and IAO based on all observations (120 km²) below threshold for Endangered, but DU is considerably undersampled, and information to support subcriteria is unknown.

C: Small and declining number of mature individuals

Not applicable.

Insufficient data to determine number of mature individuals and/or continuing decline.

D: Very small or restricted population

Not applicable.

Number of mature individuals and vulnerability to rapid and substantial population decline are unknown.

E: Quantitative analysis

Not applicable.

Analysis not conducted.

Reasons for Data Deficient

Records of occurrence are too infrequent to make any conclusions about extent of occurrence, population size, threats, or trends.

Technical summary: DU4 - Saskatchewan-Nelson Rivers population

Salvelinus confluentus

Bull Trout (Saskatchewan-Nelson Rivers population)

Omble à tête plate (Population de la rivière Saskatchewan et du fleuve Nelson)

Range of occurrence in Canada: Alberta

Demographic information

Generation time (usually average age of parents in the population)

10 years

Is there an [observed, estimated, inferred, or projected] continuing decline in number of mature individuals?

Yes

Extirpations observed on East Slopes of Alberta; majority of the decline likely happened prior to the most recent 3 generations (Figure 7), but projections imply continuing decline

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 3 years [or 1 generation; whichever is longer up to a maximum of 100 years]

Unknown

Declines likely as projected by the cumulative effects analyses but % within this time frame unknown

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 5 years [or 2 generations; whichever is longer up to a maximum of 100 years]

Unknown

Declines likely as projected by the cumulative effects analyses but % within this time frame unknown

[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last 10 years [or 3 generations; whichever is longer]

Unknown

Declines likely but % over the last 3 generations unknown

[Projected, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations, up to a maximum of 100 years]

Unknown

1. Projection from Threats Calculator 10 to 100%, but insufficient data to reliably project, infer, or suspect population trends
2. Cumulative-effects models project reductions, but % ranges widely, 0.1 to 80.7% (Table 5)

[Observed, estimated, inferred, projected, or suspected] percent [reduction or increase] in total number of mature individuals over any period of 10 years [or 3 generations; whichever is longer, up to a maximum of 100 years], including both the past and future (up to a maximum of 100 years in future)

Unknown

Are the causes of the decline clearly reversible?

No

Are the causes of the decline clearly understood?

Reasonably

Are the causes of the decline clearly ceased?

No

Are there extreme fluctuations in number of mature individuals?

No

Extent and occupancy information

Estimated extent of occurrence (EOO)

57,577 km²

Figure 12; given good recent search effort and potential extirpations since earlier observations, EOO 2012 to 2023 likely best estimate of current EOO

Index of area of occupancy (IAO), reported as 2x2 km grid value

2,024 km²

664 km² is most likely (plausible range: 444 to 1,992 km²)

Figure 12; discrete, 2x2 grid cells (2012 to 2023 observations)

IAO of most restrictive life stage, based on 2x2 grid cells summed over all estimated subpopulation spawning areas using most likely estimate of 1 (and plausible range of 0.6 to 3) spawning area per HUC10

Is the population “severely fragmented”, that is, is >50% of individuals or >50% of the total area “occupied” (as a proxy for number of individuals) in habitat patches that are both (a) smaller than required to support a viable subpopulation, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. Yes
2. Unknown

1. 77% of core areas contain <500 adults
2. Not separated by a distance larger than migratory fluvial subpopulations could disperse, although with extirpation of many of the downstream fluvial subpopulations, core areas could be separated distances greater than dispersal distance of resident Bull Trout

Number of “locations” (use plausible range to reflect uncertainty if appropriate)

2 to 9

Based on climate change effects on upstream and downstream subpopulations (2 locations; Figure 14), climate change combined with subpopulations accessible and inaccessible to fishing (4 locations), up to 9 locations based on combined threats in major watersheds

Is there an [observed, inferred, or projected] continuing decline in extent of occurrence?

Yes

Extirpations observed on East Slopes of Alberta; majority of the decline likely happened prior to the most recent 3 generations (Figure 7), but projections imply continuing decline

Is there an [observed, inferred, or projected] continuing decline in area of occupancy?

Yes

Extirpations observed on East Slopes of Alberta; majority of the decline likely happened prior to the most recent 3 generations (Figure 7), but projections imply continuing decline

Is there an [observed, inferred, or projected] continuing decline in number of subpopulations?

Yes

Extirpations observed on East Slopes of Alberta; majority of the decline likely happened prior to the most recent 3 generations (Figure 7), but projections imply continuing decline

Is there an [observed, inferred, or projected] continuing decline in number of “locations”?

No

Is there an [observed, inferred, or projected] continuing decline in [area, extent and/or quality] of habitat?

Yes

Observed, inferred, and projected decline in habitat extent and quality due to hydroelectric development, oil and gas development, transport systems, forest harvesting and mining activity

Are there extreme fluctuations in number of subpopulations?

No

Are there extreme fluctuations in number of “locations”?

No

Are there extreme fluctuations in extent of occurrence?

No

Are there extreme fluctuations in index of area of occupancy?

No

Number of mature individuals (by subpopulation)

Alberta (excluding Banff NP)

56,000 to 84,000

Based on data from 2013 to 2022 (see text)

Banff NP

Unknown

Total

>56,000 to 84,000

Does not include unknown BNP subpopulations

Quantitative analysis

Is the probability of extinction in the wild at least 20% within 20 years [or 5 generations], or 10% within 100 years]

Unknown

Analysis not conducted

Threats

Was a threats calculator completed for this species?

Yes (see Appendix 1)

Overall assigned threat impact: Very high-High

Key threats were identified and ranked as:

5 Biological resource use - High-Medium

7 Natural system modifications - High-Medium

11 Climate change and severe weather - High-Medium

6 Human intrusions and disturbance - Medium

9 Pollution - Medium

8 Invasive and other problematic species and genes - Medium-Low

3 Energy production and mining - Low

4 Transportation and service corridors - Low

What limiting factors are relevant?

• Low reproductive rate
• Very specific habitat requirements

Rescue effect (from outside Canada)

Status of outside population(s) most likely to provide immigrants to Canada.

Declining

Montana – Imperilled (S2) Idaho – Apparently Secure (S4)

Is immigration known or possible?

Very little possible

Most of the rivers in the DU flow east and north and in Montana they flow east and south so only minor connections within the mountains

Would immigrants be adapted to survive in Canada?

Yes

Is there sufficient habitat for immigrants in Canada?

Yes

Are conditions deteriorating in Canada?

Yes

Are conditions for the source (that is, outside) population deteriorating?

Yes

Is the Canadian population considered to be a sink?

No

Is rescue from outside Canada likely, such that it could lead to a change in status?

No

While immigration is possible, there would be insufficient immigration from the at-risk subpopulations in the United States to reduce risk of extirpation of the population in Canada

Wildlife species with sensitive occurrence data (general caution for consideration)

Could release of certain occurrence data result in increased harm to the Wildlife Species or its habitat?

No

Current status

Year of previous assessment

2012

COSEWIC status history

Designated Threatened in November 2012. Status re-examined and confirmed in May 2025.

Status and reasons for designation

Status: Threatened

Alpha-numeric codes: B2ab(i,ii,iii,v)

Reason for change in status: n/a

Reasons for designation: This large freshwater fish is broadly distributed east of the Rocky Mountains in southern Alberta. It is a slow-growing and late-maturing species that thrives in cold, pristine waters and often requires long, unimpeded migratory routes that connect spawning and adult habitats. Historical range contractions now limit the species to the foothills and east slopes of the Rocky Mountains, and there is evidence of continuing habitat deterioration and a decline in area of spawning habitat and number of mature individuals. The species is particularly vulnerable to negative effects from increases in water temperature, drought, fishing mortality, dams and water management activities, pollution, and competition and hybridization with non-native Brook Trout. If these threats cannot be mitigated, they could lead to this population becoming Endangered.

Applicability of criteria

A: Decline in total number of mature individuals

Not applicable.

The results of the cumulative-effects model projected declines that encompass thresholds for Threatened and Endangered, but they range widely (0.1 to 80.7%).

B: Small range and decline or fluctuation

Meets Threatened, B2ab(i,ii,iii,v).

IAO of most restrictive life stage (spawning) is below threshold for Threatened (664 km²), the population occurs at <10 locations, and is experiencing a continuing decline in EOO, IAO, extent and quality of habitat, and number of mature individuals.

C: Small and declining number of mature individuals

Not applicable.

Estimate of mature individuals (56,000 to 84,000) above thresholds.

D: Very small or restricted population

Not applicable.

Estimate of mature individuals (56,000 to 84,000) exceeds thresholds, and population is not vulnerable to rapid and substantial decline.

E: Quantitative analysis

Not applicable

Analysis not conducted.

Reasons for Special Concern, Data Deficient, Extirpated, or Extinct

Not applicable

Technical summary: DU5 – Pacific population

Salvelinus confluentus

Bull Trout (Pacific population)

Omble à tête plate (Population du Pacifique)

Range of occurrence in Canada: British Columbia

Demographic information

Generation time (usually average age of parents in the population)

10 years

Is there an [observed, estimated, inferred, or projected] continuing decline in number of mature individuals?

Unknown

No evidence of decline but surveys are incomplete, and there are contrasting trends across subpopulations

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 3 years [or 1 generation; whichever is longer up to a maximum of 100 years]

Not applicable

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 5 years [or 2 generations; whichever is longer up to a maximum of 100 years]

Not applicable

[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last 10 years [or 3 generations; whichever is longer]

Not applicable

[Projected, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations, up to a maximum of 100 years]

Unknown

Projection from Threats Calculator 3 to 70%, but insufficient data to reliably project, infer, or suspect population trends

[Observed, estimated, inferred, projected, or suspected] percent [reduction or increase] in total number of mature individuals over any period of 10 years [or 3 generations; whichever is longer, up to a maximum of 100 years], including both the past and future (up to a maximum of 100 years in future)

Not applicable

Are the causes of the decline clearly reversible?

Not applicable

Are the causes of the decline clearly understood?

Not applicable

Are the causes of the decline clearly ceased?

Not applicable

Are there extreme fluctuations in number of mature individuals?

No

Extent and occupancy information

Estimated extent of occurrence (EOO)

714,549 km²

Figure 13; all observations

Index of area of occupancy (IAO), reported as 2x2 km grid value

11,076km²

Figure 13; discrete, 2x2 grid cells summed over all observations.

No information is available on the number of subpopulation spawning sites, and therefore IAO of the most restrictive life stage is unknown but likely above thresholds

Is the population “severely fragmented”, that is, is >50% of individuals or >50% of the total area “occupied” (as a proxy for number of individuals) in habitat patches that are both (a) smaller than required to support a viable subpopulation, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. No
2. No

Population not concentrated within one subpopulation and the majority of subpopulations are likely sufficiently large to be viable

Number of “locations” (use plausible range to reflect uncertainty if appropriate)

>10

Based on combination of five spatially patchy primary threats

Is there an [observed, inferred, or projected] continuing decline in extent of occurrence?

No

Is there an [observed, inferred, or projected] continuing decline in area of occupancy?

No

Is there an [observed, inferred, or projected] continuing decline in number of subpopulations?

No

Is there an [observed, inferred, or projected] continuing decline in number of “locations”?

No

Is there an [observed, inferred, or projected] continuing decline in [area, extent and/or quality] of habitat?

No

Are there extreme fluctuations in number of subpopulations?

No

Are there extreme fluctuations in number of “locations”?

No

Are there extreme fluctuations in extent of occurrence?

No

Are there extreme fluctuations in index of area of occupancy?

No

Number of mature individuals (by subpopulation)

British Columbia has abundance estimates from 33 out of 73 core areas within this DU (see Appendix 1)

20,000 to 62,000

See Appendix 1

Total

>20,000 to 62,000

Substantial underestimate for this DU

Quantitative analysis

Is the probability of extinction in the wild at least 20% within 20 years [or 5 generations], or 10% within 100 years]

Unknown

Analysis not conducted

Threats

Was a threats calculator completed for this species?

Yes (see Appendix 2)

Overall assigned threat impact: Medium-High

Key threats were identified and ranked as:

4 Transportation and service corridors - Medium-Low

5 Biological resource use - Medium-Low

7 Natural system modifications - Medium-Low

8 Invasive and other problematic species and genes - Medium-Low

9 Pollution - Medium-Low

11 Climate change and severe weather - Medium-Low

Unknown threats were:

3 Energy production and mining

6 Human intrusions and disturbance

10 Geological events

What limiting factors are relevant?

• Low reproductive rate
• Very specific habitat requirements

Rescue effect (from outside Canada)

Status of outside population(s) most likely to provide immigrants to Canada.

Declining

The southern extension of this DU in Washington State is listed as “Species of Greatest Conservation Need” and Critically Imperilled S1

Is immigration known or possible?

Yes

Possible through the Columbia River system that crosses the international border; however, several dams in the area block access to all but a very small part of the river network

Would immigrants be adapted to survive in Canada?

Yes

Is there sufficient habitat for immigrants in Canada?

Yes

Are conditions deteriorating in Canada?

Unknown

Are conditions for the source (that is, outside) population deteriorating?

Yes

Is the Canadian population considered to be a sink?

No

Is rescue from outside Canada likely, such that it could lead to a change in status?

No

Wildlife species with sensitive occurrence data (general caution for consideration)

Could release of certain occurrence data result in increased harm to the Wildlife Species or its habitat?

No

Current status

Year of previous assessment

2012

COSEWIC status history

Designated Not at Risk in November 2012. Status re-examined and designated Special Concern in May 2025.

Status and reasons for designation

Status: Special Concern

Alpha-numeric codes: n/a

Reason for change in status: II.iv; II.v; II.vii; II.viii; II.ix; II.xi; IV.iii

Reasons for designation: This large freshwater fish is broadly distributed throughout Pacific drainages in British Columbia. It is a slow-growing and late-maturing species that thrives in cold, pristine waters, and requires unimpeded migratory routes that connect spawning and adult habitats. Although never abundant, there are many dispersed sub-populations across this area. There is no evidence of overall decline in abundance of mature individuals or distribution, but the species is particularly vulnerable to negative effects from road development associated with forest harvesting and mining exploration, dams and water management activities, increases in water temperature, drought, fishing mortality, and pollution. The revised status reflects these increasing threats; if they are not reversed or managed effectively, this distinctive Canadian population could become Threatened.

Applicability of criteria

A: Decline in total number of mature individuals

Not applicable.

Insufficient data to reliably infer, project, or suspect population trends.

B: Small range and decline or fluctuation

Not applicable.

EOO (714,549 km²) and IAO (11,076km²) exceed thresholds.

C: Small and declining number of mature individuals

Not applicable.

Estimate of mature individuals (20,000 to 62,000) exceeds thresholds.

D: Very small or restricted population

Not applicable.

Estimate of mature individuals (20,000 to 62,000) exceeds thresholds, and population is not vulnerable to rapid and substantial decline.

E: Quantitative analysis

Not applicable.

Analysis not conducted.

Reasons for Special Concern

(b) the Wildlife Species may become Threatened if factors suspected of negatively influencing the persistence of the Wildlife Species are neither reversed nor managed with demonstrable effectiveness.

Preface

Since the previous COSEWIC assessment in 2012, there have been many initiatives to fill information gaps, analyses to better understand temporal and spatial trends, development of recovery strategies, and initiation of legal protections provincially and federally. British Columbia has enhanced information on abundance and trends at the scale of core areas within the four DUs that completely (South Coast) or partially (Pacific, Upper Yukon Watershed, Western Arctic) occur there. Alberta has also enhanced information on abundance and trends organized across Hydrological Unit Codes (HUC) for the two DUs that occur completely (Saskatchewan-Nelson Rivers) or partially (Western Arctic) in the province. Information on distribution in the Data Deficient Upper Yukon Watershed DU has been improved using environmental DNA (eDNA) sampling. For the first time, estimates of biological area of occupancy have been assembled for the Saskatchewan-Nelson Rivers DU.

Bull Trout was listed within the Species At Risk Act in 2019: Saskatchewan-Nelson Rivers - Threatened; South Coast - Special Concern; Western Arctic - Special Concern. In Alberta, Bull Trout is listed as Threatened under Alberta’s Wildlife Act as of 2014. In British Columbia, Bull Trout is on the Provincial Blue List and is an Identified Wildlife Species at Risk under the Identified Wildlife Management Strategy. Critical Habitat has been identified in the federal recovery strategy and protected by a federal Order in Council in 2021. Bull Trout in national park waters is protected under the Canada National Parks Act. Critical Habitat for Bull Trout was identified and is legally protected in Banff, Jasper, and Waterton Lakes National Parks as of 2021.

A Recovery Potential Assessment was developed by Fisheries and Oceans Canada for the Threatened Saskatchewan-Nelson Rivers population. Alberta has developed a Bull Trout Recovery Plan, and British Columbia has developed a Management Plan for Bull Trout.

COSEWIC history

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions

(2025)

Wildlife species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years

Extinct (X)

A wildlife species that no longer exists

Extirpated (XT)

A wildlife species no longer existing in the wild in Canada, but occurring elsewhere

Endangered (E)

A wildlife species facing imminent extirpation or extinction

Threatened (T)

A wildlife species likely to become endangered if limiting factors are not reversed

Special concern (SC)*

A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats

Not at risk (NAR)**

A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances

Data deficient (DD)***

A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction

*

Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.

**

Formerly described as “Not In Any Category”, or “No Designation Required”

***

Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.

Wildlife species description and significance

Name and classification

Current classification

Class: Actinopterygii

Order: Salmoniformes

Family: Salmonidae

Genus: Salvelinus

Species: Salvelinus confluentus (Suckley 1859)

Common names

English: Bull Trout

French: Omble à tête plate

Synonyms and notes

The taxonomy of North American char (Salvelinus), to which Bull Trout (Salvelinus confluentus) belongs, has a tangled history. Many of the systematic uncertainties stem from limitations of morphological analyses in a group of fishes with extensive phenotypic plasticity. This Holarctic genus has been heavily influenced by Pleistocene glaciations, with periodic episodes of range fragmentation also confounding its complex intraspecific relationships. Historical processes, including fragmentation within refugia (Taylor et al. 1999; Brunner et al. 2001), as well as introgression between species within refugia or in subsequently recolonized deglaciated areas (Bernatchez et al. 1995; Wilson and Bernatchez 1998; Phillips et al. 1999; Redenbach and Taylor 2002), have likely contributed to conflicting phylogenies from morphological, mitochondrial DNA, and nuclear markers (Grewe et al. 1990; Phillips et al. 1999; Redenbach and Taylor 2002; Crespi and Fulton 2004).

For many years, Dolly Varden (Salvelinus malma) and Bull Trout were considered to be geographic variants within the Arctic Char (Salvelinus alpinus) species complex. Even when morphological analysis showed them to be sufficiently divergent from Arctic Char to be designated a separate species, S. confluentus remained part of the S. malma “species complex” (McPhail 1961). Once often assumed to be the landlocked form of Dolly Varden, subsequent analysis revealed Bull Trout to be sufficiently morphologically diverged from Dolly Varden to warrant designation as an individual species in 1978 (Cavender 1978; Haas and McPhail 1991). Molecular phylogenies now reveal Bull Trout and Dolly Varden are, in fact, not even sister species; the two species probably last shared a common ancestor more than 1 million years ago (Grewe et al. 1990; Crane et al. 1994; Phillips et al. 1994). Subsequent genetic evidence of these two char species maintaining distinct gene pools in sympatry, despite some ongoing hybridization and gene flow (Baxter et al. 1997; Taylor et al. 2001; Redenbach and Taylor 2003), provides the most compelling evidence yet that Dolly Varden and Bull Trout are distinct biological species.

Description of wildlife species

Bull Trout is a long slender fish with a comparatively large head and jaws, hence the derivation of its common name “bull.” Its body size at maturity depends on life history strategy. The average length and range of the resident form is 250 [140 to 410] mm; fluvial form is >400 [240 to 730] mm; and adfluvial form is >400 [330 to 900+ (reviewed in Pollard and Down 2001; Rodtka 2009; Mochnacz et al. 2013). Although under-reported in the literature, it may be that anadromous Bull Trout attain the largest sizes of all (Brenkman et al. 2007).

Bull Trout is olive-green to blue-grey in colour, with adfluvial fish often displaying silvery sides (Nelson and Paetz 1992). Pale round spots along its flank and back that are pink, lilac, yellow-orange, or red distinguish it from other salmonids: Brook Trout (Salvelinus fontinalis) has distinct, light-coloured, worm-like markings on top of the head, back, and dorsal fin, while Rainbow Trout (Oncorhynchus mykiss), Cutthroat Trout (O. clarkii), and Brown Trout (Salmo trutta) have dark spots (Nelson and Paetz 1992; McPhail 2007). Bull Trout usually has a pale belly, which may turn red or orange in spawning males (Nelson and Paetz 1992). Its tail fin is slightly forked, and pelvic or anal fins may have a leading white edge, but this is not followed by black as it is in Brook Trout (Nelson and Paetz 1992). Bull Trout larvae may be distinguished from other larval char by the presence of a prominent fleshy ridge underneath the chin (Gould 1987).

Bull Trout is morphologically very similar to Dolly Varden. Although no single character can consistently distinguish between them, the two species do differ across a suite of several characters. Generally, Bull Trout has a larger, broader, and flatter head than Dolly Varden, with a more slender, ventrally flattened body (Cavender 1978; Haas and McPhail 1991). Together, branchiostegal ray number, anal fin ray number, and the ratio of total upper jaw length to standard body length consistently distinguish between the two species. Bull Trout tends to have a larger upper jaw in proportion to its body length compared with Dolly Varden. It also has more anal fin rays and branchiostegal rays (Haas and McPhail 1991). A morphometric identification protocol using these four variables is presented in Haas and McPhail (1991).

Designatable units

The phylogeography of Bull Trout has been well studied and provides strong evidence for two major genetic lineages of Bull Trout in northwestern North America: a southern coastal group (henceforth called Genetic Lineage 1) and an interior group (henceforth called Genetic Lineage 2). The first genetic evidence came from mitochondrial DNA (mtDNA); a survey of mtDNA variation (115 restriction sites over 410 base pairs) from Bull Trout at 47 sites (N = 348) spanning the geographical range of the species revealed a sharp discontinuity in the geographical distribution of haplotypes (sets of alleles of closely linked loci; Taylor et al. 1999). While most of Genetic Lineage 1 based on mtDNA occurs at or west of the Coast and Cascade mountain crests, most of Genetic Lineage 2 based on mtDNA is found east of these (Figure 1). The sequence divergence (d) between these lineages is comparable to that found in other northern Holarctic fishes (d = 0.8% [Taylor et al. 1999, 2001] compared to average maximum intraspecific d of approximately 1.2% from 25 other species [Bernatchez and Wilson 1998]). Subsequent surveys of nuclear DNA (microsatellites) across Bull Trout’s geographical range have consistently corroborated the presence and distribution of these groupings (Spruell et al. 2003; Taylor and Costello 2006). Morphological and comparative life-history (Haas and McPhail 2001) evidence has also substantiated this major subdivision of Bull Trout into Genetic Lineages 1 and 2.

Figure 1. Distribution of two major Bull Trout mitochondrial DNA lineages identified by restriction fragment length polymorphism of Bull Trout from 47 sites (N = 348); SD = sequence divergence. The solid black line dividing groups A (Genetic Lineage 1) and B (Genetic Lineage 2) is the approximate location of the Cascade/Coast Mountain crest. From Taylor et al. (1999) and COSEWIC (2012).

This pattern of an inland/coastal genetic split corresponding to the Coast and Cascade mountain ranges is one that is repeated in other northwestern North American fishes (for example, Rainbow Trout: McCusker et al. 2000; Cutthroat Trout: Allendorf and Leary 1988; Chinook Salmon, Oncorhynchus tshawytscha: Teel et al. 2000; Coho Salmon, Oncorhynchus kisutch: Small et al. 1998; and Longnose Sucker, Catostomus catostomus: McPhail and Taylor 1999), as well as other taxa (for example, amphibians: Carstens et al. 2005). It is likely explained by the Bull Trout’s historical isolation in, and subsequent post-glacial dispersal from, two distinct glacial refugia at the southern edges of the Cordilleran ice sheet during the late Pleistocene: the Chehalis Refuge and the Columbia Refuge (Taylor et al. 1999).

The Chehalis Refuge is a region dominated by drainages of the Chehalis River between the Columbia River and Puget Sound that was ice-free during much of the Pleistocene. Based on the distribution of endemic species and differentiated populations in fishes and plants, it was likely independent from the nearby Columbia Refuge (Taylor et al. 1999). It was the probable refuge for Genetic Lineage 1 Bull Trout, given the localization of this lineage around the southern region of British Columbia (the lower Fraser River below Hell’s Gate Canyon and Squamish River systems), Puget Sound and the Olympic Peninsula in western Washington, the lower Columbia River, and the Klamath River in southwestern Oregon (Figure 1). Postglacial dispersal from this refuge into the lower Fraser or Columbia rivers or adjacent coastal systems may have occurred via freshwater connections through the Puget lowlands (McPhail 1967; Thorson 1980), or even via the sea given this group’s anadromous behaviour. The Columbia Refuge probably served as the source of Bull Trout’s Genetic Lineage 2 postglacial colonists. Well-documented postglacial connections among the upper Columbia in the U.S.A. and Canada right through to more northern and eastern draining systems (that is, Liard River in British Columbia, lower Peace, Athabasca, and South Saskatchewan rivers in Alberta) would have aided the dispersal of this group across the Continental Divide into interior regions (Lindsey and McPhail 1986; McPhail and Lindsey 1986).

Patterns of postglacial dispersal from these refugia can also account for peculiarities in the geographical distribution of the two lineages. For example, headwater faunal exchanges between interior and coastal drainages likely explain why all large coastal-draining systems north of the Squamish River (for example, Skeena, Stikine, Nass, Klinaklini) carry Genetic Lineage 2 Bull Trout mtDNA and microsatellite DNA alleles (Figure 2). Interdigitation of these rivers’ extensive headwater tributaries is strongly suspected to be the route of past faunal exchanges (Lindsey and McPhail 1986; McPhail and Lindsey 1986) and was the likely conduit for the expansion of Genetic Lineage 2 west of the Coast mountains’ divide at its mid-northern end (Taylor et al. 1999; Taylor and Costello 2006).

Figure 2. UPGMA (unweighted pair group method with arithmetic mean) dendrogram of pairwise sequence divergence estimates from 21 restriction fragment length polymorphism mitochondrial DNA haplotypes, including 348 Bull Trout samples analyzed from 47 sites. For each haplotype, geographical locations in which it occurred are listed. Genetic lineages and probable anadromous populations are indicated (*). From Taylor et al. (1999) and COSEWIC (2012). DUs for Canadian sites within Genetic Lineage 2 are: Western Arctic – upper and lower Peace, upper and lower Liard; Saskatchewan-Nelson – South Saskatchewan; Pacific – Nass, Skeena, upper Columbia, Klinaklini, middle and upper Fraser.

Another anomaly occurs at the southern end of the Bull Trout’s range. Here, Genetic Lineage 1 predominates in the lower Columbia area at or west of the Cascade Crest (Figure 1; Taylor et al. 1999; Spruell et al. 2003) despite the presumed role of the Lower Columbia River valley as a glacial refuge for the Genetic Lineage 2 lineage. Fish in the Columbia refuge likely concentrated east of this divide and dispersed mostly inland into the upper Columbia, Fraser, and other northern interior drainages, while Bull Trout from the Chehalis Refuge went on to colonize the lower reaches of the Columbia River valley (Taylor et al. 1999). The hypothesis of the lower Columbia River not being a single faunal unit in terms of postglacial dispersal of freshwater fishes was, in fact, postulated to account for the curious absence of several other species that occur widely elsewhere in this river system (McPhail and Lindsey 1986).

A further transition between Bull Trout Genetic Lineage 1 and 2 occurs abruptly in the Fraser River at an area known to be difficult for fish passage, the Fraser Canyon (Figures 1 and 2). The Fraser Canyon is associated with abrupt shifts in the distribution of genetic variation within some other fish species (see Taylor et al. 1999), as well as changes in the geographical distribution of several others (McPhail and Lindsey 1986). Evidently, this point of biogeoclimatic change from coastal wetlands to dry interior represents a strong natural barrier to fish dispersal and has maintained a bimodal contact zone between the two Bull Trout lineages, which have colonized this river from opposite directions.

There is a potentially significant gap in our understanding of Bull Trout phylogeography due to the lack of sampling in and around the Nahanni glacial refuge and limited sampling of mtDNA variation in Alberta. There is potential that an additional, distinct lineage exists with a relatively small distribution in the Northwest Territories (NWT), and less likely but possibly in northern Alberta, which may have implications for DU delineation.

In addition to the major division of Bull Trout into two evolutionary lineages, the hierarchical division of genetic variation among local populations (where, in this context (and this section), “populations” refers to groups of individuals of the same species that are spatially, genetically, or demographically separated from other groups) provided insights into the extent and origin of diversity within Bull Trout. Throughout Bull Trout’s range, most genetic variation resides at the interpopulation and inter-region level. For example, a mtDNA survey (115 restriction sites over 410 base pairs) of 47 populations (N = 348) sampled from across its geographical range revealed that 55% of the variation was found between Genetic Lineage 1 and 2, 33% between populations within these groups, and only 12% within them (P < 0.00005; Taylor et al. 1999). Similarly, a comprehensive survey of microsatellites (N = 7) among 57 populations (N = 1561) sampled from across its range found most variation (46%) between the two lineages, 21% among populations within groups, and 33% within them (P < 0.001; Taylor and Costello 2006).

Not surprisingly, therefore, there is a high degree of substructure within geographical lineages; overall F_ST among populations (N = 8 to 37) within lineages but spanning many hundreds of kilometres have been consistently estimated as lying between 0.30 and 0.39 (P < 0.005) in microsatellite (N ≥ 5) studies (Taylor et al. 2001; Costello et al. 2003; Whiteley et al. 2004; Taylor and Costello 2006). Significant microsatellite differentiation among populations (P < 0.05) within localized areas is even common (Spruell et al. 1999; Taylor et al. 2001; Costello et al. 2003; Taylor and Costello 2006; Carroll and Vamosi 2021). However, caution is warranted in defining Bull Trout populations according to a priori stream-of-origin designations. As for other stream-spawning fishes, fine-scale population structure in Bull Trout has traditionally been explored by designating genetic populations according to where individuals were captured. However, not all streams-of-origin may represent genetically distinguishable units and, even though levels of gene flow are low amongst Bull Trout populations, one cannot assume that individuals sampled at a site originated there.

The restricted gene flow suggested by the high degree of substructure found within geographical lineages of Bull Trout will favour divergence among different selective environments (Lenormand 2002). Given empirical evidence that estimates of neutral genetic divergence provide conservative estimates of adaptive divergence (Pfrender et al. 2000; Morgan et al. 2001), microsatellite assays of neutral genetic variation are likely to be conservative estimates of Bull Trout biodiversity. As is common among salmonids (Quinn and Dittman 1990), Bull Trout most likely diverges in quantitative traits important to population persistence in specific environments. Local adaptation will likely be most evident at larger scales, for example among populations inhabiting the four different National Freshwater Biogeographic Zones (NFBZs) that the Bull Trout’s range straddles (NFBZ 4 [Saskatchewan-Nelson Rivers Watershed], 6 [Yukon River Watershed], 11 [Pacific] and 13 [Western Arctic]; Figure 3). The disjunction between two groupings of these ecozones (Areas 4 and 13, and 11 and 6) by the Rocky Mountains is likely to foster adaptive divergence.

Figure 3. Canadian Bull Trout distribution overlain on the National Freshwater Biogeographic Zones. The Designatable units are identified by number: 1 – Southcoast BC population; 2 – Western Arctic population; 3 – Upper Yukon River population; 4 – Saskatchewan-Nelson Rivers population; 5 – Pacific population. Adapted from COSEWIC (2012) and Sawatzky (2016).

Given the plethora of historical, contemporary landscape, and biological influences, it is no surprise that there is considerable variation in genetic structure across the range of Bull Trout at the fine scale. Within the broad pattern of low genetic diversity within and high differentiation between populations, there are significant differences in mean H_E, number of alleles, and pairwise F_ST among river basins (Whiteley et al. 2006). This indicates the varying roles of genetic drift and gene flow at this scale.

Two recent studies have focused on discreteness across the boundary of the Western Arctic and Saskatchewan-Nelson Rivers National Freshwater Biogeographic Zones (Carrol and Vamosi 2021; Franks 2024). The larger of the two studies analysed genetic markers from a Bull Trout RADcapture SNP (single nucleotide polymorphism) panel (Franks 2024). After filtering, it used 582 variable loci of 962 Bull Trout captured from populations distributed across the Eastern Slopes of Alberta. Admixture analysis showed: (1) strong differentiation between the Western Arctic and Saskatchewan-Nelson Rivers NFBZs (that is, K = 2, where K is the number of clusters representing genetic populations); and (2) a hierarchical structure that also identifies differentiation among major river systems within DUs (K = 5) and among smaller scale hydrological units (K = 24) (Franks 2024; Figure 4).

Considering COSEWIC’s “discreteness” and “significance” criteria (COSEWIC 2020), where discrete means that there is little or no transmission of heritable information between it and other DUs, and evolutionarily significant means that the unit harbours an evolutionary history and/or heritable adaptive traits that cannot be reasonably expected to be practically reconstituted, we recognize five Designatable units for Bull Trout in Canada (Figure 3, Table 1):

Genetic Lineage 1

DU1 South Coast British Columbia population

Genetic Lineage 2

DU2 Western Arctic population

DU3 Upper Yukon Watershed population

DU4 Saskatchewan-Nelson Rivers population

DU5 Pacific population

Figure 4. Admixture analysis of 962 Bull Trout genotypes across 24 Hydrologic Unit Code (HUC) 8 watersheds using: K = 2 Designatable units; K = 5 (based on five distinct HUC2 watersheds); and K = 24 (based on 24 HUC8 watersheds). Watersheds with individuals are arranged from north to south in Alberta. Note that the Peace and Athabasca rivers are in the Western Arctic DU and the North Saskatchewan, Red Deer, and South Saskatchewan rivers are in the Saskatchewan-Nelson Rivers DU. From Franks (2024).

D1 – Heritable traits or markers, indicating limited transmission

• Genetic Lineage 1 (DU1) vs Genetic Lineage 2 (DU2, DU3, DU4, DU5), mtDNA RFLP haplotypes (Figure 2); anadromy unique to DU1
• DU2 vs DU4, RADcapture SNP panel of 582 variable loci (Figure 4)

D2 – Natural geographic disjunction such that transmission has been severely limited for an extended time

• DU1, DU5 (Pacific, west to Pacific Ocean) vs DU2 (Western Arctic, north into Mackenzie R and Arctic Ocean) vs DU3 (Upper Yukon, north and west through Alaska into Bering Sea) vs DU4 (Saskatchewan-Nelson, east and north into Hudson Bay) National Freshwater Biogeographic Zones
• Separated for approximately 10,000 years since the retreat of the continental glaciers

S1 – Direct evidence or strong inference of independent evolutionary trajectories for an evolutionarily significant period, usually origins in separate Pleistocene refugia

• Genetic Lineage 1 (DU1; Chehalis Refugium) vs Genetic Lineage 2 (DU2, DU3, DU4, DU5; Columbia Refugium)

S2 – Inference of adaptive heritable traits that cannot be practically reconstituted if lost (for example, in ecological settings where a selective regime is likely to have given rise to DU-wide local adaptations)

• Temperature regimes: DU2 (colder, shorter growing season) vs DU4 (warmer, longer growing season)
• Distinctive ecological and zoogeographic settings in each NFBZ: DU1, DU5 (unique fauna, esp. secondary freshwater fishes) vs DU2 (fast‐flowing streams with headwaters in the Rocky Mountains; unique mix Bering and Great Plains fishes) vs DU3 (species-poor Bering fauna) vs DU4 (temperate upland rivers, large prairie rivers, and large and small lakes mix of Rocky Mountain foothill fishes and prairie fishes)

Evidence for discreteness

Discreteness criterion D1 (evidence from genetic markers indicating limited transmission of heritable information) is supported by the occurrence of two pre-Pleistocene lineages differentiating Genetic Lineage 1 (South Coast British Columbia) from Genetic Lineage 2 encompassing the remainder of the distribution of Bull Trout in Canada. Within Genetic Lineage 2, discreteness criterion D1 is also supported by the genetic information available across the boundary of the Western Arctic and Saskatchewan-Nelson Rivers NFBZs (Franks 2024; Figure 4), and the Bull Trout in these two NFBZs have been separated for approximately 10,000 years since the retreat of the continental glaciers (Post et al. 2016). Although equivalent genetic evidence of discreteness does not exist for all pairwise borders across NFBZs, one might infer that they are also discrete from one another as they have also been separated since de-glaciation (Post et al. 2016). Thus, Bull Trout in all four NFBZs within Genetic Lineage 2 meet discreteness criterion D2 (natural geographic disjunction such that transmission of information between these range portions have been severely limited for an extended time).

Evidence for evolutionary significance

Evolutionary significance criterion S1 is supported through evidence that Genetic Lineage 1 and 2 have separate Pleistocene refugia. Evolutionary significance criterion S2 is supported through occupancy within NFBZs which represents natural disjunctions, with little or no possibility of natural dispersal between these zones following the retreat of the Pleistocene ice sheets approximately 10,000 years ago (McPhail and Lindsey 1970). Following criterion D2, it can be inferred that sufficient time has passed (approximately 1,000 generations) that natural selection and genetic drift are likely to have produced evolutionarily discrete units. The DUs occupy hydrologically discrete watersheds: Pacific NFBZ flows west into the Pacific Ocean; the Upper Yukon Watershed NFBZ flows north and west through Alaska; the Western Arctic NFBZ flows northward into the Mackenzie River and into the Arctic Ocean; and the Saskatchewan-Nelson Rivers NFBZ flows eastward and north into Hudson Bay. As such, there are no contemporary connections, so exchanges among Bull Trout populations in these NFBZs are very unlikely (Post et al. 2016).

Evolutionary significance criterion S2 is supported by various aspects of the zoogeography, ecology, and evolutionary history of Bull Trout within the discrete NFBZs. Genetic divergence leading to adaptive and heritable traits is inferred for the NFBZs based primarily on differences in environmental temperature and fish community composition over the last approximately 10,000 years of spatial separation.

The Pacific NFBZ encompasses, in part, Bull Trout subpopulations east of the Coastal-Cascade Mountain crest that are tributary to the North Pacific Ocean. Their extinction would constitute a loss of approximately 50% of the range of Bull Trout, and the vast majority (> 90%) of the range west of the Continental Divide. This assemblage of subpopulations is also the only one to contain representatives of Genetic Lineage 1, the major evolutionary Bull Trout lineage which dominates subpopulations south of about 50 degrees north latitude. Genetic Lineage 1 contains the only anadromous Bull Trout, a major life history difference with attendant adaptations for survival in marine waters relative to inland populations. The Pacific NFBZ also includes representatives of the other major evolutionary Bull Trout lineage, Genetic Lineage 2, with the transition between Genetic Lineage 1 and 2 occurring abruptly in the Fraser River at the Fraser Canyon. These lineages are distinguished by mtDNA and corroborated by a diverse and independent set of traits (neutral nuclear DNA markers, other inherited traits, and biogeographical patterns). Two DUs for the Pacific NFBZ are, therefore, proposed: Genetic Lineage 1: South Coast British Columbia population, and Genetic Lineage 2: Pacific population.

The other four DUs contain only representatives of Genetic Lineage 2, each existing in a distinctive ecological and zoogeographic setting. The Western Arctic NFBZ encompasses a proposed DU whose subpopulations are from the Mackenzie River system (and major tributaries such as the Liard, Peace, and Athabasca rivers). These rivers have a distinctive zoogeographic assemblage of fishes, a variable mix of largely Bering and Great Plains species (Post et al. 2016). Over 50 species are known from this ecoregion (Mandrak et al. 2023), including at least 30 species whose distributions overlap with that of Bull Trout (Watkinson pers. comm. 2024). Many of the rivers in this biogeographic zone begin as fast‐flowing streams with headwaters in the Rocky Mountains (Post et al. 2016). The stream temperature map produced by Weller et al. (2023) shows that the thermal environment in north-central British Columbia, where most core Bull Trout populations occur, aligns with the thermal environment occupied by core populations in the Northwest Territories by Mochnacz et al. (2023), both within the Western Arctic NFBZ. Conversely, data from core populations in the Saskatchewan-Nelson DU show that Bull Trout occupy warmer thermal environments than those reported in the Western Arctic DU. Together, these data suggest that the thermal environment in the Western Arctic DU differs from the Saskatchewan-Nelson DU. Loss of Bull Trout in this NFBZ would eliminate approximately 30% of the range of Bull Trout in Canada and the few that occur north of the Arctic Circle.

The Upper Yukon Watershed NFBZ encompasses a proposed DU within the Yukon River drainage. This represents the only assemblage of Bull Trout subpopulations west of the Continental Divide in a system that is tributary to the Bering Sea. The Yukon River watershed in northern British Columbia and southern Yukon Territory has a distinctive and relatively species-poor freshwater fauna derived from the Bering Glacial Refuge (Walker 1976; Lindsey and McPhail 1986) such that these subpopulations of Bull Trout exist in an ecological setting that is unusual for the species as a whole.

The Saskatchewan-Nelson Rivers NFBZ consists of a proposed DU whose subpopulations are tributary to the western headwaters of the North and South Saskatchewan rivers. This large biogeographic zone includes temperate upland rivers, large prairie rivers, and large and small lakes covering much of the Canadian prairies. These systems, particularly the latter, are dominated by a Great Plains fish fauna within an environmental setting that is warmer with longer growing seasons and quite distinct compared to north-flowing Arctic drainages (Post et al. 2016). Over 100 species—a mix of Rocky Mountain foothill fishes and prairie fishes—are known from this NFBZ (Post et al. 2016; Mandrak et al. 2023), including at least 20 species whose distributions overlap with that of Bull Trout (Watkinson pers. comm. 2024). Loss of these subpopulations would eliminate the only component of the Bull Trout assemblage in Canadian watersheds that are tributary to the Hudson Bay drainage.

The Pacific DU exists in a distinctive ecological and zoogeographic setting compared to the other Genetic Lineage 2 populations. Composed of temperate catchments flowing into the Pacific Ocean, it experiences a wide range of climates, and diverse freshwater habitats ranging from small fast‐flowing streams to large rivers such as the Fraser as well as small lakes and large lakes (Post et al. 2016). Over 70 species are known from this NFBZ (Post et al. 2016; Mandrak et al. 2023), including more than 40 species whose distributions overlap with that of Bull Trout (Watkinson pers. comm. 2024), especially secondary freshwater fishes such as Pacific salmon, lampreys, and sticklebacks (Post et al. 2016).

In summary, recognition of five DUs of Bull Trout in Canada is based on the discreteness inherent in two phylogenetic lineages occupying four NFBZs. Each of these DUs is also evolutionarily significant due to inferred likely local adaptations in response to their distinctive ecological and zoogeographic settings, primarily driven by temperature and fish community differences among DUs. Consequently, this report recognizes five DUs for Bull Trout in Canada (Figure 3).

Special significance

Most of the global distribution of Bull Trout is within Canada. It is often a top predator in depauperate unproductive freshwater ecosystems in western and northern Canada. As an obligate resident of cold, clear, and clean aquatic ecosystems, Bull Trout is a useful indicator species of habitat quality degradation.

Aboriginal (Indigenous) knowledge

Aboriginal Traditional Knowledge (ATK) is relationship-based. It involves information on ecological relationships between humans and their environment, including characteristics of species, habitats, and locations. Laws and protocols for human relationships with the environment are passed on through teachings and stories and Indigenous languages and can be based on long-term observations. Place names provide information about harvesting areas, ecological processes, spiritual significance or the products of harvest. ATK can identify life history characteristics of a species or distinct differences between similar species.

Cultural significance to Indigenous peoples

This species is culturally significant to Indigenous Peoples who hold detailed knowledge on the evolving, dynamic nature of the species. Bull Trout is important to Indigenous Peoples who recognize the interrelationships of all species within the ecosystem. There are two publicly available sources of ATK on Bull Trout in Canada. The Bull Trout subpopulation of the Nak’azdli Whut’en Territory within the Pacific DU is considered stable and the fish healthy in the Nation River and its tributaries (Pearce et al. 2019). Bull Trout redd surveys have identified active spawning behaviour in the upper Oldman River in the Saskatchewan-Nelson Rivers DU within the Blackfoot Confederacy (Siksikaitsitapi Blackfoot Confederacy, no date supplied).

Distribution

Global range

Bull Trout is endemic to western Canada and the US Pacific Northwest like many other taxa that have recolonized formerly glaciated areas (Figure 5). Its range in the contiguous U.S., however, has become greatly restricted in recent times with its southernmost occurrence now lying at the Oregon-California border (Haas and McPhail 1991; USFWS 2008).

Figure 5. Approximate historical and current global range of Bull Trout. Historical range from McPhail and Baxter (1996) and current range modified from Figure 5 in USFWS (2008) and Rodtka (2009).

Canadian range

The largest portion of Bull Trout’s global range occurs in western Canada (about 80% of its global range; Rieman et al. 1997), occurring in British Columbia, Alberta, Yukon, and the Northwest Territories across four NFBZs (Figures 3 and 4). In British Columbia, it occurs in 26 of 36 Ecological Drainage Units (EDUs). This classification level represents distinct major water drainages that contain unique fish assemblages based on broad zoographic, physiographic, and climate patterns (Ciruna et al. 2007). Bull Trout is found in the cool waters of most major mainland drainages of this province; it is distributed throughout interior river drainages (for example, upper Columbia, Peace, Liard, and Yukon river drainages) and in those major coastal drainages that penetrate the Coast Mountains into the interior of the province (Haas and McPhail 1991; McPhail 2007; Hagen and Decker 2011). The species is absent, however, from Vancouver Island, Haida Gwaii, and three adjacent, warm water drainages in the southern interior (Haas and McPhail 1991; McPhail 2007; Hagen and Decker 2011). Bull Trout records on Vancouver Island were initially found on iMap / Habitat Wizard from the 1990s and early 2000s, but these appear to be misidentifications. There are no verified records of Bull Trout on Vancouver Island (Czembor pers. comm. 2025; Taylor pers. comm. 2025), and these earlier records were likely either Dolly Varden misidentifications or the “BT” in the records is a data entry error that was intended to indicate Brown Trout (Czembor pers. comm. 2025).

Bull Trout is found in all of the major eastern slope drainages in Alberta (Figure 3; Haas and McPhail 2001; Rodtka 2009). Historically, Bull Trout was more widely distributed in this province (Figures 6 and 7), with anecdotal information and limited historical records suggesting a large decline in distribution in all the river systems occupied in Alberta since the early 1900s. Where Bull Trout was found as far downstream as Fort McMurray in the 1970s, it now tends to occupy primarily the upstream reaches of the major drainages, although occasional downstream captures of Bull Trout are recorded (Sullivan and Reilly 2023). Most subpopulations are now found within the Rocky Mountain and Foothills natural regions, as well as a small portion of the Peace River Parkland and Dry Mixedwood subregions (Rodtka 2009).

Bull Trout occurs in the western portion of the Northwest Territories, in Mackenzie River drainages north to the central Sahtú Settlement Area (Reist et al. 2002). Since then, work has continued to strengthen our knowledge about the northern extent of this species and has confirmed that Bull Trout is widely distributed, but typically at low densities, throughout much of southern and central Northwest Territories in drainages west of the Mackenzie River (Mochnacz et al. 2006; Mochnacz and Reist 2007; Mochnacz et al. 2013) (Figure 8). To date, the northernmost known record is from the Gayna River (Mochnacz et al. 2009). While this summarizes the most up-to-date understanding of Bull Trout’s distribution in the Northwest Territories, new information from this area will continue to refine our knowledge as it becomes available (Reist and Sawatzky 2010; Mochnacz et al. 2013).

Figure 6. Historical (<1950) distribution of adult Bull Trout in Alberta assessed at the scale of HUC8s. The colour coding is based on density relative to an Alberta standard and ranges from very high to functionally extirpated (from Alberta Government 2018).

Figure 7. Current (2013) distribution of adult Bull Trout in Alberta assessed at the scale of HUC8s. The colour coding is based on density relative to an Alberta standard and ranges from very high to functionally extirpated (from Alberta Government 2018). Common names for the functionally extirpated HUCs (> 3 generations ago) are from Reilly (pers. comm. 2025), and they were mapped using their HUC8 codes from Alberta Government (2018).

Figure 8. Distribution and sampling sites of Bull Trout and Dolly Varden in Northwest Territories. From Mochnacz et al. (2013).

In Yukon Territory, Bull Trout occurs mainly in the Liard River drainage basin but is also thought to occupy the upper Yukon River watershed (Figure 8). Because a Bull Trout sample from the Liard River (which drains into the Mackenzie River) corroborated its presence in the southeast of this territory (Haas and McPhail 1991), Bull Trout has been confirmed in numerous drainages and lakes of the Liard River watershed in southeast Yukon (Can-nic-a-nick Environmental Sciences 2004). Although the overall distribution of Bull Trout in this remote area remains somewhat unclear, a recent modelling exercise and site visits reveal that Bull Trout is likely widespread in this drainage basin. On the other hand, little is known about the distribution of Bull Trout in the upper Yukon River watershed. Bull Trout has been found in the extreme headwaters of this drainage in northwestern British Columbia (Haas and McPhail 1991) and a traditional knowledge study undertaken by the Teslin Tlingit Council in the late 1990s indicated that fish of the Dolly Varden/Bull Trout complex could be found in rivers of the Yukon River drainages within their Traditional Territory (Connor et al. 1999). Anecdotal reports also report char from this area. However, a thorough survey failed to capture any from this vicinity (Connor et al. 1999).

Population structure

There is broad evidence in salmonids, and in Bull Trout specifically, of substantial genetic heterogeneity within DUs due to the tendency of reproductive adults to return to their natal spawning areas (Frank 2024). Sampling and data compilation by jurisdictions demonstrates this spatial heterogeneity, but the approaches differ among jurisdictions. British Columbia reports data from individual lakes and streams, nested within Core Areas and Ecologically Distinct Units (EDUs). In contrast, Alberta uses scaled hydrological units (HUC) at the HUC8 scale to report Bull Trout information. It is assumed that this spatial scale reasonably approximates subpopulations sharing natal spawning habitat with minor contemporary straying.

Extent of occurrence and area of occupancy

Extent of occurrence (EOO) is estimated as the area included in a polygon without concave angles that encompasses the geographic distribution of all known subpopulations for each DU. For those DUs where sampling effort has varied between time periods (so that the EOO from a single time period appears to be based on a subset of all occupied sites), current EOO is based on all data unless there is an indication that Bull Trout no longer exists at previous sites.

For riverine organisms, index of area of occupancy (IAO) is typically estimated by placing a 2x2 km grid over all known occupied sites and then summing the number of grid cells (Discrete IAO) or interpolating and summing over the extent of the stream network between the observation records (Continuous IAO). Discrete IAO will underestimate area of occupancy where sampling is insufficient, while Continuous IAO will overestimate area of occupancy where suitable habitat is patchy. Because sampling for Bull Trout in all but DU3 has been reasonably extensive, Discrete IAO is used here. Even then, for stream-spawning salmonids, Discrete IAO using a 2x2 km grid approach over all occupied sites is inadequate to capture the area of habitat necessary for the crucial life stages of spawning and early rearing. The actual area of occupancy for the most restrictive life stage has been estimated only in DU4 (see below), but in this case, it was found to be approximately 32% of the IAO determined from all occupied sites.

DU1 South Coast British Columbia population (figure 9)

Current EOO: EOO based only on 2012 to 2023 records is 13,533 km², but sampling appears to have been limited outside the 2012 to 2023 polygon. Thus, with no indication that these drainages are no longer occupied by Bull Trout, the EOO calculated using all observations (21,841 km²) is likely the best estimate of current EOO.

Current IAO: IAO estimated by placing a 2x2 km grid over all 2012 to 2023 observations is 200 km², but with no indication that the drainages outside the 2012 to 2023 polygon are no longer occupied, the current IAO is estimated using all observations (648 km²). No information is available on the number of subpopulation spawning sites, and therefore IAO of the most restrictive life stage is unknown, but it is very likely below the threshold of 500 km² based on estimates from DU4 (that is, where IAO based on subpopulation spawning sites is approximately one-third of IAO based on all observations).

Declines in EOO or IAO? With limited recent sampling outside the 2012 to 2023 polygon, it is unknown whether EOO or IAO have declined.

Figure 9. Distribution of Bull Trout in DU1 South Coast British Columbia population summarized into 2012 to 2023 observations and three earlier time periods. EOO and IAO (based on a 2x2 km grid placed on each observation) are mapped and indicated for pre-2012, 2012 to 2023, and for all observations.

DU2 Western Arctic population (figure 10)

Current EOO: EOO based only on 2012 to 2023 records is 542,950 km², which is higher than the pre-2012 EOO largely due to increased sampling effort in the Northwest Territories. However, recent sampling in northern BC and Yukon appears to have been limited outside the 2012 to 2023 polygon, which suggests that this is an underestimate of current EOO. Overall EOO (606,655 km²) potentially will overestimate current EOO given the observation that some subpopulations on the East Slopes of Alberta have become functionally extirpated (Figure 7). Therefore, current EOO is between 542,950 and 606,655 km².

Current IAO: IAO estimated by placing a 2x2 km grid over all 2012 to 2023 observations is 2,680 km² (a likely underestimate given recent undersampling in northern BC and Yukon); 9,792 km² for pre-2012 (an underestimate given that it does not include recent Northwest Territories records); and 11,136 km² (a likely overestimate given the extirpation of some subpopulations on the East Slopes of Alberta). Therefore, current IAO is between 9,792 and 11,136 km². No information is available on the number of subpopulation spawning sites, and therefore IAO of the most restrictive life stage is unknown. However, if it is approximately one-third of IAO based on a 2x2 km grid from all observations (as in DU4; see below), it is unlikely to be below the threshold of 2,000 km².

Declines in EOO or IAO? Given the extirpations observed on the East Slopes of Alberta (Figure 7), EOO and IAO have declined, although the majority of the decline likely happened prior to the most recent three generations.

Figure 10. Distribution of Bull Trout in DU2 Western Arctic population summarized into 2012 to 2023 observations and three earlier time periods. EOO and IAO (based on a 2x2 km grid placed on each observation) are mapped and indicated for pre-2012, 2012 to 2023, and for all observations.

DU3 Upper Yukon Watershed population (figure 11)

Current EOO: EOO based on all observations (including eDNA detections) is 5,885 km², but this region is substantially undersampled.

Current IAO: IAO is 120 km² estimated by placing a 2x2 km grid over all observations, but it is substantially undersampled. No information is available on the number of subpopulation spawning sites, and therefore IAO of the most restrictive life stage is unknown.

Declines in EOO or IAO? Unknown.

Figure 11. Distribution of Bull Trout in DU3 Upper Yukon Watershed population (including eDNA detections) summarized into 2012 to 2023 observations and two earlier time periods. EOO and IAO (based on a 2x2 km grid placed on each observation) are mapped and indicated for pre-2012 and for all observations.

DU4 Saskatchewan-Nelson Rivers population (figure 12)

Current EOO: EOO based only on 2012 to 2023 records is 57,577 km²; EOO is 58,136 km² based on pre-2012 records and 61,689 km² based on all observations. As with DU2, observations that some subpopulations on the East Slopes of Alberta have become functionally extirpated suggests that overall EOO is an overestimate, although the differences among time periods are relatively modest. Given the search effort that has occurred in Alberta in the most recent time period, the 2012 to 2023 EOO is likely a good estimate of current EOO.

Current IAO: IAO estimated by placing a 2x2 km grid over all 2012 to 2023 observations is 2,024 km²; given the recent search effort that has occurred in Alberta, this is likely a good estimate of current IAO.

IAO, as defined as the availability of habitat necessary for successful spawning, has been estimated for the Saskatchewan-Nelson Rivers DU (Sullivan and Reilly 2023). The area of streambed occupied by Bull Trout spawning redds in this DU was estimated using two approaches. The first estimate was derived by the product of the current estimated number of mature females and the average size of redds. This approach yielded a biological area of occupancy estimate of 1.9 km² (range bootstrapped means of 0.2 to 6.1 km²). This likely underestimates the available area for this crucial life stage in the DU as suitable area might not be currently occupied for spawning, and it estimates only area of occupancy. It does not use the 2x2 km grid to provide an index of area of occupancy. Therefore, a second estimate was derived assuming that the DU is comprised of a number of Bull Trout subpopulations, each requiring a certain area of streambed for redds. Considering the available information on Bull Trout genetic variability in Alberta (Warnock et al. 2010; Carroll and Vamosi 2021), evidence of genetic distinction is often observed at a spatial scale that roughly translates to a HUC10 watershed. Bull Trout are currently found in 166 HUC10 watersheds. The number of spawning areas within all watersheds is unknown, so four scenarios were considered to estimate a reasonable range based on available information from studied watersheds and expert opinion. This included a low estimate (2/3 of the watersheds have one spawning area), a moderate estimate (one spawning area/watershed), a high estimate (two spawning areas/watershed), and a very high, and relatively unlikely, estimate (three spawning areas/watershed). Next, the 2x2 km grid system approach was applied by multiplying the total number of spawning areas within the DU by 4 km² to estimate the average area of streambed occupied by Bull Trout redds, representing the area necessary for success of the most critical life stage for the population. This approach yielded a spawning IAO estimate of 444 to 1,992 km² based on assumptions of 0.67 to 3 spawning areas per HUC10, with expert opinion suggesting that one spawning area per HUC 10 watershed is most likely, and this assumption yields an IAO of 664 km² (Sullivan and Reilly 2023).

Declines in EOO or IAO? Given the extirpations observed on the East Slopes of Alberta (Figure 7), EOO and IAO have declined, although the majority of the decline likely happened prior to the most recent three generations.

Figure 12. Distribution of Bull Trout in DU4 Saskatchewan-Nelson Rivers population summarized into 2012 to 2023 observations and three earlier time periods. EOO and IAO (based on a 2x2 km grid placed on each observation) are mapped and indicated for pre-2012, 2012 to 2023, and for all observations.

DU5 Pacific population (figure 13)

Current EOO: EOO based only on 2012 to 2023 records is 510,288 km², but with no indication that the drainages outside the 2012 to 2023 polygon are no longer occupied by Bull Trout, the current EOO is taken as the overall EOO (714,549 km²).

Current IAO: IAO estimated by placing a 2x2 km grid over all 2012 to 2023 observations is 2,352 km², but with apparently less extensive recent sampling, the overall IAO (11,076 km) likely better represents the current IAO. No information is available on the number of subpopulation spawning sites, and therefore IAO of the most restrictive life stage is unknown, but it is likely above thresholds.

Declines in EOO or IAO? No.

Figure 13. Distribution of Bull Trout in DU5 Pacific population summarized into 2012 to 2023 observations and three earlier time periods. EOO and IAO (based on a 2x2 km grid placed on each observation) are mapped and indicated for pre-2012, 2012 to 2023, and for all observations.

Fluctuations and trends in distribution

DU1 South Coast British Columbia population – No evidence of fluctuations or trends.

DU2 Western Arctic population – No evidence of fluctuations. In terms of trends, a contraction in distribution has been reported with extirpation or functional extirpation of some subpopulations on the East Slopes of Alberta (Figure 7; see Abundance). However, the majority of the decline happened prior to the most recent three generations. For example, there are no records of adult Bull Trout in the Athabasca River above Whitecourt since the 1970s and, following significant sampling effort over time periods representing three generations (1993 to 2002, 2003 to 2012, and 2013 to 2022), the species is now considered functionally extirpated from 12 HUCs in this DU (Figure 7). In some of these watersheds, a single adult Bull Trout was captured during the 1993 to 2002 window (for example, one in each of the Beaverlodge and Redwillow River HUCs in 1999 and 2001, respectively), but in such low numbers and in watersheds already experiencing some serious habitat issues (for example, frequent and high sedimentation, likely higher temperatures) that the declines are concluded to have happened > 30 years ago (Reilly pers. comm. 2025). Projections imply continuing declines in distribution in the Alberta portion of the DU.

DU3 Upper Yukon Watershed population – No evidence of fluctuations or trends.

DU4 Saskatchewan-Nelson Rivers population – No evidence of fluctuations. In terms of trends, a contraction in distribution has been reported with extirpation or functional extirpation from seven HUCs on the East Slopes of Alberta (Figure 7). As in DU2, the majority of the decline likely happened prior to the most recent three generations (Reilly, pers. comm. 2025). Projections imply continuing declines in distribution.

DU5 Pacific population – No evidence of fluctuations or trends.

Biology and habitat use

Information in this section was sourced from several reviews that together represent the most recent comprehensive assessments for Bull Trout across its Canadian range (Alberta: Rodtka 2009; BC: McPhail 2007; Hagen and Decker 2011; NT: Stewart et al. 2007a,b; Alberta Environment and Parks 2020; Fisheries and Oceans Canada 2020a,b). The many facets of Bull Trout biology are discussed below, with geographical variation highlighted. The strongest differences are shown among the divergent life history patterns and a general north to south trend may also be evident correlated to spatial patterns in water temperature. The description given herein refers to all DUs unless specified otherwise.

Life cycle and reproduction

Bull Trout expresses substantial diversity of life history patterns that can be summarized into four types. A non-migratory stream resident form spends its entire life cycle in small rivers and streams and is often, but not exclusively, isolated by barriers either physical (for example, waterfalls, dams; Latham 2002; Sawatzsky 2016), physiological (for example, unfavourably high temperatures; Rieman and McIntyre 1993; Rieman et al. 1997), or biological (for example, presence of non-native competitor species; Paul and Post 2001; Nelson et al. 2002). Migratory forms also spawn and rear in small rivers and streams but migrate to other water bodies as adults. The fluvial form spends its entire life in flowing water, making migration between spawning and juvenile-rearing natal streams and larger streams and rivers (often the mainstem of large rivers) in which it feeds, matures, and overwinters between breeding seasons. An adfluvial form matures in lakes but migrates up tributaries to natal streams to spawn. These three forms are common throughout Bull Trout’s Canadian range (Stewart et al. 2007a). In contrast to these three forms, which reside solely in fresh water, a fourth anadromous form migrates between freshwater and the sea. This life history form is restricted to the southwestern portion of British Columbia and northwestern Washington. Despite this diversity, there is no evidence of genetic subdivision between different life histories (Homel et al. 2008). Indeed, females of one migratory type may produce offspring of a different migratory type indicating plasticity in key life history traits (Brenkman et al. 2007).

Bull Trout usually reaches sexual maturity between 5 and 7 years of age, with the extreme range between 3 and 8 years. Maximum age is unknown but ages up to 24 years have been recorded. The generation time estimated for seven Bull Trout subpopulations in British Columbia displaying different life history strategies was approximately 7 years (Pollard and Down 2001). It was this estimate of 7 years that was adopted as generation time within the previous status report (COSEWIC 2012). However, this estimate is likely biased low for many Bull Trout subpopulations that mature at 6 to 8 years and have longevity of 15 to 24 years (Johnston and Post 2009; Mochnacz et al. 2013). Age-structured populations with these characteristics most likely have generation times greater than 7 years and may be best estimated at 10 years (Post pers. comm. 2024).

Although Bull Trout is an iteroparous species, there is strong evidence that it displays alternate-year spawning or resting periods between consecutive spawning events, at least in some systems (Pollard and Down 2001; Johnston and Post 2009; Mochnacz et al. 2013). This reproductive strategy, which is often condition- and survival-dependent, may enable Bull Trout to accrue sufficient energy for reproduction in colder, less productive systems (reviewed in Johnston and Post 2009). This strategy can also show a density-dependent response, with the proportion spawning annually declining with increasing density in a population recovering from severe overfishing (Johnston and Post 2009).

Like all char, Bull Trout spawns in the fall, from mid-August to late October (Sinnatamby et al. 2018). Except for stream-resident subpopulations that spawn locally, this is preceded by a migration. Younger individuals may enter the spawning ground first. Their gonads are usually not fully mature, so gamete development may be completed in their spawning stream over a month or so before breeding at the same time as older fish. At least in some areas, actual spawning does not occur until water temperatures drop below about 10^oC and spawning has been observed as cold as 4^oC (Sullivan and Reilly 2023). As a result, southern subpopulations appear to have a later, more protracted spawning window than northern ones (Pollard and Down 2001).

Digging of the spawning site, or redd, and spawning is similar to that of other salmonids. Larger females typically use larger substrate toward the centre of the channel when spawning and bury their eggs deeper. This presumably provides better protection against the impacts of low flows (that is, sediment deposition) and freezing. A dominant male usually accompanies each spawning female and vigorously defends her from other satellite males who try to compete for fertilizations (Kitano et al. 1994; Baxter 1997). Some subpopulations also have jacks or “sneakers” that dash in at the moment of egg release, often succeeding in fertilizing some eggs. Sometimes these small precocious males mimic females’ colour, behaviour, and morphology (lacking a kype, the hook-like lower jaw modification which is a secondary sex characteristic in most males), aiding their approach to a spawning pair just before gamete release (Kitano et al. 1994; Baxter 1997). Their presence may contribute to the skewed sex ratios that are sometimes observed in spawning runs, although higher rates of repeat spawning amongst females likely also contribute to the pattern of female predominance (McPhail and Baxter 1996; Pollard and Down 2001). The sex ratio of the entire population, on the other hand, is commonly close to 1:1 (McPhail and Baxter 1996; Pollard and Down 2001).

Spawning usually occurs during the day, but in some disturbed systems spawning occurs at night. Like most fishes, fecundity (egg number) in Bull Trout depends on female body size; the larger fluvial and adfluvial females produce more eggs (typically 2,000 to 5,000+) than the smaller stream-resident females (<1,000). Fertilized eggs incubate in the gravel over winter before fry typically hatch from March onwards (at a total length of about 25 mm). The incubation period is temperature dependent and can range from 35 days to more than 4 months.

Habitat requirements

Key habitat features for Bull Trout success by life stage have been summarized by DFO 2020 (Table 2). In general, Bull Trout is a coldwater species generally found in water below 18^oC, but most commonly in temperatures less than about 12^oC (Dunham et al. 2003; Isaak et al. 2015). Indeed, its southern range is limited by temperature (Dunham et al. 2003). Bull Trout habitat requirements go far beyond temperature, however, being more specific than other salmonids (Rieman and McIntyre 1993). Characteristic requirements are habitat that is cold, clean, complex, and connected (USFWS 2008). Their habitat use is also strongly influenced by the presence, or absence, of other species.

All life history stages need complex forms of cover, with Bull Trout tending to conceal themselves by remaining near or closely associating with the sub-strate, submerged wood, or undercut banks (Rieman and McIntyre 1993; Watson and Hillman 1997). Bull Trout also has specific requirements regarding channel and hydrologic stability that include depth, velocity, and substrate parameters (Rieman and McIntyre 1993; Watson and Hillman 1997). The association with substrate appears more important for Bull Trout than for many other species (Nakano et al. 1992).

Although Bull Trout may be present throughout large river basins, its specific and changing habitat requirements mean that it will only be found in patches of a system (Rieman and McIntyre 1995). Large scale studies of spatial patterns of habitat patch occupancy show that persistence in stream networks is strongly dependent on patch size (stream or watershed size), connectivity, and quality (Rieman and McIntyre 1995; Dunham and Rieman 1999; Isaak et al. 2015; Mochnacz et al. 2021). The importance of habitat size and connectivity is further supported by models of Bull Trout population dynamics investigating the temporal processes driving these patterns, such as dispersal, demographic variation, and environmental variability (Rieman and Allendorf 2001). Molecular genetic studies also show that disruption of connectivity can lead to lower effective size of local subpopulations by simultaneously reducing dispersal and local adult population sizes (Costello et al. 2003; Taylor and Costello 2006; Whiteley et al. 2006).

These specific habitat requirements are, in fact, Bull Trout’s most significant natural limiting factor (reviewed in Rieman and McIntyre 1993; Dunham et al. 2003). Such specificity makes Bull Trout particularly vulnerable to human-induced habitat change and makes it less able to persist in the face of such change (Rieman and McIntyre 1993, 1995). Its habitat utilization varies according to both life-history stage and migratory form of the adult, as well as shifting on a daily and seasonal basis. Major transitions in habitat use over the Bull Trout’s life history are the norm (Table 2). Important associations are summarized below. Habitat requirements appear to be largely similar for Bull Trout across its range (Stewart et al. 2007a; DFO 2020) and the description given herein refers to all Canadian Bull Trout DUs.

Bull Trout natal streams tend to be shallow, structurally diverse headwater or tributary streams with stable channels found at higher elevations (Burrows et al. 2001; Ripley et al. 2005; Decker and Hagen 2008). Their structural diversity not only meets habitat requirements of spawning adults but also provides for the changing habitat needs of rearing juveniles. These natal habitats occur as discrete patches of suitable habitat in a matrix of the larger stream network (Baxter 1997; Dunham and Rieman 1999; Decker and Hagen 2008; Hagen et al. 2020). Watershed size appears to be a significant factor in providing essential connectivity between these habitats (Rieman and McIntyre 1995). Once in their natal streams (following migration for adfluvial and fluvial forms), Bull Trout undergo a behavioural transition in habitat use towards a pattern of daytime concealment and nighttime emergence (Jakober et al. 2000). Concealment cover includes woody debris and substrate crevices (Jakober et al. 2000).

Winter incubation sites are particularly vulnerable to anchor ice accumulations, as well as scouring and low flows. Females, therefore, often select spawning sites associated with groundwater sources that stabilize temperatures through the winter (Baxter 1997; Baxter and McPhail 1999; Baxter and Hauer 2000; Ripley et al. 2005). Within these areas of upwelling, they tend to select localized spots of strong downwelling and high interstitial flows (Baxter and Hauer 2000). These occur over coarse gravel-cobble substrates that have low levels of fine sediment, for example, the tail-outs of pools at the heads of riffles (Baxter and Hauer 2000). The specific selection of these characteristics increases aeration of eggs. Successful incubation is dependent on several stream characteristics, including appropriate temperature, gravel composition, permeability, and surface flow.

The preference of young Bull Trout for coarser substrate than is used by spawning adults appears to be heavily influenced by avoidance of predation and competition. In the spring, newly emerged young-of-the-year Bull Trout fry are denser than water and seek cover in shallow, slow-flowing stream margins with coarse cobble-boulder substrate (Pollard and Down 2001; Spangler and Scarnecchia 2001). As these juveniles grow, they tend to shift to deeper, faster flowing water, preferring pools over riffles (Bonneau and Scarnecchia 1998; Pollard and Down 2001; Spangler and Scarnecchia 2001). During the early months and years of life, when juvenile Bull Trout are rearing in their natal streams, microhabitat use shifts both daily and seasonally. During all seasons, juveniles are secretive during the day, remaining close to cover, and disperse more at night (Bonneau and Scarnecchia 1998; Jakober et al. 2000). This pattern of daytime concealment and nighttime emergence is particularly pronounced in winter (Bonneau and Scarnecchia 1998; Jakober et al. 2000). Juveniles tend to shift to deeper, slower-flowing water in the fall, where they stay in contact with coarse substrates and remain closer to cover (Bonneau and Scarnecchia 1998; Spangler and Scarnecchia 2001). This provides ice-free refuges for them throughout winter. Evidently, both shallow stream margins and deep water with low velocities provide important rearing areas for growing juveniles. Cover use varies with latitude and elevation. As the diversity of cover type diminishes with increasing latitude and/or elevation (for example, woody debris), juveniles have less opportunity to use shade, undercut banks, and large woody debris (Mochnacz et al. 2006). Instead, they make more use of pocket pools, rootwads, cobbles, boulders, and overhanging vegetation for shelter (Mochnacz et al. 2006).

Maturing and adult Bull Trout tend to use habitat for foraging and overwintering that has the appropriate combination of temperature, shelter, and foraging opportunities. However, while stream habitat use by Bull Trout has been studied in detail, the specifics of habitat use of rivers, lakes, and coastal waters by these fish are poorly understood. Both fluvial and resident Bull Trout prefer low-velocity water, often associated with the tail-outs of pools, and tend to remain close to cover (McPhail 2007). Resident forms find this habitat not far from their spawning grounds.

While radiotelemetry indicates Bull Trout need only move a few kilometres in the fall to find ice-free overwintering sites (Jakober et al. 1998), those in northern latitudes may move further into larger tributaries. Just as groundwater upwellings are a preferred locality for spawning, these sites that have more stable temperature regimes than areas of surface-water recharge (that is, warmer during winter, colder during summer) can also provide resident Bull Trout with suitably cold water throughout the year (Baxter and Hauer 2000). In streams at least, Bull Trout undergo a behavioural transition in habitat use during winter towards a pattern of daytime concealment and nighttime emergence. This is negatively correlated to temperature and fish size (Jakober et al. 2000). Migratory forms (fluvial and anadromous) seek suitable habitat out in the larger streams and rivers (or even the sea) that they both migrate through and eventually settle in to forage and overwinter (Burrows et al. 2001; Muhlfield and Marotz 2005). Based on fishing patterns, adfluvial adult Bull Trout appear to remain in deeper, cooler water during the day (mostly resting on the bottom) and then move to littoral areas for foraging at night (McPhail 2007).

Movements, migration, and dispersal

The movements of young-of-year and small juveniles are not well described, in part because these secretive fish are difficult to catch or survey. The timing of fluvial and adfluvial juvenile migration appears to be highly variable among systems. While juveniles may inhabit their natal streams for 1 to 4 years, migration at age 2 or older is most common (Post and Johnston 2002; Johnston and Post 2009; Sinnatamby et al. 2018). Migrations are common during high spring flows in the late spring/summer and when temperatures decline in the fall and winter. This timing may reduce juvenile predation risk and expose them to higher quality food resources at a time when adults occupy spawning grounds. When juvenile adfluvial Bull Trout move into lakes, they are rarely taken in the littoral zone, suggesting that they move into deep water.

Often isolated above natural barriers, adult resident Bull Trout typically disperse only short distances to spawn, rear, feed, and overwinter. Migratory forms (fluvial, adfluvial, anadromous) undergo migrations between feeding areas and overwintering habitat, and their distant natal habitat. The timing of spawning migrations differs among subpopulations, being partly dependent on the distance to be travelled, which varies widely (up to several hundred kilometres; Pillipow and Williamson 2004; Hagen and Decker 2011). The onset of migration is thought to be triggered by a hierarchy of environmental cues, including changes in river discharge and water temperature. Migratory movements generally occur nocturnally, and fluvial subpopulations usually begin spawning migrations when temperatures are relatively high and water levels are declining, from May to August. After spawning, migratory Bull Trout generally move rapidly back to their overwintering habitats by September or October. Bull Trout typically display high fidelity to both natal streams to spawn and overwintering habitat, although there is some evidence of straying, at least at the local scale (Swanberg 1997a; O’Brien 2001; Bahr and Shrimpton 2004).

Upstream pre-spawning migrations are generally slower than the downstream post-spawning movements, with patterns of migration also dependent on age and life-stage; evidence suggests that larger adults consistently migrate quickly, whereas smaller individuals show more diverse and less predictable behaviour (Post and Johnston 2002; Muhlfield and Marotz 2005; Monnot et al. 2008; Sinnatamby et al. 2018). Bull Trout may congregate at tributary mouths or estuaries before the onset of spawning migrations (Taylor and Costello 2006; Brenkman et al. 2007). Coupled with their tendency to gather below barriers before spawning, this habit renders them highly catchable and susceptible to overharvesting (Paul et al. 2003; Post et al. 2003).

Although not thoroughly investigated, there is evidence of anadromy in Bull Trout in the southwest of British Columbia, as well as the northwest of Washington. Bull Trout have been collected in the near shore marine areas of Howe Sound, British Columbia and Puget Sound, Washington (Cavender 1978; Haas and McPhail 1991), and anglers refer to sea-run subpopulations in the Squamish River and Pitt River, of which the latter is part of the Fraser drainage. More recently, radiotelemetry and otolith chemistry have verified that anadromy is a primary life history form in some coastal U.S.A. Bull Trout (Brenkman and Corbett 2005; Brenkman et al. 2007). The life history of anadromous Bull Trout appears variable; some make only single migrations after a prolonged residence in freshwater, but many move annually between freshwater and salt water after their first seaward migration around ages 3 or 4 (Brenkman et al. 2007). This suggests that they are largely iteroparous like non-anadromous Bull Trout. These anadromous fish co-occur with non-anadromous fish, and life history plasticity results in both types of females being able to produce anadromous progeny (Brenkman et al. 2007).

Interestingly, an anadromous life history is not expressed in any of the numerous Genetic Lineage 2 populations that have access to the sea (Cavender 1978; Haas and McPhail 1991, 2001). Its confinement to Genetic Lineage 1 subpopulations suggests that anadromy in Bull Trout originated, or at least persisted, in the Chehalis Refugium, from where Bull Trout (and anadromous Dolly Varden) is thought to have post-glacially recolonized these localities (Haas and McPhail 2001).

Interspecific interactions

Diet

Bull Trout is an opportunistic forager. While individual prey species may change across Bull Trout’s broad range of latitudes and elevations, the general taxonomic groupings preyed upon by each life stage are similar across its range (Stewart et al. 2007b). Throughout their distribution, they feed on a diversity of vertebrate and invertebrate prey, selecting for larger-bodied prey when available. Little is known about the seasonal changes in Bull Trout diet, but they most likely alter their diet in response to seasonal abundance of prey, given their opportunistic nature.

Various life history stages of aquatic and terrestrial insects (mainly mayflies, caddisflies, stoneflies, and chironomids) are commonly consumed by both adults and juveniles. Where other fish species are absent, typically in the highest reaches of the stream habitats or in isolated mountain lakes, juveniles and resident adult Bull Trout feed primarily on these macroinvertebrates. When feeding during the day, juvenile fish are secretive and remain close to the bottom, with most feeding movements directed towards insects drifting nearby (McPhail 2007). At night, they will disperse and forage more on benthic organisms. Little if any surface foraging has been observed. Larger juveniles and resident adults will take fish when available (including young of their own species) but their relatively low-piscivorous diet accounts for their low growth rates relative to migratory adult Bull Trout.

Juvenile Bull Trout become increasingly piscivorous as they approach adulthood. Bull Trout’s relatively large gape enables them to consume prey up to 50% of their own length (Beauchamp and Van Tassell 2001). While adults may continue to eat a wide variety of invertebrates, they become increasingly piscivorous with size in the presence of other fish species. They are often the top aquatic predator where they live and some adfluvial subpopulations are almost exclusively piscivorous as adults (Post and Johnston 2002). Salmonids are important prey species for both adfluvial and fluvial populations, including smaller juvenile Bull Trout, as well as trout, kokanee (Oncorhynchus nerka), whitefish (especially Mountain Whitefish, Prosopium williamsoni), and Arctic Grayling (Thymallus arcticus). Other fishes, such as a variety of suckers, minnows, sculpins, and sticklebacks are also consumed. When the chance presents, Bull Trout will even consume suitably sized frogs, snakes, ducklings, and small mammals. A Bull Trout has been observed with North American Porcupine (Erethizon dorsatum) quills in its mouth (Cott and Mochnacz 2007). The feeding habits of anadromous Bull Trout at sea are unknown.

Interspecific competition with native salmonids

Interspecific competition with other native salmonids is likely an important factor in excluding Bull Trout or regulating their coexistence. One example which has received particular attention is Bull Trout’s interaction with Dolly Varden in areas of sympatry. Dolly Varden is generally more coastal in nature than Bull Trout and ranges further north; it is found from the western Pacific to Alaska, east to the Mackenzie River, and south to the Olympic peninsula, northwest Washington (Haas and McPhail 1991). Their largely parapatric distributions, however, come into contact along the Cascade/Coastal mountain crests from northwestern Washington to Northern BC. This zone of overlap is broadest in Northern British Columbia, where it crosses the Continental Divide north of the Skeena watershed in the headwaters of the Peace and Liard river systems (Taylor et al. 1999).

In addition to historical introgression, genetic analysis has shown these two species currently hybridize across much of this area of sympatry (Baxter et al. 1997; Taylor et al. 2001; Redenbach and Taylor 2003; Taylor and Costello 2006). Asymmetric introgression of mtDNA shows that this hybridization is typically unidirectional, with most F₁ hybrids resulting from a Bull Trout female mating with a Dolly Varden male (Baxter et al. 1997; Redenbach and Taylor 2003). This ongoing hybridization may result from the smaller Dolly Varden males acting as jacks that sneak fertilizations during Bull Trout spawning (Baxter et al. 1997; Hagen and Taylor 2001; Redenbach and Taylor 2003).

Current patterns of sympatry and hybridization are due to ancient introgression within, and co-dispersal from, a common refuge, as well as ongoing hybridization resulting from secondary contact between previously allopatric populations across parts of their ranges. While evidence of historical introgression indicates that the most southerly sympatric populations have probably been exchanging genes for 100,000 years, others have come into contact more recently, about 15,000 years ago at the end of the last glaciation (Redenbach and Taylor 2002). Such disparate durations of contact could result in regional differences in levels of reproductive isolation. Longer periods of co-evolutionary history between “southern” Dolly Varden and Genetic Lineage 1 Bull Trout may have strengthened reproductive isolation between them through reinforcement, resulting in lower hybridization along the south coast. A quantitative assessment of this awaits more extensive sampling, with preliminary data revealing no significant relationship between areas of secondary contact and range expansion, and the highly variable levels of hybridization detected among sites (Redenbach and Taylor 2003). There is, however, a qualitative suggestion that contemporary hybridization may be more extensive in central and northern coast subpopulations than in those along the southern coast; despite being broadly sympatric in southwestern BC and northwestern Washington (for example, Leary and Allendorf 1997), present day hybridization has only been detected in Canada in the Skagit River (McPhail and Taylor 1995).

Sympatry between Bull Trout and Dolly Varden extends to the northernmost tip of Bull Trout’s known distribution, and the most southerly range of Dolly Varden in Northwest Territories: the Gayna River (Mochnacz et al. 2009, 2013). Although they co-occur in the same river system, they are largely not syntopic, with Bull Trout occupying downstream areas and Dolly Varden isolated above barriers (Mochnacz et al. 2009, 2013). Not surprisingly, then, sequencing of mitochondrial and nuclear genes has not uncovered any genetic evidence of hybridization (Mochnacz et al. 2013).

Despite this ongoing hybridization and gene flow, Bull Trout and Dolly Varden maintain distinct gene pools in sympatry (Baxter et al. 1997; Taylor et al. 2001; Redenbach and Taylor 2003). Although postzygotic selection against juvenile hybrids appears to be limited, prezygotic isolation barriers are likely strong thanks to strikingly different adult life histories where they coexist (Hagen and Taylor 2001). Typically, adult Bull Trout are large (40 to 90 cm fork length), migratory or adfluvial, and piscivorous, whereas adult Dolly Varden are small (12 to 21 cm fork length), stream residents, and feed on drift (Hagen and Taylor 2001; Redenbach and Taylor 2003). These disparities in sympatry likely limit interspecific pairings because of size assortative pairing and size-dependent reproductive habitat use (Hagen and Taylor 2001). This contrasts sharply to life-history strategies adopted by each species in allopatry, where each broadens its trophic and habitat niches to include resources that overlap with the other species in sympatry. As has been suggested for other salmonids (for example, Campton and Utter 1985), life history differences such as these may also contribute to extrinsic post-zygotic selection against later-stage hybrids.

While Bull Trout occurs sympatrically with Dolly Varden over only a small part of its range, it is naturally sympatric with either Rainbow Trout or Cutthroat Trout across most of its range. Interactions with these, or kokanee, may be beneficial to Bull Trout in providing them with high quality food resources (Beauchamp and van Tassell 2001), although there is also potential for strong competitive interactions (for example, with Cutthroat Trout; Nakano et al. 1992; Jakober et al. 2000). Although these interactions have received little research attention compared to those with other char (reviewed in Dunham et al. 2008), temperature may affect the ability of Bull Trout to compete with these species (reviewed in Stewart et al. 2007b); Bull Trout are more abundant than Rainbow Trout when they occur in sympatry at temperatures below 13°C, but the situation is reversed at higher temperatures. Also, Bull Trout occur allopatrically, rather than sympatrically, with Westslope Cutthroat Trout (Oncorhynchus lewisi) in warmer water (Pratt 1984). Furthermore, in the coldwater streams of watersheds that have glacial influence, Bull Trout may preferentially select larger, lower gradient tributary reaches for spawning that have abundant gravel and cobble substrates. However, in non-glacial systems dominated by Rainbow Trout or Pacific salmon in their lower reaches, Bull Trout commonly spawn in the furthest upstream reaches they can access, which are often higher gradient, and above obstructions that block the migration of these other species (reviewed by Hagen and Decker 2011).

Bull Trout’s interaction with another native salmonid, Lake Trout (Salvelinus namaycush), has received less attention. Lake Trout, whose adults are also primarily piscivorous, occur over most of continental North America north of 45^oN. They overlap with about 40% of Bull Trout’s range, along its eastern and northern parts (Donald and Alger 1993). Competition resulting from substantial niche overlap in food utilization and growth, as well as opportunistic predation upon one another, may contribute to their somewhat disjunct distribution; small northern lakes tend to contain only one of these species, while larger lakes often carry both (Donald and Alger 1993). An exception to this is the large Babine Lake in the Skeena system, BC, which is inhabited only by Lake Trout despite it appearing to be good Bull Trout habitat. Bull Trout are common, however, in the Babine River immediately below it, indicating that Bull Trout are apparently competitively superior in flowing water but Lake Trout are competitively superior in the lake (McPhail 2007). Further evidence that Lake Trout can displace Bull Trout from lakes comes from the southern part of the zone of sympatry. Here, adfluvial Bull Trout tend to be found in higher elevation lakes (>1500 m) and Lake Trout in lower ones (<1500 m; Donald and Alger 1993), often accompanied by allopatric fluvial or stream resident Bull Trout in tributary streams. When non-native Lake Trout were introduced into two higher elevation lakes in this region, they displaced native Bull Trout (Donald and Alger 1993). In an Alberta reservoir, Abraham Lake, the fish species composition has shifted from one dominated by Bull Trout to Lake Trout.

Interspecific competition with non-native salmonids

While ongoing hybridization with native Dolly Varden presents no risk to the integrity of Bull Trout populations, direct interactions (for example, hybridization, competition) with several species of introduced salmonids may displace Bull Trout subpopulations and threaten to extirpate them from many habitats throughout broad areas of its range (Manning et al. 2022). In western North America, Rainbow Trout, Brown Trout, and Brook Trout are the most widespread non-native salmonids (Fuller et al. 1999). In particular, Brook Trout is considered a substantial threat to Bull Trout populations. Occupying habitats similar to those used by native trout and char, Brook Trout are commonly found downstream of, or overlapping with, Bull Trout (Paul and Post 2001; Earle et al. 2007; Rieman et al. 2007; Warnock and Rasmussen 2013a; Voss et al. 2023). This pattern of segregation is likely influenced by direct interactions. Brook Trout competes with Bull Trout for food and space (Nakano et al. 1998; Gunkel et al. 2002; McMahon et al. 2007; Warnock and Rasmussen 2013b; Pallard 2022). The absence of resource partitioning or a niche shift by Bull Trout in the presence of Brook Trout (Gunkel et al. 2002) makes them vulnerable to displacement, especially when resources are scarce. Life history characteristics of Brook Trout (faster maturation, shorter-lived, and higher densities compared to Bull Trout; Earle et al. 2007; McPhail 2007) will tend to compound this effect. Bull Trout occurrence has been negatively associated with the presence of Brook Trout (Rich et al. 2003; Warnock and Rasmussen 2013a), and hierarchical analysis supports the hypothesis that Brook Trout displace Bull Trout upstream (Rieman et al. 2006). Nevertheless, the ecological impacts of non-native Brook Trout on Bull Trout are highly variable and likely depend on environmental conditions, such as water temperature, as well as the spatial and temporal scales of observation (for example, Dunham and Rieman 1999; Rich et al. 2003; Rieman et al. 2006; Earle et al. 2007; McMahon et al. 2007; Warnock and Rasmussen 2013a,b). In addition, Cutthroat Trout have been extensively stocked throughout the Red Deer and North Saskatchewan river basins, and there are many self-sustaining non-native Cutthroat Trout subpopulations in sympatry with Bull Trout. Non-native Cutthroat Trout impacts on Bull Trout are not well understood (Sullivan and Reilly 2023).

Competitive displacement of Bull Trout by Brook Trout may be exacerbated by gamete wastage resulting from hybridization (Leary et al. 1993). Although the geographical extent of hybridization is not well defined, genetic evidence has documented extensive hybridization in British Columbia (McPhail and Taylor 1995) and Montana (Leary et al. 1993; Kanda et al. 2002). This suggests that it may be widespread and common wherever the two species co-occur. Their F₁ hybrids are predominantly partially sterile males (Leary et al. 1993; Kanda et al. 2002), although some backcrosses identified by molecular analyses indicate that F₁ reproduction does occur (McPhail and Taylor 1995; Kanda et al. 2002). Reduced survival and fecundity of these hybrids likely contributes to the prevention of hybrid swarms forming (Kanda et al. 2002), but their frequent production represents wasted reproductive effort. In such an instance, one parental species should be favoured over the other, causing displacement or extinction. Not only will Brook Trout’s earlier maturation and higher densities be to its advantage, but the predominance of female Bull Trout x male Brook Trout pairings (Leary et al. 1993; Kanda et al. 2002) results in greater wasted reproductive effort for Bull Trout. Recent genetic research in Alberta does identify hybridization with Brook Trout throughout Bull Trout range, but the incidence of backcrosses was low, suggesting that there are likely barriers to the formation of hybrid swarms (Franks 2024).

Physiological, behavioural, and other adaptations

Bull Trout’s pattern of high genetic differentiation between subpopulations and low diversity within them suggests that subpopulations experience limited gene flow. They are, therefore, likely to be locally adapted to their spatially heterogeneous environment. While Bull Trout have many specific habitat requirements, including depth, velocity, substrate, and cover, it is its thermal sensitivity that is its most notable physiological characteristic. The influence of temperature on Bull Trout distribution has been recognized more consistently than any other factor (reviewed in Rieman and McIntyre 1993; Dunham et al. 2003; Mochnacz et al. 2023). Low temperatures are important to the survival and development of all life history stages, from incubation through to breeding, but a narrow range of cold water is particularly critical during incubation and juvenile rearing. High water temperatures, and the resulting low dissolved oxygen levels, increase the rate of yolk absorption and decrease the size of fry. The optimal incubation temperature for survival to hatching is 2 to 4^oC (Austin et al. 2019). Groundwater inflows are important in providing stable temperature for egg development (Baxter and McPhail 1999).

Given a natural thermal gradient (8 to 15°C), juvenile Bull Trout selects the coldest water available (Bonneau and Scarnecchia 1996). Similarly, adult Bull Trout are generally found in water below 18^oC, but most commonly in temperatures less than about 12^oC (Dunham et al. 2003). In the northernmost areas of Bull Trout distribution, they may select for the warmest temperatures available, but these are still well below 12^oC (Mochnacz et al. 2023). Laboratory tests of thermal tolerance confirm field reports of Bull Trout having one of the lowest upper thermal limits and growth optima of North American salmonids (Hass 2001; Selong et al. 2001). Although the low temperatures typical of Bull Trout habitat lead to relatively low optimum growth rates, such temperature preferences discourage or exclude the invasion of species with higher temperature requirements, which may otherwise compete with Bull Trout.

The specific habitat requirements of Bull Trout result in its patchy distribution within a landscape (Rieman and McIntyre 1993; DFO 2020). This, coupled with life history attributes (including top aquatic predator and high site fidelity) that result in relatively low population densities and restricted gene flow (Taylor et al. 2001; Taylor and Costello 2006), means that local extinctions through stochastic processes can be considered natural, even common, events for Bull Trout (Rieman and McIntyre 1993, 1995). Bull Trout has evolved strategies that help it to cope with such natural disturbances, including phenotypic plasticity and density dependent changes in life history traits, such as faster maturation and more frequent reproductive events at lower density (Johnston and Post 2009). This coldwater specialist may be especially vulnerable to climate change (Rieman and McIntyre 1993; Rieman et al. 1997, 2007). Subpopulations near its southern limit will be most susceptible, given that this limit is defined by high temperature, but the thermal and precipitation effects of global climate change are likely to exacerbate fragmentation of Bull Trout subpopulations throughout much of the range (Keleher and Rahel 1996; Rahel et al. 1996; Isaak et al. 2015).

Another notable aspect of the Bull Trout’s physiology is the ability of at least some subpopulations to tolerate salt water.

Limiting factors

The natural limiting factors for Bull Trout discussed herein are considered universal across their range and, therefore, relevant to all DUs. Bull Trout’s specific habitat requirements are its most significant natural limiting factor (reviewed in Rieman and McIntyre 1993; Dunham et al. 2003; Hagen and Weber 2019). Its need for cold water (most commonly less than 12^oC) in particular, as well as the very specific habitat required for spawning and rearing, strongly influence its occurrence and result in its characteristic patchy distribution (Rieman and McIntyre 1993; Dunham et al. 2003). A warmer climate in the southern and eastern margins of its global range influences Bull Trout’s spotty distribution here (Dunham et al. 2003). This sensitivity makes it an excellent indicator of environmental disturbance. Interactions with other fish species are likely another important determinant of Bull Trout distribution and abundance; interference competition from other species, such as Rainbow or Cutthroat trout, also appears to be mediated by water temperature, while the abundance of prey species, such as kokanee, likely also influences Bull Trout growth and survival (Isaak et al. 2015).

Bull Trout are also limited by their low reproductive potential. Within suitable reaches, density-dependent survival appears to limit production of age–1+ Bull Trout parr (Hagen 2008 and references therein). This density-dependent survival at the juvenile life stage can be an important determinant of abundance at later life stages (Johnston et al. 2007). Other life history attributes, such as it being a top aquatic predator and showing high site fidelity, can contribute to relatively low densities. Together with its restricted gene flow (Taylor et al. 2001; Taylor and Costello 2006) and natural pattern of fragmentation, these factors make Bull Trout vulnerable to local extinctions through stochastic processes. Such natural extinctions may even be common (Rieman and McIntyre 1993, 1995). The pattern of depauperate neutral genetic variation within Bull Trout subpopulations and high differentiation between them indicates a historical demographic pattern of bottlenecks and local extinctions.

The lower productivity of the colder waters in Bull Trout’s northern extent (that is, the more northerly subpopulations of the Western Arctic and Upper Yukon Watershed populations) likely limits its population density (Mochnacz and Reist 2007; Mochnacz et al. 2009). In addition, they may recover more slowly from adverse impacts compared to their more southerly counterparts, given their tendency for slower growth and less frequent mating (Stewart et al. 2007a; Mochnacz et al. 2013). Given this likely susceptibility to perturbations, there is concern about the potential impact of development activities (Cott et al. 2008) on Bull Trout habitat in the Northwest Territories (Mochnacz et al. 2013).

These limiting factors render Bull Trout vulnerable to human activities and their impacts (Rieman and McIntyre 1993, 1995). On the other hand, strategies that Bull Trout has evolved to persist in the face of variable environmental conditions may also offer some compensation when dealing with human-induced changes. For example, phenotypic plasticity and density-dependent changes in life history traits, such as faster maturation and more frequent reproductive events at lower density, may offer some resilience to perturbations (Johnston and Post 2009).

Population sizes and trends

Data sources, methodologies, and uncertainties

Visual counts of redds have been the primary stock assessment tool for adult Bull Trout populations in the U.S.A. (Dunham et al. 2001; USFWS 2008). This is one of the least expensive and non-invasive adult population assessment methods. The characteristic form and bright, clean appearance of redds, as well as the low water conditions generally present during the early fall mean that they can be a reliable indicator of spawner abundance. Tight correlation of redd counts with independent estimates of population size have verified their usefulness (Dunham et al. 2001; Al-Chokhachy et al. 2005), although a number of caveats about their reliability and repeatability apply to their use (USFWS 2008). Errors (omissions and false identifications) must be reasonably low if redd counts are to accurately indicate the status of a population and provide an index of population trends. High levels of inter-observer variability can be a significant source of error in redd count accuracy and precision (Dunham et al. 2001) and will confound the ability to detect trends in streams over limited time scales (Rieman and Myers 1997). These discrepancies can be greatly reduced, however, when detailed criteria for redd identification and experienced observers are used (Muhlfeld et al. 2006; Decker and Hagen 2008; Hagen and Spendlow 2019). In addition to inconsistent methodology, variations in detection rate among streams and at different times also contribute to inconsistencies. While a near complete count of redds may be possible under certain environmental conditions, weather and stream type may result in underestimates (Decker and Hagen 2008). For example, high flows may delay redd counts and lead to underestimates, as redds become more difficult to identify with the passing of time after spawning (Decker and Hagen 2008). Also, in areas of limited gravel or high redd abundance, or where spawning site selection is highly specific, superimposition of redds upon one another can occur (Baxter and McPhail 1996). Redd counting in these instances can only be based on a subjective evaluation (Decker and Hagen 2008).

Caution must be applied when estimating the number of adults from the number of redds counted. The Bull Trout’s propensity for alternate-year spawning or resting periods between consecutive spawning events (Pollard and Down 2001; Johnston and Post 2009) means that only the number of spawning adults can be estimated. In addition, the expansion factor from redds to number of spawners can vary among populations as a result of single females constructing more than one redd (Leggett 1980) and sneak fertilizations from inconspicuous satellite and sneaker males in some populations (Kitano et al. 1994; Baxter 1997; McPhail 2007). Some males may fertilize more than one redd, while some redds may be fertilized by more than one male (Fraley and Shepard 1989). A review of three calibration studies that used independent estimates of population size (two in British Columbia and one in Idaho) found the average number of Bull Trout spawners/redd to be 2.2 (Decker and Hagen 2008), while two other BC rivers yielded expansion factors of 1.5 and 3 (Pollard and Down 2001). This range is confirmed by Al-Chokhachy et al. (2005), whose review of five studies in the Columbia River basin suggested an average expansion factor of 2.7 (range of 1.2 to 4.3).

Alternative methods for estimating the spawning population include trapping migratory subpopulations, electro-fishing, snorkel surveys, aerial surveys and, more recently, resistivity counters (Johnston et al 2009; Hagen and Decker 2011). All of these methodologies are more labour-intensive than redd counting, and each comes with its own potential drawbacks. For example, trap avoidance may bias trapping estimates. Electro-fishing gear is size-selective, and its capture efficiency diminishes in Bull Trout’s preferred habitat of flowing waters with low conductivity and high cover (Bonneau et al. 1995; Peterson et al. 2004). Day and night snorkel counts can compensate for diel shifts in Bull Trout habitat use, but counting errors depend partly upon water clarity and habitat type (Thurow and Schill 1996; Dunham et al. 2001; Thurow et al. 2006). The deployment of resistivity counters is a costly procedure, and their reliability has yet to be evaluated for Bull Trout (Decker and Hagen 2008). Quantitative estimates of adult densities in lakes are rare, but hydroacoustic surveys have been used (McPhail and Baxter 1996). The diel differences in habitat use by adfluvial subpopulations needs to be taken into consideration when selecting sampling localities and techniques.

Electro-fishing and snorkeling surveys are most commonly used to estimate juvenile Bull Trout densities. As with all surveying techniques, their potential drawbacks make them vulnerable to bias. The diel and seasonal shifts in habitat use by juvenile Bull Trout, in particular, will affect the density of fish in sampling localities and the effectiveness of these techniques (Jakober et al. 2000). Juvenile Bull Trout’s preference for cover during the day makes it difficult to assess their populations from daytime surveys (Jakober et al. 2000). Size selection and variable capture efficiency suggest electrofishing-based estimates are more biased than those from night-time snorkeling surveys (Decker and Hagen 2005). However, occupancy-based surveys and modelling allow users to account for imperfect detection and can provide more reliable estimates of juvenile distribution (Rodtka et al. 2015; Mochnacz et al. 2021). In addition, electrofishing surveys with replicates (spatial or temporal) can provide more reliable juvenile estimates of abundance than those that lack replication (for example, single pass electrofishing) (Royle et al. 2005.)

Extensive electrofishing surveys have been conducted over the previous three decades in the East Slopes of the Rockies in Alberta. Data from these surveys have been used to estimate adult Bull Trout abundance using the combination of catch-per-unit-area data coupled with a measure of electrofishing efficiency (Sullivan and Reilly 2023). Best estimates and confidence intervals are calculated using statistical bootstrapping and expressed as adults (with 95% confidence intervals) for entire DUs (Sullivan and Reilly 2023).

Abundance, trends, and conservation status of Bull Trout have been assessed within “core areas,” somewhat differently defined among jurisdictions and over time within jurisdictions. In Alberta, the approach has been to define watershed core areas at hydrological scales of Hydrologic Unit Code (HUC). The HUC is a hierarchical system created by the United States Geological Survey that is based on surface hydrologic features in a standard, uniform geographical framework (Fredenberg et al. 2005). Alberta has classified Bull Trout abundance, trends, and status at the HUC8 scale (Sullivan and Reilly 2023). British Columbia has summarized data at several scales: management regions (8), ecological drainage units (26), and core areas (98) (Hagen and Decker 2011; Hagen and Spendlow 2019; but see Appendix 1 for updates). Substantial data collection and compilation have occurred since the Threatened and Special Concern recommendations by COSEWIC in 2012, although not consistently across the national range, and many updates are available since the previous assessment in 2012 (COSEWIC 2012).

The genetic population structure of the vast majority of these core areas has not been defined in either Alberta or British Columbia. Given that genetic differentiation has been detected among Bull Trout subpopulations at the fine scale, for example, within watersheds over distances as small as a few kilometres (Spruell et al. 1999; Taylor et al. 2001; Costello et al. 2003; Taylor and Costello 2006; Carrol and Vamosi 2021), it is possible, or even likely in some instances, that the number of genetically distinct Bull Trout subpopulations exceeds the core areas identified within each DU. A summary of our current knowledge for each of the Canadian DUs is outlined below.

Abundance

DU1 South Coast British Columbia population

The Bull Trout population of this DU is restricted to southwest British Columbia and found in three ecological drainage units (EDUs) and five Core Areas (Appendix 1). This is the only area in Canada that is known to contain Genetic Lineage 1 Bull Trout. Population estimates for the DU are 4,800 to 6,500 adult Bull Trout, but this is likely a minimum estimate as not all areas have been sampled (Appendix 1). Of the five Core Areas, none are considered C1 High Risk, one is C2-At Risk, two are C3-Potential Risk, one is C4-Low Risk, and one is Unranked (Table 3 and Appendix 1).

DU2 Western Arctic population

Bull Trout subpopulations from the vast area of the Mackenzie River drainage basin are found in two Canadian provinces and two territories (Figure 3). Of the 75 Bull Trout core areas (HUC8 scale) that have been identified in Alberta, 32 fall within this DU. Abundance and stream occupancy has been assessed by core areas using electrofishing sampling within the Athabasca and Peace-Smoky river basins. Note that the Province of Alberta analyses do not include HUCs within national parks. Alberta has taken two complementary approaches to assessing abundance: (1) relative abundance by HUC in relation to a historical benchmark (MacPherson 2023) and (2) an absolute abundance estimate derived from catch-per-unit stream distance and an efficiency estimate (called catchability in the literature) (Sullivan and Reilly 2023). The first approach provides an informative spatial picture of both the historical and present-day relative abundance across the DU (Sullivan and Reilly 2023). Historical data were derived from a combination of data and expert opinion collected up to and including the 1950s. Current primarily refers to data from 2013 to 2022 but does include a small number of earlier observations when no more recent data are available for a few of the HUCs. The second approach provides a DU-wide estimate (exclusive of national parks) of population size of adult Bull Trout across three 10-year periods, 1993 to 2002, 2003 to 2012, and 2013 to 2022, although abundance from each time period cannot be compared directly as the sampling methods, sample sites, and intended goals of sampling differed substantially across time.

The relative abundance of Bull Trout in the East Slopes of Alberta have been assembled at the scale of HUC8 watersheds for both historical (<1950) and current (2013) periods using a diversity of data and historical reports (Sullivan and Reilly 2023; McPherson et al. 2024). Historically, the Eastern Slopes of Alberta within this DU had a high proportion of areas with high to very high adult Bull Trout densities (Figure 6). In stark contrast, current distributions show both substantial spatial contraction and substantial reductions in abundance with a high proportion of areas with low, very low adult abundance, and substantial areas are now extirpated (Figure 7). DU-wide population estimates for the three previous 10-year time periods (equivalent to three sequential generation lengths) were 1993 to 2002: 13,956 (95% CI 8,178 to 20,558), 2003 to 2012: 51,149 (95% CI 31,307 to 73,497), 2013 to 2022: 34,131 (95% CI 11,127 to 58,565) (Sullivan and Reilly 2023). No trend in numbers across generations can be applied to these estimates because of biased sampling of watersheds across generations (Sullivan and Reilly 2023). Similar estimates have not yet been completed for Jasper National Park but are certainly over 2,000, most likely between 3,000 and 4,000 and almost certainly fewer than 7,000 (Sullivan and Reilly 2023).

The British Columbia section of the Western Arctic DU has three EDUs and 17 Core Areas, but most don’t have sufficient data to assess abundance. The sum of Core Areas with population estimates is > 8,410 mature Bull Trout, but this should be considered a substantial underestimate for the DU because eight of 17 have no estimates, and most of the rest are recorded as minimum estimates (Appendix 1). Of the 17 Core Areas, none are considered C1-High Risk or C2-At Risk, one is C3-Potential Risk, two are C4-Low Risk, and 14 are Unranked (Table 3 and Appendix 1).

Little is known about the abundance of the Western Arctic Bull Trout population of the Northwest Territories, where only now recent surveying is establishing the northern range of this species distribution (Mochnacz et al. 2013, 2023). Two surveys (electro-shocking, angling, and set lines) of 29 streams in the southern (Dehcho) and central (Sahtú) Northwest Territories found Bull Trout represent 1 to 4% of the total catch (Mochnacz and Reist 2007; Mochnacz et al. 2009). This is in line with the general observation that Bull Trout typically comprise less than 5% of the total catch from broad faunal surveys (reviewed in McPhail and Baxter 1996). In southeast Yukon, Bull Trout are known to occur in numerous drainages and lakes of the Liard River (Can-nic-a-nick Environmental Sciences 2004) and are likely widespread in this drainage basin. These northern subpopulations are likely to be small in size, although there is currently no information available on the number or size of these Western Arctic Bull Trout subpopulations. Given that productivity generally decreases with increasing latitude due to colder temperature and shorter growing seasons, the initial indication of a small but wide-ranging population is likely an accurate reflection of the Bull Trout population in its northern reaches.

Although substantial areas within the Western Arctic DU do not have estimates of adult Bull Trout abundance, information from those areas that have been assessed suggest reasonable estimates for the whole Western Arctic DU certainly are in the many hundreds of thousands.

DU3 Upper Yukon Watershed population

Bull Trout subpopulations from the Yukon River watershed are believed to be found in both Yukon Territory and British Columbia, although there is very little information on their distribution or abundance within the Yukon Watershed DU (Appendix 1). Distribution of Bull Trout in this DU was further investigated by Schonewille and Costello (2018). eDNA survey results indicate the presence of Bull Trout outside previously delineated watersheds (upper Swift River) (EDI 2022). Range extensions to the Jennings and Morley watersheds are predicted in eDNA results, and further sampling will be conducted in these watersheds in future years to further refine distribution and life history information. In British Columbia, all core areas are unranked (Table 3 and Appendix 1).

DU4 Saskatchewan-Nelson Rivers population

Bull Trout subpopulations from this DU are restricted to Alberta where 43 core areas have been identified. Abundance and stream occupancy has been assessed by core areas using electrofishing sampling within the Oldman, Bow, Red Deer, and North Saskatchewan river basins at the spatial scale of HUC8. Alberta has taken two complementary approaches to assessing abundance: (1) relative abundance by HUC in relation to a historical benchmark and (2) an absolute abundance estimate derived from catch-per-unit stream distance and an efficiency estimate (called catchability in the literature) (Sullivan and Reilly 2023). The first approach provides an informative spatial picture of both the historical and present-day relative abundance across the DU. Historical data were derived from a combination of data and expert opinion collected up to and including the 1950s. Current data are primarily from 2013 to 2022 but include a small number of earlier observations when no more recent data are available for a few HUCs. The second approach provides a DU-wide estimate of population size of adult Bull Trout across three 10-year timeframes, 1993 to 2002, 2003 to 2012, and 2013 to 2022. It is important to note that these three-generation length time slices cannot be compared directly as the sampling methods, sample sites, and intended goals of sampling differed substantially across time. For future considerations, the approach used within the most recent period, 2013 to 2022, will now be used consistently to provide a benchmark for future estimates of abundance for use in temporal and spatial trend analyses.

Historically (<1950), the Eastern Slopes of Alberta within this DU had a high proportion of areas with high to very high adult Bull Trout densities (Figure 6 and 7). In stark contrast, current distributions (2013) show both substantial spatial contraction and substantial reductions in abundance with a high proportion of areas with low and very low adult abundance and substantial areas now extirpated.

DU wide population estimates for the three previous 10-year time periods (equivalent to three sequential generation lengths) were 1993 to 2002: 48,575 (95% CI 46,386 to 51,208), 2003 to 2012: 34,743 (95% CI 27,647 to 46,302), 2013 to 2022: 67,807 (95% CI 56,290 to 83,834) (Sullivan and Reilly 2023). No trend in numbers across each generation can be applied to these estimates because of the biased sampling of watersheds in each generation. Instead, the data are best described as showing the numbers of mature Bull Trout in the Saskatchewan-Nelson DU are most likely in the tens of thousands, and almost certainly not in the hundreds of thousands (Sullivan and Reilly 2023).

DU5 Pacific population

In British Columbia, there are 20 EDUs with 73 Core Areas identified (Appendix 1). This summary is derived from Hagen and Decker (2011) and updated (Warnock pers. comm. 2024). Population abundance has been estimated for 33 of the Core Areas. Although a number of short and longer-term monitoring initiatives for Bull Trout have been undertaken within this DU (Hagen and Spendlow 2019; Hagen et al. 2020), the majority occur within the Columbia drainage (Appendix 1). Little information about abundance exists for north coastal watersheds, the Thompson River, or the mid- and upper Fraser River within this DU (Appendix 1). A sum of population estimates from Core Areas results in approximately 20,000 to 62,000 adult Bull Trout. Of the 73 Core Areas, five are considered C1-High Risk, eight are C2-At Risk, 26 are C3-Potential Risk, 17 are C4-Low Risk, and 17 are Unranked (Table 3 and Appendix 1).

Fluctuations and trends

Bull Trout populations have experienced declines in abundance across their range in recent decades, but particularly in southern and eastern parts of its range in the U.S.A. (Rieman et al. 1997; USFWS 1999, 2008) and Alberta (Rodtka 2009; Sawatzky 2016; Alberta Environment and Parks 2020; DFO 2020). For the most part, this range reduction is comprised of localized extirpations, although it is known to have become extirpated from two large systems in the U.S.A. (McCloud, California; Willamette, Oregon; McPhail and Baxter 1996). The status of Bull Trout populations appears to show a general south to north trend, with decreasing abundance trends towards its southern margins and no trends in the more pristine and suitable environments in northerly regions and National Parks (Haas and McPhail 1991; McPhail 2007).

In addition to this general trend of declining abundance, there is evidence to suggest that the full range of life histories is also being lost from populations. There is particular concern that migratory Bull Trout may be especially susceptible to declines in larger, highly fecund, individuals (Nelson et al. 2002; Post et al. 2003; Rodtka 2009). For example, large-bodied fluvial or adfluvial Bull Trout were common in southwestern Alberta prior to 1950, but many extant subpopulations are now comprised of small-bodied residents that only occupy a fraction of their former range (Fitch 1997). It has also been noted that adfluvial Bull Trout populations in the upper Columbia Basin frequently include individuals that are larger and older than those found in more southerly subpopulations, suggesting these more northerly subpopulations experience less exploitation as well as lower growth rates (Hagen 2008).

Although this general pattern of decline in abundance is clear, two factors make it difficult to quantify. Firstly, broad natural fluctuations in abundance (Paul et al. 2000) make it difficult to assess population trends over short periods of time. This natural variation, combined with the limitations of surveying methods (Rieman and Myers 1997; Dunham et al. 2001; USFW 2008; Al-Chokhachy et al. 2009), means that considerable resource and temporal commitments are required to detect moderate changes in abundance of Bull Trout. Evidence from long-term studies conducted in the U.S.A. suggests that more than a decade of methodologically consistent Bull Trout monitoring may be required to detect a large population decline (Rieman and Myers 1997; Al-Chokhachy et al. 2009). Given this sensitive species’ tendency towards naturally low population sizes, it may not be possible to prove significant trends for many monitored Bull Trout subpopulations before they drop below a critically low level (Rieman and Myers 1997). Despite these hurdles, monitoring is often proposed as a mechanism to assess trends in abundance in order to recognize and mitigate land management effects. Standardized quantitative information gathered from Bull Trout subpopulations over a period of decades will be necessary for a thorough evaluation of the trends and status of Bull Trout in each DU. However, few long-term monitoring efforts on the abundance of Bull Trout exist in Canada, and the limited long-term quantitative data that are available are often confounded by non-standardized sampling techniques. Therefore, some of the current knowledge of population trends actually relies on qualitative expert opinion (Rodtka 2009; Hagen and Decker 2011; Alberta Environment and Parks 2020; Sullivan and Reilly 2023; Wilson pers. comm. 2023).

Nevertheless, more monitoring and baseline assessments are now being established, post-recommendations of Threatened (TH) and Special Concern (SC) from COSEWIC (2012). Even so, substantial gaps remain in the availability of data to determine trends at any time scale, and in particular, at the three-generation time scale. Here are summarized the available information on abundance trends at time scales limited by data availability by DU. In British Columbia, where sufficient data exist, annual and three-generation population trends have been estimated at the scale of stream or lakes within Core Areas (Appendix 1). Unfortunately, there is no viable approach to roll these individual estimates up to the scale of complete DUs, but the estimates can be used as indices of general patterns within DUs. In addition, an Alberta initiative has developed a cumulative-effects model at the HUC10 scale that predicts future trends in abundance over three generations that can be rolled up at the scale of the whole Saskatchewan-Nelson Rivers DU and the portion of the Western Arctic DU that is in Alberta. The approach projects the change in Bull Trout system capacity from current (2010 to 2020) to future (2040 to 2050) for the Alberta portion of the Western Arctic DU (Table 4) and the Saskatchewan-Nelson Rivers DU (Table 5). Habitat changes are predicted from ALCES model and include flow, sediment, and nutrients (ALCES 2020), angler effort which is assumed to double, and competition involving a presumed 5% increase in Brook Trout competition for habitat and hybridization. The models are built from variables from 170 and 132 HUC10s from the Saskatchewan-Nelson Rivers and Western Arctic DUs (Alberta only and exclusive of national parks). The projections presented here are a subset of those presented in Alberta Future Projections (Sullivan and Reilly 2023).

DU1 South Coast British Columbia population

Short-term trend data are available for two local subpopulations from two Core Areas within this DU (Skagit River and Cheakamus River, Appendix 1). Positive but non-significant trends in adult abundance were observed for both. This trend in the Skagit River reflects population recovery following the implementation of more restrictive fishing regulations (Hagen and Decker 2011). A similar response was observed in the Cheakamus River until 2006, a year after a caustic soda spill into the river. Since this time, adult numbers steadily declined but most recent counts (2010 to 2011) indicate the subpopulation is increasing (Hagen and Decker 2011). In summary, no consistent trend is apparent from either the limited quantitative data or expert opinion assessment for this DU; status appears to vary by major watershed according to local pressures and threats.

• Continuing decline in number of mature individuals: no evidence of declines
• Evidence for continuing decline (1 generation or 3 years, whichever is longer, usually up to 100 years): no evidence of declines
• Evidence for continuing decline (2 generations or 5 years, whichever is longer, usually up to 100 years): no evidence of declines
• Evidence for past decline (3 generations or 10 years, whichever is longer) that has either ceased or is continuing (specify): no evidence of declines
• Evidence for projected or suspected future decline (next 3 generations or 10 years, whichever is longer, up to a maximum of 100 years): 10 to 70% reduction projected from the Threats Calculator
• Extinction risk based on quantitative analysis: not conducted
• Long-term trends: unknown
• Population fluctuations, including extreme fluctuations: no evidence of fluctuations

DU2 Western Arctic population

Substantial declines in Bull Trout subpopulations have been observed in the Alberta portion of this DU since the middle of last century (Figures 6 and 7). While the most severe declines and extirpations of Bull Trout have been observed in the downstream extent, all areas have suffered declines in abundance. The most recent rigorous assessment of subpopulation sizes in Alberta for the previous three generations provides updated population estimates but unfortunately can’t be used for trend assessments because methods, sample sites, and sampling design goals differ substantially over the last 30 years. However, it is reasonable to infer that declines over the last several generations are common and substantial within subpopulations across the DU.

Within the BC component of the DU, adult Bull Trout trend estimates are available from six time series for six of 14 Core Areas. One core area has a significant negative trend, and five have non-significant positive and negative trends (Appendix 1). Unfortunately, no data exist for the majority of core areas regarding either abundance or distribution of Bull Trout. In summary, both negative and positive abundance trends are evident from the limited quantitative data.

There is also not sufficient quantitative information on the abundance to estimate trends in the Northwest Territories where surveying is only now establishing the northern range of this species distribution (Mochnacz et al. 2013). Distribution remains even less clear in Yukon Territory, but recent eDNA analysis has expanded the known range (Schonewille and Costello 2018). However, these northerly subpopulations (which generally inhabit less productive habitat than their more southerly counterparts) are likely to be smaller, and hence more susceptible to perturbations, than those found further south. Indeed, Bull Trout are thought to be the most sensitive species in the upper Liard River basin (Can-nic-a-nick Environmental Sciences 2004).

A rigorous baseline assessment has been completed in one watershed in NWT (Mochnacz et al. 2021). This assessment can be used in the future to assess trends over generational time scales. Although it doesn’t measure adult abundance directly, occupancy estimates can be used as a surrogate for relative abundance estimates of both adults and juveniles (Mochnacz pers. comm. 2024).

Overall trends in abundance, restricted to the previous three-generation time period, are uncertain within this DU. Large areas within the DU remain poorly sampled, and in the areas in Alberta that have received more extensive sampling, much of the declines may have preceded the most recent three generations.

Future projections of Bull Trout system capacity at the three-generation time scale have been assessed using a cumulative-effects model (Joe model) for the Alberta portion of this DU (Sullivan and Reilly 2023), and a subset of the projections are summarized in Table 4. Joe models are semi-quantitative, static models that are composed of a series of stressor-response curves that describe the relationship between Bull Trout system capacity and threats (MacPherson et al. 2019; MacPherson et al. 2023). System capacity is defined as the potential for a system to support adult individuals, ranging from 0 to 100% of a pristine reference condition. MacPherson et al. (2023) found a significant correlation between system capacity and average backpack electrofishing catch per unit effort for mature Bull Trout before and after recovery actions, so declines in number of mature individuals can be inferred from projected declines in system capacity.

Overall, Sullivan and Reilly (2023) explored Bull Trout system capacity under 11 future scenarios (that is, to 2040 to 2050 to reflect change after three generations) comprising either a single threat increasing in severity or multiple threats (Table 4). Current threat values reflect severity for the 2010 to 2020 time period (see Government of Alberta 2023). The future severity of competition was modelled at “moderate” (5% increase from the current competition/hybridization estimate) or “high” (20% increase from current competition/hybridization estimate). The future severity of the habitat-related threats was generated using information from ALCES (2020) simulations considering natural disturbances, successional trajectories, and anthropogenic footprints associated with projected resource production rates. Two angling effort scenarios were modelled, including a two-fold and four-fold increase from the current estimate, and climate change scenarios were modelled for two Representative Concentration Pathways (RCPs)—a “moderate” scenario (RCP 4.5) where the world shifts to clean energy sources and reduces carbon emissions and a “high” scenario (RCP 8.5) where humans are heavily dependent on fossil fuels and emissions grow.

Of the 11 model scenarios, only increased competition alone showed no projected declines (Table 4); the impact of habitat-related threats and angler effort doubling showed projected declines of 11 to 11.6%, and all models with two or more threats projected declines of at least 21.2%. A moderate climate change stressor (scenario RCP 4.5) alone projected a 40.9% decline, and a full model with habitat variables, angler effort projections, and moderate climate change projected a 69.2% decline over the next three generations. Finally, an aggregate model with a higher angler effort and more extreme climate change projections yields an 86.6% decline in Bull Trout system capacity. Thus, under five of the 11 scenarios, thresholds for Threatened (two) or Endangered (three) using Criterion A3 would be met. Note that these future projections are only for the Alberta portion of the DU. At the scale of the whole Western Arctic DU, existing data and future projections of abundance suggest declines but at unknown rates.

Note that, although there is no direct evidence available that there have been declines at the whole DU scale, the suite of threats assessed within the cumulative-effects models is directional and cumulative, so negative projections into the future imply negative impacts within the last three-generation time period, as well. With this rationale, it is reasonable to infer negative trends during this three-generation time period.

• Continuing decline in number of mature individuals: yes
• Evidence for continuing decline (1 generation or 3 years, whichever is longer, usually up to 100 years): uncertain but likely
• Evidence for continuing decline (2 generations or 5 years, whichever is longer, usually up to 100 years): uncertain but likely
• Evidence for past decline (3 generations or 10 years, whichever is longer) that has either ceased or is continuing (specify): uncertain but likely
• Evidence for projected or suspected future decline (next 3 generations or 10 years, whichever is longer, up to a maximum of 100 years): 10 to 70% reduction projected from the Threats Calculator; a suite of cumulative-effects models for the Alberta portion of the DU projects a 0 to 86.6% decline (Table 4)
• Extinction risk based on quantitative analysis: not conducted
• Long-term trends: substantial long-term decline (> 3 generations) in the Alberta component of the DU, but less clear elsewhere
• Population fluctuations, including extreme fluctuations: no evidence of extreme fluctuations

DU3 Upper Yukon Watershed population

Estimates of abundance, fluctuations and trends are unknown in this DU.

• Continuing decline in number of mature individuals: unknown
• Evidence for continuing decline (1 generation or 3 years, whichever is longer, usually up to 100 years): unknown
• Evidence for continuing decline (2 generations or 5 years, whichever is longer, usually up to 100 years): unknown
• Evidence for past decline (3 generations or 10 years, whichever is longer) that has either ceased or is continuing (specify): unknown
• Evidence for projected or suspected future decline (next 3 generations or 10 years, whichever is longer, up to a maximum of 100 years): unknown
• Extinction risk based on quantitative analysis: not conducted
• Long-term trends: unknown
• Population fluctuations, including extreme fluctuations: unknown

DU4 Saskatchewan-Nelson Rivers population

Substantial declines in Bull Trout subpopulations in this DU have been observed across their distribution since the middle of last century (Figures 6 and 7). While the most severe declines and extirpations of Bull Trout have been observed in the downstream extent, all areas have suffered declines in abundance. A summary for 45 HUCs in the DU showed 7 stable, 0 increasing, 31 decreasing, and 7 extirpated (Sawatzky 2016). The time scales of these trend assessments are variable, depending on data availability, so they can’t readily be assessed at the three-generation norm for COSEWIC assessments. The most recent rigorous assessment of population sizes by DU for each of the previous three generations provides updated population estimates but unfortunately can’t be used for trend assessments because methods, sample sites and sampling design goals differ substantially over the last 30 years. However, it is reasonable to infer that declines over the last several generations are common and substantial across the DU.

Future projections of Bull Trout system capacity at the three-generation time scale have been assessed using a cumulative-effects model (Sullivan and Reilly 2023), and a subset of the projections are summarized in Table 5. Of the 11 model scenarios, only increased competition alone showed insubstantial projected declines; the impact of angler effort doubling and quadrupling showed projected declines of 12.9 and 23.5%, respectively; the model for habitat-related variables projected a 30.3% decline; and a moderate climate change stressor (scenario RCP 4.5) projected a 30.7% decline. A high climate change stressor (scenario RCP 8.5) projected a 53.9% decline, and all models with two or more threats projected declines of at least 40.2%. A full model, with both habitat variables plus climate change projected a 66.0% decline over the next three generations. Lastly, a full model with higher projection for angler effort and more extreme climate change projections yielded an 80.7% decline in Bull Trout system capacity. Thus, under seven of the 11 scenarios, thresholds for Threatened (four) or Endangered (three) using Criterion A3 would be met.

Note that although there is no direct evidence available that there have been declines over the previous three generations, the suite of threats assessed within the cumulative-effects models are directional and cumulative, so negative projections into the future imply negative impacts within the last three-generation time period as well. With this rationale, it is reasonable to infer negative trends during the previous three-generation time period.

• Continuing decline in number of mature individuals: yes, an inferred and projected continuing decline
• Evidence for continuing decline (1 generation or 3 years, whichever is longer, usually up to 100 years): yes, projected continuing decline from the cumulative-effects analyses
• Evidence for continuing decline (2 generations or 5 years, whichever is longer, usually up to 100 years): yes, projected continuing decline from the cumulative-effects analyses
• Evidence for past decline (3 generations or 10 years, whichever is longer) that has either ceased or is continuing (specify): yes, projected continuing decline from the cumulative-effects analyses
• Evidence for projected or suspected future decline (next 3 generations or 10 years, whichever is longer, up to a maximum of 100 years): yes, projected at 0-86.6% decline from a suite of cumulative-effects analyses and 10 to 100% decline over three generations projected by the Threats Calculator
• Extinction risk based on quantitative analysis: not conducted
• Long-term trends: substantial long-term (> 3 generations) declines in abundance and spatial extent
• Population fluctuations, including extreme fluctuations: no evidence of extreme fluctuations

DU5 Pacific population

This DU contains 73 Core Areas, of which 16 have time series of adult abundance, although most are relatively short-term (Appendix 1). Overall, there are 23 time series of adult abundance across the 16 Core Areas, of which 14 are non-significant positive or negative trends. Of those series with significant trends, six are positive and three are negative.

In summary, no consistent trend is apparent from either the limited quantitative data or expert opinion assessment for this DU. Given the lack of quantitative data for most core areas within this DU, however, it would be inappropriate to consider existing trend data as representative of larger geographic areas. That said, the greatest concerns occur in the Moyie, Pend d’Oreille, and Columbia (downstream of Arrow Lakes Reservoir) rivers where expert opinion considers both low abundance and declining trends to be of significant concern in all three core areas (Hagen and Decker 2011). In contrast, most other core areas in the Columbia basin (upper Kootenay River and Koocanusa) are considered to be stable to increasing with large numbers of adults.

• Continuing decline in number of mature individuals: local evidence of declines but no overall evidence of decline at the DU scale
• Evidence for continuing decline (1 generation or 3 years, whichever is longer, usually up to 100 years): no evidence of DU wide decline
• Evidence for continuing decline (2 generations or 5 years, whichever is longer, usually up to 100 years): no evidence of DU wide decline
• Evidence for past decline (3 generations or 10 years, whichever is longer) that has either ceased or is continuing (specify): no evidence of DU wide decline
• Evidence for projected or suspected future decline (next 3 generations or 10 years, whichever is longer, up to a maximum of 100 years): 3 to 70% decline from Threats Calculator
• Extinction risk based on quantitative analysis: not conducted
• Long-term trends: no evidence of general long-term declines at the scale of the DU
• Population fluctuations, including extreme fluctuations: no evidence of extreme fluctuations

Severe fragmentation

Considering whether severe fragmentation applies to the Saskatchewan-Nelson Rivers DU, the majority (77%) of core areas contain fewer than 500 adults (Sawatzky 2016), which is considered the necessary threshold for Bull Trout subpopulations to be viable and persist long-term (Reiman and Allendorf 2001). However, severe fragmentation would require these core areas to be discrete and separated by a distance larger than Bull Trout can be expected to disperse. Migratory fluvial subpopulations within this DU would likely have been capable of dispersing over these distances, although many of the downstream fluvial subpopulations have been extirpated due to the development of dams and other barriers, and these core areas may be separated by distances larger than many resident subpopulations of Bull Trout can disperse. Thus, although evidence of severe fragmentation is not conclusive with the information available, most of the subpopulations are small and likely not viable long term without immigration. Within the other DUs, most population size estimates, where available, exceed the minimum viable threshold and therefore would not be considered severely fragmented.

Rescue effect

In theory, a diminished or extirpated population of Bull Trout could experience rescue from neighbouring subpopulations in the U.S.A., although rescue would not be possible for DUs 2 and 3, which are endemic to Canada. The potential for rescue for the other DUs will depend on several factors, including the amount of migration between subpopulations, the viability of immigrants in their new environment, and the status of neighbouring subpopulations. However, genetic studies on Bull Trout indicate low levels of gene flow between subpopulations. Significant genetic differentiation is common even within watersheds, although the degree of divergence is more pronounced at a more regional scale (Spruell et al. 1999; Taylor et al. 2001; Costello et al. 2003; Whiteley et al. 2004; Taylor and Costello 2006; Carroll and Vamosi 2021). Strong site fidelity to spawning area and overwintering habitat revealed by radiotelemetry studies (Swanberg 1997a; Bahr and Shrimpton 2004) further suggests that migration between subpopulations is low. This diminishes the likelihood of immigration providing significant rescue for Bull Trout subpopulations. Significant dispersal between watersheds seems particularly unlikely, although some evidence of straying at the local level (Swanberg 1997a; O’Brien 2001; Bahr and Shrimpton 2004), and at least one account of dispersal between watersheds (Brenkman and Corbett 2005), does suggest a potential role for dispersal from nearby sources in the repopulation of a declining or extirpated subpopulation, but recovery within a DU is different from rescue from the U.S.A. Within the Saskatchewan-Nelson Rivers DU, rescue across major basins is not possible due to dams on the mainstem rivers. However, even prior to dam construction, there is no evidence of contemporary connectivity among these subpopulations of Bull Trout.

While local adaptation of Bull Trout will reduce the viability of immigrants in new environments (Nosil et al. 2005) and diminish the possibility of rescue from neighbouring subpopulations, phenotypic plasticity may counterbalance this to some extent. Divergence in quantitative traits will likely be most evident across different environments at larger scales, for example, among subpopulations comprising different DUs. However, local adaptation may exist even at the fine scale, given that microsatellite-based differentiation, which likely provides conservative estimates of adaptive divergence (Pfrender et al. 2000; Morgan et al. 2001), has commonly been detected among subpopulations within localized areas (Spruell et al. 1999; Taylor et al. 2001; Costello et al. 2003; Taylor and Costello 2006).

Any rescue to be had will most likely occur between close, adjacent subpopulations that are connected by contiguous habitat suitable for Bull Trout migration. This could include several watersheds that have transboundary Bull Trout populations, such as the Flathead River, upper Kootenay River, Kootenay Lake, Salmo River, Skagit River and Chilliwack watershed within the Pacific DU. The southmost distribution of Bull Trout in the Saskatchewan-Nelson Rivers DU have many fewer cross boundary watersheds, so rescue from US subpopulations is not likely. In addition, the direction of any transboundary rescue effect is most likely to be from Canadian subpopulations to U.S.A. waters because Canadian Bull Trout are likely more numerous in both the number of subpopulations and abundance than their U.S.A. counterparts. Transboundary movement of Bull Trout between Canada and the U.S.A. in the South Coast DU is possible, particularly if there is any movement in and out of the marine environment. With very few strong or protected subpopulations near the border in the US, rescue of South Coast or Pacific DUs is possible but unlikely (Rieman et al. 1997), and it is therefore very unlikely that a Bull Trout subpopulation in the U.S.A. could contribute to rescue within Canada.

Threats

Historical, long-term, and continuing habitat trends

While the decline of Bull Trout in developed areas over the last century (Rieman et al. 1997; USFWS 1999, 2008; Rodtka 2009; Sawatzky 2016) clearly indicates their environmental sensitivity, the reasons underlying this vulnerability are not clearly understood. Most evidence is correlative in nature and identification of causal mechanisms is needed. Nevertheless, three main anthropogenic factors are likely responsible for their decline: loss of habitat network through degradation and fragmentation, interaction (hybridization and competition) with introduced species, and overexploitation (Rieman and McIntyre 1993; Brewin 2004; Rodtka 2009; Sawatzky 2016). These broad categories apply to Bull Trout across its range, and the descriptions given in each category’s subsection are relevant to all Canadian Bull Trout DUs. However, the type and extent of specific threats will vary at regional and local scales.

It can be difficult to predict and quantify the influences of anthropogenic specific threats, and their interactions with other threats and natural limiting factors. For example, increasing connectivity in landscapes that have become fragmented through human disturbance may reduce extinction risk by facilitating movement. However, it may simultaneously foster invasion by other non-native species (Fausch et al. 2008) or threaten previously isolated resident subpopulations with replacement by larger, migratory ones (Hagen 2008). In another example, the Bull Trout’s ability to resist invasion and persist in watersheds may be strengthened where intact habitat allows the expression of a full range of life histories, including large, highly fecund, migratory individuals (Nelson et al. 2002). When these migratory individuals are lost (for example, through habitat loss or fragmentation or overfishing), non-native fishes may be better able to displace or replace remaining resident Bull Trout (Dunham et al. 2008). Although understanding of such interactions is limited, it is undisputed that this combination of anthropogenic threats forms a formidable obstacle to the persistence of many Bull Trout subpopulations (Rieman and McIntyre 1993; Brewin 2004; Rodtka 2009; Sawatzky 2016).

Loss of habitat

The degradation and fragmentation of freshwater habitat associated with disruptive land use practices, such as commercial forestry, hydroelectric, oil, gas and mining development, agriculture, urbanization, and all of their associated road development has been widely documented (reviewed in Rieman and McIntyre 1993; Ripley et al. 2005; Rodtka 2009). The gradual decline of Bull Trout in developed areas over the last century (Rieman et al. 1997; USFWS 1999, 2008; Rodtka 2009) suggests a trend of negative biological response to this environmental disruption. Indeed, road density, as a general, indirect measure of habitat disturbance, has frequently been found to significantly negatively correlate (P < 0.05) with Bull Trout occurrence (Rieman et al. 1997; Baxter et al. 1999; Dunham and Rieman 1999; Ripley et al. 2005; Scrimgeour et al. 2008).

Habitat degradation

The environmental sensitivity of Bull Trout is not surprising given their very specific habitat requirements. Variables such as temperature, depth, velocity, substrate, and cover are critical to the persistence of this coldwater specialist. Bull Trout’s long overwinter incubation and rearing phase make these early life stages particularly vulnerable. For example, the occurrence of Bull Trout is negatively correlated to the percentage of fine sediment filling interstitial spaces (Weaver and White 1985; Ripley et al. 2005). Groundwater is key to providing the high-quality habitat required for this stage, as well as overwintering, in many Bull Trout subpopulations (Baxter 1997; Baxter and McPhail 1999; Baxter and Hauer 2000; Ripley et al. 2005). As well as direct impacts, habitat degradation that impacts the availability and abundance of prey species will also likely have a bottom-up effect on this top predator.

The exact mechanisms by which disruptive land use practices adversely affect the occurrence and abundance of Bull Trout are not well understood. Their impacts on habitat quality are likely related to changes in forest composition and age that alter the input of groundwater and woody debris, loss of deep pools, channel simplification, decreased vegetation cover, and increase surface runoff, sediment inputs, and nutrient pulses. These effects can lead to diminished water quality, reduced cover, increased thermal and light regimes, increased sedimentation, and altered flow regimes that destabilize streambeds (reviewed in Rieman and McIntyre 1993; Ripley et al. 2005; Tonina et al. 2008; Rodtka 2009). For example, increased stream temperatures are a common result of watershed developments when they result in loss of riparian vegetation (Holtby 1988; Johnson and Jones 2000; Post and Johnston 2002).

Climate change

Bull Trout’s susceptibility to increasing water temperatures extends beyond the localized effects of altered patterns of forest cover, to global climate change (Rieman and McIntyre 1993; Rieman et al. 1997, 2007). Climate change and associated global warming in northern North America is likely to exceed global means with recent models projecting 2.0 to 4.5^oC between now and 2080 (Government of Canada 2024). Such temperature changes would limit the availability of suitable Bull Trout habitat and increase the risk of invasion and displacement, by other species that require warmer water (Keleher and Rahel 1996; Rahel et al. 1996; Porter and Nelitz 2009; Eby et al. 2014). An increase in winter precipitation and a decrease in summer rainfall are also expected in western regions (Christensen et al. 2007). Subsequent winter flooding caused by heavy precipitation or glacial floods could damage Bull Trout spawning and rearing habitat. Changes like these are likely to have their biggest impact on Bull Trout subpopulations in the south of its range, where temperature already defines its southern limit (Dunham et al. 2003). Here, simulations of predicted 5^oC warming result in a 69% decrease in the length of streams having thermally suitable habitat for cold water salmonids in a Wyoming drainage of the Rocky Mountains (Rahel et al. 1996), and a loss of 92% of thermally suitable Bull Trout natal habitat area over 50 years in the interior Columbia River basin of the U.S.A. (Rieman et al. 2007). However, mountain streams tend to show slower climate change velocities than sites further downstream and may therefore provide a mid-term refuge for coldwater species such as Bull Trout (Isaak et al. 2015, 2016; LeMoine et al. 2020). In addition, there has been little consideration of potential impacts, including potential range extensions, at the northern limits of the species’ range as extreme cold temperatures can also be limiting at their northern limit (Mochnacz et al. 2021).

Habitat fragmentation

As well as having very specific habitat requirements, migratory subpopulations need uninterrupted migratory corridors that connect spawning grounds with feeding and overwintering habitats. The viability of these subpopulations, therefore, is linked to their need to access this diversity of habitat at different stages throughout their life cycle (Rieman and McIntyre 1993). Several activities can fragment Bull Trout’s habitat. Hydroelectric dams are obvious barriers to movement that can threaten the viability of Bull Trout subpopulations across their range (U.S.A.: Neraas and Spruell 2001; BC: Decker and Hagen 2008; Hagen 2008; AB: reviewed in Rodtka 2009). They can isolate subpopulations and prevent migration between productive juvenile and adult rearing environments (Swanberg 1997b; Neraas and Spruell 2001; Decker and Hagen 2008; Hagen 2008), as well as alter and degrade Bull Trout habitat (Brown 1995; Decker and Hagen 2008; Hagen 2008).

Road construction and other linear development features can also lead to fragmentation of Bull Trout habitat via numerous smaller blockages and hanging culverts (reviewed in Rieman and McIntyre 1993; Ripley et al. 2005; Rodtka 2009; Cott et al. 2015). Other obstructions to movement can be more subtle than these obvious physical impacts; degraded habitat resulting from, for example, increased water temperatures and velocities, can also ruin and fragment suitable habitat patches (Rieman and McIntyre 1993; Hagen 2008).

Existing fragmentation restricts gene flow, making isolated subpopulations more susceptible to local extinction from stochastic and deterministic risks (Lande 1993; Dunham and Rieman 1999). With less chance of recolonization through regional connectivity, extinction at the regional scale becomes more likely (Rieman et al. 1997). As a result of such fragmentation, Bull Trout’s distribution may diminish in a way that is not directly proportional to the loss of habitat area. Rather, rates of extinction may accelerate beyond rates of habitat loss (Rieman and McIntyre 1995).

Interaction with introduced species

The expansion of introduced fish species also poses a significant threat to Bull Trout (Donald and Alger 1993; Leary et al. 1993). Although many introduced species (for example, Lake Trout, Yellow Perch (Perca flavescens), Smallmouth Bass (Micropterus dolomieu), Largemouth Bass (Micropterus salmoides), Walleye (Sander vitreus), and Northern Pike (Esox lucius)) may pose a threat to Bull Trout subpopulations, the greatest threat likely comes from non-native Brook Trout, which now broadly overlap spatially with Bull Trout. There are documented negative consequences of Brook Trout as the result of direct interactions with Bull Trout. The introduction of this recreational fish across the Pacific Northwest from its native eastern North American range began in the late 1800s. Ongoing introductions and its subsequent invasion have led to its wide establishment throughout much of Bull Trout’s range (Fuller et al. 1999) and its presence in many of the same basins (Rieman and McIntyre 1993).

Anecdotal evidence of Bull Trout’s occurrence being negatively associated with the presence of Brook Trout strongly implicates this non-native fish in the decline in Bull Trout subpopulations across much of its range (Paul and Post 2001; Rich et al. 2003; Rieman et al. 2006; McCleary and Hassan 2008; Sawatzky 2016; Sinnatamby et al. 2023). Hierarchical analysis confirms that Brook Trout can influence upstream displacement of Bull Trout, although the extent of displacement is strongly influenced by environmental conditions (including elevation and temperature; Rieman et al. 2006). While complete elimination of Bull Trout is not a foregone conclusion of Brook Trout invasion, even partial upstream displacement of Bull Trout by Brook Trout may pose a serious threat in these low-density subpopulations. Bull Trout occurrence decreases with stream width (Rieman and McIntyre 1995; Earle et al. 2007; McCleary and Hassan 2008) so, as Bull Trout are displaced upstream, smaller and more isolated Bull Trout subpopulations will likely become more vulnerable to local extinction through other causes (Lande 1993; Dunham and Rieman 1999).

The potentially devastating and unpredictable impact of non-native species on Bull Trout is illustrated by the crash in the early 1990s of Bull Trout in Flathead Lake and the Moyie River system in northwest Montana. The collapse of these Bull Trout subpopulations that were previously considered to be abundant and secure resulted from the introduction of the combination of Lake Trout and the non-native invertebrate, Opossum Shrimp (Mysis relicta; Spencer et al. 1991). These species caused major ecosystem changes and cascading food web interactions (Spencer et al. 1991).

Overexploitation

Bull Trout were once considered “junk” fish because of their tendency to prey on other salmonids (McPhail 2007; Dunham et al. 2008). Active eradication plans combined with easy road access resulted in Bull Trout being “fished out” of some areas, including parts of southern Alberta, British Columbia, and Montana (Mackay et al. 1997; McPhail 2007; Dunham et al. 2008; Fredenburg 2014). Changing attitudes and management practices mean that the threat of extirpation from overharvesting has been reduced for many Canadian Bull Trout subpopulations (McPhail 2007; Johnston et al. 2007; Johnston and Post 2009; Johnston et al. 2011). Johnston et al. (2007) provides evidence that rapid recovery to high abundance can occur following restrictive angling regulations. Strong density-dependent compensation in juvenile survival has been observed in several subpopulations, indicating strong scope for recovery from overexploitation occurs where habitat quality and quantity are not severely limiting, though data are limited across the species’ range (Chudnow et al. 2019).

Nevertheless, not all subpopulations that have been subject to strict angling regulations have shown signs of recovery (reviewed in Rodtka 2009; Hagen and Decker 2011). The lack of change in some systems may be partly attributed to Bull Trout’s high catchability. Angler-mediated mortality from hooking, poaching, and non-compliance to fishing regulations still poses a significant threat in some areas (Post et al. 2003; Hagen and Decker 2011; Thorley et al. 2017; Joubert et al. 2020). The infrastructure of road networks developed to support urban and industrial activities can exacerbate this threat by increasing accessibility (reviewed in Rieman and McIntyre 1993; Ripley et al. 2005; Rodtka 2009). Simulations using reasonable estimates of fishing effort, mortality from catch-and-release, and illegal harvest demonstrate that many Bull Trout subpopulations will continue to require restrictive angling regulations if they are to be sustained (Post et al. 2003).

Although there is no published information on the extent of mortality of Bull Trout in rivers where intensive fisheries exist for other salmonids, incidental by-catch mortality from commercial and recreational fisheries directed at these other fishes poses a risk to Bull Trout (Sinnatamby et al. 2023). This may be borne out not just through increased hooking mortalities (Paul et al. 2003), but also through misidentification with other char and trout species (Rodtka 2009); many anglers remain unaware of a key distinguishing morphological feature in Bull Trout, the absence of spotting on the dorsal fin (Rodtka 2009). The introduction of sport fish, such as Brook Trout, adds to this threat (Paul et al. 2003).

Features of Bull Trout life history, including late age-at-maturity, low fecundity, and a tendency towards non-consecutive year spawning, will hamper recovery from anthropogenic disturbances (Paul et al. 2003; Post et al. 2003; Johnston et al. 2007; Johnston and Post 2009). Its high catchability also renders Bull Trout particularly vulnerable to overharvesting, even when angling effort and harvest limits are low (Paul et al. 2003; Post et al. 2003; Brenkman et al. 2007).

DU1 South Coast British Columbia population

Threats have been assessed for this DU (Table 6 – Hagen and Decker 2011; MLWRS 2022). Significant threats include:

Loss of habitat

The numerous hydroelectric projects and their associated dams in the Lower Mainland (BCME 2011), as well as extensive urbanization, agricultural, and transportation system development (and, to a lesser extent, forestry) may degrade and/or fragment Bull Trout habitat within this DU (Hagen and Decker 2011).

Introduced species

Brook Trout in Canada are concentrated in southeastern British Columbia, as well as southwestern Alberta (Fuller et al. 1999; McPhail 2007). British Columbia’s Brook Trout Stocking Program supplies these fish to fewer than 100 lakes (Pollard and Down 2001). Several initiatives in British Columbia attempt to address concerns about the threat Brook Trout pose to Bull Trout. For example, BC’s draft Brook Trout Stocking Policy, developed in 1998, calls on no further expansion of its stocking program, sterilization of all stocked fish, and pilot projects investigating their replacement with less risky stocking practices (Pollard and Down 2001). Although ill effects of Brook Trout presence on Bull Trout are not always observed, sometimes Bull Trout can be almost extirpated following introduction, and there has been a recent influx of Brook Trout into the Skagit River system, which makes up approximately 75% of this DU (Appendix 2). Also, the lower Fraser River has more than 10 other invasive fish species; for example, Smallmouth Bass has recently invaded Cultus Lake where Bull Trout occur.

Overexploitation

Anadromous Bull Trout may be particularly susceptible to incidental by-catch, given their multiple migrations between fresh water and salt water, and their tendency to congregate in estuaries (Taylor and Costello 2006; Brenkman et al. 2007). Incidental by-catch of anadromous Bull Trout has been documented in terminal gill-net fisheries directed at Pacific salmon in north-west Washington State (Brenkman et al. 2007). Although protective regulations are in place, illegal harvest is thought to be a potential threat to Bull Trout subpopulations in the Lillooet provisional core area in particular (Hagen and Decker 2011).

DU2 Western Arctic population

Threats have been assessed for the BC component of this DU (Table 6 – Hagen and Decker 2011; MLWRS 2022) and through a cumulative threats assessment in Alberta (see Table 4 and Sullivan and Reilly 2023). Significant threats that have been identified include:

Loss of habitat

Habitat disturbances from intense development pressure in the Lower Peace River basin within British Columbia and Alberta warrant particular attention for this DU. Exploration for and extraction of oil and gas, as well as mining developments and timber harvesting, and their accompanying developments (for example, roads, urbanization) are of the most concern (Rodtka 2009; Hagen and Decker 2011). To a lesser extent, similar concerns extend to the Lower Liard River basin within British Columbia (Hagen and Decker 2011) and Yukon (Connor et al. 1999). The Site C dam on the Peace River is likely a threat to the subpopulations in the Halfway-Peace, Murray, Moberly, and Pine/Sukunka core areas. Conversion of river to reservoir habitats and associated changes in species assemblages, and changes to life history strategies are likely if fish passage facilities are ineffective.

Despite the significant potential of industrial and transportation development in watersheds to be detrimental to Bull Trout subpopulations, little evidence of this has been effectively documented. Scrimgeour et al. (2008) is an exception to this; they found the occurrence of Bull Trout in the Kakwa and Simonette watersheds of west central Alberta negatively related to percent disturbance from exploration and extraction of oil and gas resources, as well as forest harvesting. Ripley et al. (2005) also found the level of commercial foresting (cumulative area of the subbasin harvested) in the Kakwa River Basin negatively correlated to Bull Trout occurrence. Both of these studies also found that road density acted as a general, indirect measure of habitat disturbance that significantly negatively correlated (P < 0.05) with Bull Trout occurrence (Ripley et al. 2005; Scrimgeour et al. 2008).

The susceptibility of Bull Trout to detrimental changes in water quality from heavy metal contaminants released from mining activities is also poorly understood (but see Hansen et al. 2002a,b,c). There is, however, concern about the contribution of mining activity in Alberta’s northeast slopes region to declining Bull Trout stocks in the area. Elevated levels of selenium, which can reduce recruitment in fish subpopulations by increasing rates of deformities during early development (Hodson et al. 1980; Hodson and Hilton 1983), occur in the region (Casey and Siwik 2000). Muscle biopsies indicate that selenium concentrations do, in fact, exceed toxicity threshold values for negatively impacting reproductive success in most Bull Trout captured downstream of coal mining activity (Palace et al. 2004). However, further analysis of Bull Trout eggs is needed to understand the impact of selenium on Bull Trout survival and recruitment in these coal impacted waters (Palace et al. 2004). Coal mine development planned for the Murray River area in the lower Peace River watershed may pose a risk to Bull Trout spawning in this area.

Although hydroelectric dams can pose a risk to Bull Trout subpopulations, there are relatively few such developments in Northern British Columbia or in Alberta. Those that exist within this DU are clustered around the Upper Peace River (Hagen and Decker 2011), although the Site C Dam on the Peace River has the potential to profoundly affect Bull Trout subpopulations in the Lower Peace EDU (Hagen and Decker 2011).

Introduced species

Although Brook Trout in Canada are most prevalent in southern British Columbia and southwestern Alberta (Fuller et al. 1999; McPhail 2007), Bull Trout’s occurrence has been negatively associated with the presence of Brook Trout within this DU (McCleary and Hassan 2008). While most Brook Trout stocking within Bull Trout’s range in Alberta has stopped, an ongoing Provincial Brook Trout Stocking Program continues to supply these fish to less than 100 lakes across British Columbia (Pollard and Down 2001). As listed under DU1 South Coast British Columbia population subsection, several initiatives attempt to address concerns about the threat to Bull Trout from this continuing Brook Trout stocking program (Pollard and Down 2001). With increasing temperatures, Brook Trout may be more problematic. Also, there are some concerns regarding Brook Trout x Bull Trout hybridization, but this does not yet appear to be pervasive (Appendix 2). An increasing abundance of Lake Trout in Williston Reservoir (Upper Peace EDU) is also a growing but low severity threat at present (Hagen and Decker 2011).

Overexploitation

There is a curious pattern of increases in some Bull Trout subpopulations within Alberta (for example, Lower Kananaskis Lake) but no change in others (for example, Kakwa River) that have been subject to strict angling regulations (reviewed in Rodtka 2009). The lack of change in some systems may be partly attributed to Bull Trout’s high catchability, with hooking mortality, poaching, and non-compliance to fishing regulations still posing a significant threat in some areas (reviewed in Rodtka 2009). The potential for overexploitation of Bull Trout is recognized as a moderately severe threat in specific locations in the Upper Peace EDU (Hagen and Decker 2011). In addition, the increase in angler-mediated mortality that may be associated with increased accessibility (Ripley et al. 2005) will likely be a threat in remote areas of this DU that have experienced recent increases in road development for primary resource extraction but where enforcement remains difficult. Exploitation was assessed as a threat within the cumulative effects analysis, but only for the Alberta component of this DU. Reasonable scenarios of increases in angler effort focused on Bull Trout includes a doubling to quadrupling over 30 years, resulting in projections of approximately 11% and 22% reductions in system capacity respectively (Table 4).

DU3 Upper Yukon Watershed population

Significant threats include:

Loss of habitat

Unlike the other DUs, there are no hydroelectric dams within this DU that threaten Bull Trout habitat (BCME 2011). Furthermore, there is very minimal road access and little (historical) mining activity (Hagen and Decker 2011).

DU4 Saskatchewan-Nelson Rivers population

An extensive threats assessment has been completed at the scale of HUC8s and rolled up into a DU wide summary by Sawatsky (2016) (Table 7). In addition, threats have been quantified through a cumulative threats assessment in Alberta (see Table 5 and Sullivan and Reilly 2023). Significant threats include:

Loss of habitat

All of the land use practices that have been identified as general threats to the integrity of Bull Trout habitat throughout their Canadian range have been associated with the decline of Bull Trout in southwestern Alberta (for example, commercial forestry, hydroelectric, oil, gas and mining development, agriculture, urbanization, their associated road development, and climate change). Although it is often difficult to attribute particular habitat losses directly to Bull Trout population declines, a long-term study (34 years) on Hidden Creek, Alberta, has documented the coincidence of large-scale clear cutting and subsequent reduction in spawning activity of Bull Trout, likely as the result of hydrological changes and sedimentation effects (Fitch 2024). Three substantial floods also occurred within this time period, which may have had short term negative consequences on reproduction, but the impact was short lived in contrast to longer term consequences of forestry practices (Fitch 2024).

Hydroelectric, water supply, and flood control dams can pose a risk to Bull Trout subpopulations, and many of the larger mainstem rivers in this DU have dams. These have the effect of fragmenting Bull Trout habitat with greatest impacts on the large-bodied fluvial migratory life history type. This is likely the primary cause of extirpations within the mid-reaches of the North Saskatchewan, Red Deer, Bow, and Oldman rivers (Figure 7). These dams have no provision for fish passage (Fernet and O’Neil 1997).

Introduced species

Brook Trout are particularly prevalent in southwestern Alberta, (Fuller et al. 1999; McPhail 2007). Brook Trout introductions in southwestern Alberta are thought to have contributed to the historical pattern of decline in Bull Trout subpopulations in this DU (Fitch 1997; Paul and Post 2001). In recognition of this, most Brook Trout (as well as Brown Trout) stocking within Bull Trout’s range in Alberta has either stopped for more than 8 years or, in a few cases, been replaced by stocking of only sterile, triploid fish.

A Brook Trout removal research project in Quirk Creek, southwestern Alberta (Paul et al. 2003; Earle et al. 2007; Sinnatamby et al. 2023), provides a cautionary note on the difficulty of removing or suppressing introduced species to promote Bull Trout recovery. Here, Brook Trout have been found to be relatively resilient to even selective harvesting, thanks to their fast growth and early maturation, and their lower catchability (that is, proportion of vulnerable population caught per unit of angling effort) compared to native salmonids, including Bull Trout (Paul et al. 2003; Earle et al. 2007; Sinnatamby et al. 2023). On the other hand, Bull Trout, with their higher catchability, slower growth, and later maturity, are extremely sensitive to overexploitation, and may even be negatively impacted from incidental mortalities resulting from such initiatives (Paul et al. 2003; Earle et al. 2007; Sinnatamby et al. 2023). However, extensive mechanical removal efforts to eradicate Brook Trout were successful in Banff National Park (Pacas and Taylor 2015), although this would not be possible if the goal was to recover a native species in situ as it would also be eradicated.

Overexploitation

The diminished threat of extirpation from overharvesting within this DU is reflected in the expansion of some previously exploited Bull Trout subpopulations since the introduction of strict angling regulations (for example, Lower Kananaskis, Jacques and Harrison lakes, and Clearwater and Sheep rivers; Johnston et al. 2007; and reviewed in Rodtka 2009). Nevertheless, not all Bull Trout subpopulations in southwestern Alberta that have been subject to strict angling regulations have shown change (for example, Elbow and Highwood rivers, and Quirk Creek; reviewed in Rodtka 2009). The lack of change in some systems may be partly attributed to Bull Trout’s high catchability, with hooking mortality, poaching, and non-compliance to fishing regulations still posing a significant threat in some areas (reviewed in Rodtka 2009). Exploitation was assessed as a threat within the cumulative effects analysis for this DU. Reasonable scenarios of increases in angler effort focused on Bull Trout includes a doubling to quadrupling over 30 years, resulting in projections of approximately 12% and approximately 24% reductions in system capacity respectively (Table 5).

DU5 Pacific population

Threats have been assessed for this DU (Table 6 – Hagen and Decker 2011; MLWRS 2022). Significant threats include:

Loss of habitat

Hydroelectric dams within this DU are concentrated in a southern central area that covers the Upper Columbia basin, the Thompson-Okanagan region, and the interior of the Cariboo-Chilcotin region (BCME 2011; Hagen and Decker 2011). Evidence from this DU indicates that hydroelectric dam projects can degrade Bull Trout habitat, as well as potentially isolating resident populations and preventing migratory fishes from moving between their spawning and feeding grounds. The inundation of streams and lakes can ruin spawning and rearing grounds, adult habitat can be degraded, and the reduced flow can degrade adult habitat downstream through sedimentation (Brown 1995; Decker and Hagen 2008; Hagen 2008). The spawning preference of Bull Trout for colder, higher elevation headwaters will, however, reduce this impact relative to other salmonids (Hagen 2008). While riparian restoration along streams and the removal of migration barriers can correct for these losses in habitat and connectivity, care must be taken to not create other negative impacts, such as threatening previously isolated resident subpopulations with replacement by larger, migratory ones (Hagen 2008).

Another serious threat to Bull Trout across parts of this DU (especially the Middle Fraser EDU but also including parts of the Homathko-Klinaklini, Bella Coola-Dean, and Thompson EDUs) is the recent massive loss of pine forest cover to the Mountain Pine Beetle (Dendroctonus ponderosae), which could lead to significantly warmer thermal regimes (Hagen and Decker 2011). While these impacts will likely be reduced in the long term as forests regenerate, climate change will likely exert an increasingly negative influence on thermal regimes for Bull Trout. Although detrimental habitat changes associated with global warming are likely to have their biggest impact on Bull Trout subpopulations in the US, where temperature already defines its southern limit (Dunham et al. 2003), an assessment of the snowmelt-dominated watersheds in the Cariboo-Chilcotin region of the Middle Fraser EDU in British Columbia suggests that the thermal and precipitation effects of global warming will produce a long-term pattern of considerably decreased cold water stream habitat by the 2080s (Porter and Neritz 2009). Indeed, the potential of climate change to be a major threat to the long-term persistence of Bull Trout is recognized for a number of provisional core areas in the Middle Fraser, Thompson, and Columbia-Arrow Lakes EDUs (Hagen and Decker 2011). It is also recognized as a potential threat to areas of the Upper and Lower Kootenays, Bella Coola-Dean, and Upper Fraser EDUs (Hagen and Decker 2011). Bull Trout streams downstream of heavily glaciated headwaters that are found in some areas of this DU (for example, some areas in the Homathko-Klinaklini, Thompson, Columbia-Arrow Lakes, and Middle Fraser EDUs) will likely be buffered against such degradation of thermal regimes (Hagen and Decker 2011).

Habitat threats related to other watershed development are also recognized in EDUs across this DU (Hagen and Decker 2011). In places these threats are potentially widespread, for example, mining in the Upper Kootenays, and forestry in the Upper Columbia and Lower Kootenays EDUs (Hagen and Decker 2011). Potentially significant threats to Bull Trout subpopulations posed by some proposed watershed developments (for example, hydroelectric projects in the Homathko-Klinaklini EDU; mining in the Upper Nass, Upper Stikine, Nakina, and Taku EDUs; and recreation resort in the Thompson EDU) are recognized as requiring immediate attention (Hagen and Decker 2011).

Introduced species

Brook Trout in this DU are concentrated in southeastern British Columbia (Fuller et al. 1999; McPhail 2007). The potential threat posed by this species is recognized for several areas in this DU (Upper Columbia, Columbia-Arrow Lakes, and Upper Fraser EDUs, Hagen and Decker 2011), and there has been a negative impact from Brook Trout throughout the Yoho and Kootenay regions (Appendix 2). While British Columbia’s ongoing Provincial Brook Trout Stocking Program supplies these fish to less than 100 lakes (as of 2001; Pollard and Down 2001), several initiatives attempt to address concerns about the threat to Bull Trout from this continuing Brook Trout stocking program (listed under DU1 [Genetic Lineage 1: Southcoast BC populations] subsection, Pollard and Down 2001). Lake Trout incursion in the Moyie EDU is considered to be a major threat (Hagen and Decker 2011). Bass are present in the Beaver-Quesnel system (including in the colder Quesnel system) and result in competition with juvenile Bull Trout, and Whirling Disease (Myxobolus cerbralis) has been detected in Yoho National Park and Kootenay Lake in B.C. (Government of British Columbia 2025).

Overexploitation

Overexploitation is likely the most significant historical impact on Bull Trout in the Middle Fraser EDU, alongside hydroelectric development (Hagen and Decker 2011). As is the case elsewhere (for example, Wigwam River, Pollard and Down 2001), at least some Bull Trout subpopulations within this EDU have recovered from past exploitation following stricter angling regulations (for example, Quesnel Lake, Porter, and Nelitz 2009). Nevertheless, concern about localized overharvest still exists for some core areas in this and other EDUs (for example, Thompson, Lower and Middle Skeena, Lakelse-Kalum, and Morice-Bulkley) (Hagen and Decker 2011).

Current and projected future threats

This section addresses threats impacts following the IUCN-CMP unified threats classification system. Bull Trout is vulnerable to the cumulative effects of various threats. The nature, scope, and severity of these threats has been described in Appendix 2, following the IUCN-CMP (International Union for the Conservation of Nature – Conservation Measures Partnership) unified threats classification system (see Salafsky et al. 2008 for definitions and Master et al. 2012 for guidelines). The threat assessment process consists of assessing impacts for each of 11 main categories of threats and their subcategories, based on the scope (proportion of population exposed to the threat over the next 10-year period), severity (predicted population decline within the scope during the next 10 years or 3 generations, whichever is longer up to approximately 100 years), and timing of each threat. The overall threat impact is calculated by taking into account the separate impacts of all threat categories and can be adjusted by the species experts participating in the threats evaluation.

South Coast British Columbia population DU1

The overall threat impact for Bull Trout is considered to be High, corresponding to an anticipated further decline of between 10 and 70% over the next three generations. The primary threats ranked are:

8 Invasive and other problematic species and genes - High-Medium

11 Climate change and severe weather - Medium

5 Biological resource use - Medium-Low

9 Pollution - Medium-Low

4 Transportation and service corridors – Medium-Low

7 Natural system modifications – Medium-Low

Unknown threats are:

3 Energy production and mining

6 Human intrusions and disturbance

10 Geological events

Western Arctic population DU2

The overall threat impact for Bull Trout is considered to be High, corresponding to an anticipated further decline of between 10 and 70% over the next three generations. The primary threats ranked are:

7 Natural system modifications - High-Medium

11 Climate change and severe weather - High-Medium

5 Biological resource use - Medium

9 Pollution - Medium

4 Transportation and service corridors - Medium-Low

3 Energy production and mining - Low

6 Human intrusions and disturbance - Low

8 Invasive and other problematic species and genes - Low

Unknown threats are:

10 Geological events

Upper Yukon Watershed population DU3

The overall threat impact for Bull Trout is Unknown with the following Unknown primary threats:

3 Energy production and mining

4 Transportation and service corridors

5 Biological resource use

9 Pollution

10 Geological events

11 Climate change and severe weather

Saskatchewan-Nelson Rivers population DU4

The overall threat impact for Bull Trout is considered to be Very high-High, corresponding to an anticipated further decline of between 10 and 100% over the next three generations. The primary threats ranked are:

5 Biological resource use - High-Medium

7 Natural system modifications - High-Medium

11 Climate change and severe weather - High-Medium

6 Human intrusions and disturbance - Medium

9 Pollution - Medium

8 Invasive and other problematic species and genes - Medium-Low

3 Energy production and mining - Low

4 Transportation and service corridors – Low

Pacific population DU5

The overall threat impact for Bull Trout is considered to be High-Medium, corresponding to an anticipated further decline of between 3 and 70% over the next three generations. The primary threats ranked are:

4 Transportation and service corridors - Medium-Low

5 Biological resource use - Medium-Low

7 Natural system modifications - Medium-Low

8 Invasive and other problematic species and genes - Medium-Low

9 Pollution - Medium-Low

11 Climate change and severe weather - Medium-Low

Unknown threats are:

3 Energy production and mining

6 Human intrusions and d

Number of threat locations

Bull Trout is a broadly but patchily distributed species that encounters a mosaic of threats that vary substantially in spatial scale. Therefore, the precise number of threat-based locations is difficult to establish. DU1 and DU5 tend to have spatially patchy primary threats (invasive and other problematic species in DU1 and a number of Medium-Low threats in DU5, including transportation and service corridors, biological resource use, and invasive and other problematic species in DU5), and therefore likely have considerably more than 10 locations. In DU2, the threat from natural systems modifications (for example, dams and water withdrawal for fire suppression and fracking) is likewise spatially patchy, suggesting more than 10 locations. Climate change was scored as an equally serious plausible threat, although the scale at which it might affect the population is hard to estimate considering that it doesn’t consider the interacting effects of water extraction and temperature or the effects of groundwater input (Appendix 2). The number of locations in DU3 is unknown.

In DU4, which is smaller and relatively homogenous compared to DU 2 and 5, the extensive spatial data collected for the Saskatchewan-Nelson Rivers population permits estimation of threats-based locations where the combination of the three Medium-High threats (biological resource use, natural systems modifications, and climate change and severe weather in the Eastern Slopes of the Rocky Mountains) suggests as few as two locations or as many as nine locations. In this DU, there are likely to be increasingly severe drought conditions that will be exacerbated by the effects of temperature extremes and expected to be widespread. A subdivision based on susceptibility to climate change into upstream and downstream areas suggests two locations. Climate modelling projections for 2055 showing maximum weekly maximum (air) temperature (MWMT) in the Saskatchewan-Nelson Rivers DU, predicted that air temperatures are likely to remain more suitable for Bull Trout in small zones at higher elevations but not at lower elevations, which are likely to be highly impacted from temperature and hydrological changes over the next three generations (Reilly pers. comm. 2025; Figure 14). Even alpine areas can get too warm in heat dome conditions and lower elevation areas are warming already. If these two locations based on the threat of climate change are further subdivided into strata characterized by high and low accessibility to fishing impacts (based on road accessibility), then there are likely four locations. If considering a combination of the three Medium-High threats (that is, including natural systems modifications, in addition to climate change and severe weather and biological resource use), there are as many as nine locations based on major watersheds: 1) North Saskatchewan and 2) Clearwater rivers, which flow through Edmonton; 3) Red Deer River flowing through the city of Red Deer; 4) Bow and 5) Elbow rivers, flowing through Calgary; 6) Highwood River, which joins the Bow/Elbow rivers downstream of Calgary; 7) Oldman River; 8) Belly River; and 9) St. Mary River, which flows through Lethbridge.

Figure 14. Climate modelling projections for 2055 showing maximum weekly maximum (air) temperature (MWMT) or 7-day maximum in the Saskatchewan-Nelson Rivers DU. Air temperatures are predicted to remain more suitable for Bull Trout in small zones at higher elevations (yellow 8.68 to 10.64 ^oC and orange 10.64 to 12.43 ^oC) but not at lower elevations (light red 12.43 to 15.4 ^oC and dark red 15.40 to 17.93 ^oC) (Reilly pers. comm. 2025; last updated March 2015).

Protection, status, and recovery activities

Legal protection and status

Bull Trout are listed under the Canadian Species at Risk Act (2019) as follows: South Coast British Columbia population DU1 – Special Concern; Western Arctic population DU2 – Special Concern; Saskatchewan-Nelson Rivers population DU4 – Threatened. In addition, the Pacific population DU5 is considered Not At Risk and the Upper Yukon Watershed population DU3 as Data Deficient. In Alberta, Bull Trout is listed as Threatened under Alberta’s Wildlife Act as of 2014 (Alberta Environment and Parks 2020). In British Columbia, Bull Trout is on the Provincial Blue List (that is, Species of Special Concern) and is an Identified Wildlife Species at Risk under the Identified Wildlife Management Strategy (Ministry of Water, Land and Air Protection 2004). Critical Habitat has been identified in the federal recovery strategy and afforded protection by a federal Order in Council in 2021 (SOR/2021 to 31). Bull Trout in national park waters are afforded protection under the Canada National Parks Act and there is a zero-possession limit for Bull Trout under the National Park Fishing Regulations. Critical Habitat for Bull Trout was identified and is legally afforded protection in Banff National Park of Canada, Jasper National Park of Canada, and Waterton Lakes National Park of Canada as of 2021.

Despite these legal protections, it appears that the protections are not fully effective in protecting Bull Trout and their critical habitat. A recent analysis in Alberta suggests that water crossings and forest harvest have eliminated substantial designated Critical Habitat and that future approved developments will continue to compromise Critical Habitat (CPAWS 2025). Therefore, despite Provincial and Federal legal protection, substantial threats to Bull Trout and their Critical Habitat remain.

Non-legal status and ranks

Nature Serve Conservation Status Global Rank is Secure (G3), Canada Rank is Vulnerable–Apparently Secure (N3N4). Provincial Ranks are: Imperilled (S2) in Alberta; Imperiled–Vulnerable (S2S3) in Northwest Territories; Vulnerable (S3) in Yukon Territory; and Vulnerable–Apparently Secure (S3S4) in British Columbia (NatureServe Explorer 2025). Adjoining states are Critically Imperilled (S1) in Washington, Imperilled (S2) in Montana, and Apparently Secure (S4) in Idaho.

Land tenure and ownership

The Fisheries Act of Canada (2019) provides protection of all fish and fish habitat including protection against the death of fish, other than by fishing, and the harmful alteration, disruption, or destruction of fish habitat. These enhanced protections were added after changes to the Fisheries Act in 2012 (Hutchings and Post 2013). Bull Trout and their habitat in national parks are protected under the Canada National Parks Act.

The majority of land in Bull Trout’s Canadian range is Crown or public (BC approximately 94%; AB approximately 72%; NT; approximately 100%; YK 98%) with the minority being privately owned (PDAC 2008).

Recovery activities

• A Recovery Potential Assessment was developed by Fisheries and Oceans Canada to provide information and scientific advice in fulfilling the requirements under SARA (Sawatzky 2016). The assessment was only for the Saskatchewan-Nelson Rivers population
• Critical Habitat for Bull Trout was identified and is legally protected in Banff National Park of Canada, Jasper National Park of Canada, and Waterton Lakes National Park of Canada as of 2021
• Information for identification of Critical Habitat of Bull Trout and candidate locations in the Saskatchewan-Nelson Rivers population has been identified (Fisheries and Oceans Canada 2020b)
• Alberta has developed a Bull Trout Recovery Plan (Alberta Environment and Parks (2020) which assesses recovery potential, develops recovery objectives and strategic approaches for recovery
• British Columbia has developed a Management Plan for Bull Trout (B.C. Ministry of Lands, Water and Resource Stewardship 2023) which outlines the current management framework, goals and objectives and approaches to meeting objectives
• Parks Canada is researching Bull Trout population structure and abundance, and is performing reproductive assessments at multiple sites as well as collecting tissue samples for future genetic testing (Parks Canada Agency 2023)
• Parks Canada, Fisheries and Oceans Canada, the University of Montana, Alberta Environment and Protected Areas, and the Blackfoot Confederacy collaborate on Bull Trout genetic analysis methods, population assessments, and delineation of Critical Habitat (Parks Canada Agency 2023)
• Parks Canada is conducting a feasibility assessment of Bull Trout recovery methods, including non-native fish removals and translocations (Parks Canada Agency 2023; Taylor and Humphries pers. comm. 2024)
• Parks Canada has removed Brook Trout from a number of headwater lakes (Devon Lakes, Badger Lake) threatening downstream Bull Trout subpopulations (Pacas and Taylor 2015; Rollheiser 2023)

Information sources

Alberta Environment and Parks. 2020. Alberta Bull Trout Recovery Plan. Alberta Species at Risk Recovery Plan No. 46. 107 pp.

Alberta Government. 2018. Bull Trout fish sustainability index maps. [Acessed September 2023 and April 2025].

ALCES. 2020. Alberta BAU forecast assumptions. October 2020. 18 pp.

Al-Chokhachy, R., P. Budy, and H. Schaller. 2005. Understanding the significance of redd counts: a comparison between two methods for estimating the abundance of and monitoring Bull Trout populations. North American Journal of Fisheries Management 25:1505-1512.

Al-Chokhachy, R., P. Budy, and M. Conner. 2009. Detecting declines in the abundance of a Bull Trout (Salvelinus confluentus) population: understanding the accuracy, precision, and costs of our efforts. Canadian Journal of Fisheries and Aquatic Sciences 66:649-658.

Allendorf, F.W., and R.F. Leary. 1988. Conservation and distribution of genetic variation in a polytypic species: the Cutthroat Trout. Conservation Biology 2:170-184.

Angers, B., and L. Bernatchez. 1998. Combined use of SMM and non-SMM methods to infer structure and evolutionary history of closely-related Brook Charr (Salvelinus fontinalis, Salmonidae) populations from microsatellites. Molecular Biology and Evolution 15:143-159.

Armbruster, P., W.E. Bradshaw, and C.M. Holzapfel. 1998. Effects of postglacial range expansion on allozyme and quantitative genetic variation of the Pitcher-Plant Mosquito, Wyeomyia smithii. Evolution 52:1697-1704.

Austin, C.S., Essington, T.E. and Quinn, T.P., 2019. Spawning and emergence phenology of Bull Trout Salvelinus confluentus under differing thermal regimes. Journal of Fish Biology 94:191-195.

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Collections examined

No collections were examined for the preparation of this report.

Authorities contacted

• Andrusak, G., Rivers Management Biologist, Ministry of Water, Land and Resource Stewardship, Victoria, British Columbia
• Cannings, S., Species at Risk Biologist, Environment and climate Change Canada, Whitehorse, Yukon
• Carrière, S., Conservation Biologist, Environment and Climate Change Canada (retired), Whitehorse, Yukon
• Czembor, C., Aquatic Species at Risk Scientist, Aquatic Ecosystems Branch, Ministry of Water, Land and Resource Stewardship, Victoria, British Columbia
• de Forest, L., Species at Risk Ecologist, Parks Canada, Lunenburg, Nova Scotia
• Diment, J., Science Advisor, Fisheries and Oceans Canada, Winnipeg, Manitoba
• Fitch, L., Fisheries Biologist (retired), Alberta Fish and Wildlife, Lethbridge, Alberta
• Grant, P., Research Scientist, Fisheries and Oceans Canada, Sidney, British Columbia
• Gutsell, R., Wildlife Status Biologist, Alberta Environment and Protected Areas, Edmonton, Alberta
• Gwich'in Renewable Resources Board (GRRB), Inuvik, Northwest Territories
• Humphries, S., Ecologist Team Leader II – Aquatics, Parks Canada, East Kootenay, British Columbia
• Jung, T., Senior Wildlife Biologist, Yukon Department of Environment, Whitehorse, Yukon
• Killeen, J., Conservation Science and Program Manager, Canadian Parks and Wilderness Society, Edmonton, Alberta
• Kissinger, B., Lead Water and Fish Program, fRI Research, Hinton Alberta
• May-McNally, S., Science Advisor, Fisheries and Oceans Canada, Ottawa, Ontario
• McPherson, L., Provincial Fish Stock Assessment Specialist, Forestry and Parks, Edmonton, Alberta
• Mochnacz, N., Fisheries Research Biologist, Fisheries and Oceans Canada, Winnipeg, Manitoba
• Morgan, J., Section Head, Fish and Wildlife, Ministry of Forests, Fort St. John, British Columbia
• Mulder, R., Data Information Specialist, Yukon Conservation Data Centre, Yukon Department of Environment, Whitehorse, Yukon
• Nisga'a Wildlife Committee (NWC) / Nisga'a Joint Fisheries Management Committee (NJFMC), Gitlaxt’aamiks, British Columbia
• Reilly, J., Provincial Recovery Specialist, Alberta Environment and Protected Areas, Edmonton, Alberta
• Sahtu Renewable Resources Board (SRRB), Tulı́t'a, Northwest Territories
• Sheppard, P., Species at Risk Coordinator, Parks Canada, Ottawa, Ontario
• Sinclair, C., Senior Fisheries Biologist, Yukon Department of Environment, Whitehorse, Yukon
• Sullivan, M., Provincial Fish Science Specialist, Alberta Environment and Protected Areas, Edmonton, Alberta
• Taylor, E., Professor, Department of Zoology, University of British Columbia, Vancouver, British Columbia
• Taylor, M., Ecologist Team Leader II – Aquatics, Parks Canada, Banff, Alberta
• Teslin Renewable Resources Council (TRRC), Teslin, Yukon
• Tla'amin Joint Fisheries Committee (TAJFC), qathet, British Columbia
• Tsawwassen Joint Fisheries Committee (TWJFC), Tsawwassen, British Columbia
• Warnock W., Aquatic Specialist, BC Ministry of Water, Land and Resource Stewardship, Cranbrook, British Columbia
• Watkinson, D., Research Biologist, Fisheries and Oceans Canada, Winnipeg, Manitoba
• Wek'eezhii Renewable Resources Board (WRRB), Yellowknife, Northwest Territories
• Wilson G., Aquatic Species at Risk Specialist, BC Ministry of Water, Land and Resource Stewardship, Victoria, British Columbia
• Yukon Fish and Wildlife Management Board (YFWMB), Whitehorse, Yukon

Acknowledgements

Funding for the preparation of this report was provided by Environment and Climate Change Canada. The authorities listed above provided valuable data and/or advice. The comments of the jurisdictional reviewers and the COSEWIC Freshwater Fishes SSC were greatly appreciated, as is the assistance of Ryan Collins (COSEWIC Secretariat) for preparation of the maps and extent of occurrence and index of area of occupancy calculations.

Biographical summary of report writer

John R. Post is Professor Emeritus, Ecology and Evolutionary Biology, Department of Biological Sciences, University of Calgary. His research program and publications span population ecology, conservation biology and social-ecological systems dynamics with a focus on temperate and northern freshwater fishes. He served for 12 years as Co-Chair of the Freshwater Fish Specialist Sub-Committee of COSEWIC.

Appendix 1. Update to appendix 2 from COSEWIC (2012) by Will Warnock (pers. Comm.) in consultation with other BC regional biologists

British Columbia core areas are listed within Ecological Drainage Units (EDU) and within COSEWIC Designatable units (DU). Estimates of adult population abundance, short-term and 3-generation trend, and conservation status ranks are listed along with field methods and sampling time spans where sufficient data exists. NA is not applicable and n.d. is no data available.

Appendix 2. Threats assessment worksheets

DU1

Species or Ecosystem scientific name: Salvelinus confluentus Bull Trout South Coast population (DU1)

Element ID: 1098567

Elcode: AFCHA0502Q

Date: 3/11/2024

Assessor(s): Dwayne Lepitzki (Facilitator). Margaret Docker (FWF SSC), John Richardson (FWF SSC), John Post (Writer), Rowshyra Castaneda (DFO), Mochnacz, Neil (DFO), Trevor Pitcher (FWF SSC/DFO), Doug Watkinson (FWF SSC/DFO), Shannan May-McNally (DFO), Jeffrey Lemieux (DFO), Shelley Humphries (PCA), Karine Robert (COSEWIC Secretariat), Patricia Woodruff (BC Government), Ian Spendlow (BC Government), Kris Maier (BC Government), Christopher Hegele (BC Government), Will Warnock (BC Government), Myles Brown (Yukon Government).

References: Draft Status Report and the 2017 BC calculator provided by report writer.

Assigned overall threat impact: B = High

Impact adjustment reasons: We considered the threat impact to be High because a number of the threats are likely overlapping, and there is no evidence of recent declines.

Overall threat comments: Generation time is 10 years so timeframe for severity and timing = 30 years (generation time assessed as 7 years in the 2012 status report but was underestimated). Total 5300 to 7000 mature individuals. Watershed counts Appendix 1: Skagit 4000 (57 to 75%); Phelix Cr (Birkenhead Lake) 200 to 1000 (3 to 19%), Gold Creek (Alouette Reservoir) 500 to 1000 (7 to 19%); Cheakamus River 77 to 469 (1 to 9%); Squamish River 500 (7 to 9%); others still unknown; no evidence of continuing decline. There is not a lot of population and trend information available for the many different Bull Trout subpopulations and Designatable units found throughout BC. Many of the subpopulations studied show stable or increasing trends; however, certain regions, such as the heavily developed lower Fraser, show overall declines. This DU is located entirely within BC, in 5 watersheds: Lillooet, lower Fraser, lower Fraser Canyon, Skagit, and Squamish. The majority of Bull Trout of this DU are found in the Skagit system. The anadromous life history form is unique to this population.

DU2

Species or Ecosystem scientific name: Salvelinus confluentus Bull Trout Western Arctic population (DU2)

Element ID: 1071873

Elcode: AFCHA05122

Date: 3/14/2024

Assessor(s): Dwayne Lepitzki (Facilitator). Margaret Docker (FWF SSC), John Richardson (FWF SSC), John Robert Post (Writer), Mochnacz, Neil (FWF SSC/DFO), Shannan May-McNally (DFO), Zing-Ying Ho (DFO), Christiane Brito Uherek (DFO), Jeffrey Lemieux (DFO), Carrie Kwok (DFO), Maggie Boothroyd (DFO), Geoff Skinner (Parks Canada), Shelley Humphries (Parks Canada), Stephanie Crowshoe (Parks Canada), Karine Robert (COSEWIC secretariat), Patricia Woodruff (Government of British Columbia), Kris Maier (Government of British Columbia), James Morgan (Government of British Columbia), Selena Hunjen (Government of British Columbia), Thomas Jung (Government of Yukon), Pascale Savage (Government of Yukon), Cameron Sinclair (Government of Yukon), Myles Brown (Government of Yukon), Cameron Sinclair (Government of Yukon), Claire Singer (Government of Northwest Territories), Robin Gutsell (Government of Alberta), Michael G. Sullivan (Government of Alberta), Jessica Reilly (Government of Alberta), Adrian Meinke (Government of Alberta).

References: Draft Status Report and the 2017 BC calculator (note the 2017 BC calculator covered only the BC component of the full DU distribution) provided by the report writer.

Assigned overall threat impact: B = High

Impact adjustment reasons: We considered the threat impact to be High since a number of the threats are likely overlapping, and the evidence of declines are concentrated in the southern portion of the DU. However, all but the highest elevation subpopulations could be extirpated within the next 30 years.

Overall threat comments: Generation time is estimated to be 10 years so timeframe for severity and timing = 30 years (note that the 2021 Status Report used 7 years). Population abundance is likely >100,000; continuing decline projected; approximately 69% decline projected model for AB (not including JNP, NWT, YK): have historical declines in AB, no data on trends for majority of core areas in BC. Estimates of the approximate distributional proportion by jurisdiction are BC approximately 42%, NWT approximately 42%, AB approximately 12% and YT approximately 3%. There is not much population and trend information available for the many Bull Trout subpopulations throughout BC. Although there are many threats to Bull Trout, especially due to its habitat requirements, many of these are localized. Many subpopulations of Bull Trout in BC persist in relatively pristine and/or inaccessible areas, and there is no evidence of decline in the number of mature individuals in BC.

Classification of Threats adopted from IUCN-CMP, Salafsky et al. (2008).

DU3

Species or Ecosystem Scientific name: Salvelinus confluentus Bull Trout Upper Yukon Watershed population (DU3)

Element ID: 1071875

Elcode: AFCHA05124

Date: 3/11/2024

Assessor(s): Dwayne Lepitzki (Facilitator). Margaret Docker (FWF SSC), John Richardson (FWF SSC), John Post (Writer), Rowshyra Castaneda (DFO), Mochnacz, Neil (DFO), Trevor Pitcher (FWF SSC/DFO), Doug Watkinson (FWF SSC/DFO), Shannan May-McNally (DFO), Jeffrey Lemieux (DFO), Shelley Humphries (PCA), Karine Robert (COSEWIC Secretariat), Patricia Woodruff (BC Government), Ian Spendlow (BC Government), Kris Maier (BC Government), Christopher Hegele (BC Government), Will Warnock (BC Government), Myles Brown (Yukon Government).

References: Draft Status Report and the 2017 BC calculator provided by report writer.

Assigned overall threat impact: U = Unknown

Impact adjustment reasons: Although there are localized threats to Bull Trout within this DU, the scope and severity of threats overall are unknown.

Overall threat comments: Generation time is 10 years, so timeframe for severity and timing = 30 years (GT assessed as 7 years in the 2012 status report). Little is known about the scope and severity of threats at the scale of the DU.

Classification of Threats adopted from IUCN-CMP, Salafsky et al. (2008).

DU4

Species or Ecosystem Scientific name: Bull Trout - Sask and Nelson Rivers Populations (DU4)

Element ID: 1098566

English Name: ELCODEAFCHA0502B

Version Date: 2019-06 to 24

Version Author(s): Dwayne Lepitzki (Facilitator). Margaret Docker (FWF SSC), John Richardson (FWF SSC), John Robert Post (Writer), Neil Mochnacz (FWF SSC/DFO), Doug Watkinson (FWF SSC/DFO), Shannan May-McNally (DFO), Zing-Ying Ho (DFO), Christiane Brito Uherek (DFO), Mark Taylor (Parcs Canada), Geoff Skinner (Parcs Canada), Joanna James (COSEWIC secretariat), Robin Gutsell (Government of Alberta), Michael G. Sullivan (Government of Alberta), Jessica Reilly (Government of Alberta).

Generation Time: 10 years

Assigned overall threat impact: AB = Very high - High

Impact adjustment reasons: Not Applicable

Overall threat comments: Following discussion of overall threat impact, decision to maintain Very high-High. AB cumulative-effects modelling suggests declines likely within this range.

Classification of Threats adopted from IUCN-CMP, Salafsky et al. (2008).

DU5

Species or Ecosystem Scientific name: Salvelinus confluentus Bull Trout Pacific population (DU5)

Element ID: 1098568

Elcode: AFCHA0502N

Date: 3/11/2024

Assessor(s): Dwayne Lepitzki (Facilitator). Margaret Docker (FWF SSC), John Richardson (FWF SSC), John Post (Writer), Rowshyra Castaneda (DFO), Mochnacz, Neil (DFO), Trevor Pitcher (FWF SSC/DFO), Doug Watkinson (FWF SSC/DFO), Shannan May-McNally (DFO), Jeffrey Lemieux (DFO), Shelley Humphries (PCA), Karine Robert (COSEWIC Secretariat), Patricia Woodruff (BC Government), Ian Spendlow (BC Government), Kris Maier (BC Government), Christopher Hegele (BC Government), Will Warnock (BC Government), Myles Brown (Yukon Government).

References: Draft Status Report and the 2017 BC calculator provided by report writer.

Assigned overall threat impact: BC = High - Medium

Impact adjustment reasons: We assigned a High-Medium overall impact. Overall, the population appears to have been stable, but a large percentage of this population is facing increased water withdrawals from their habitat as well as threats from Brook Trout and Whirling Disease.

Overall threat comments: Generation time is 10 years so timeframe for severity and timing = 30 years (GT assessed as 7 years in the 2012 status report). There is not a lot of population and trend information available for the many different Bull Trout subpopulations and DUs found throughout BC. Many of the subpopulations for which there is information show stable or increasing trends; however, certain regions show overall declines. This DU is located only in BC, with more than 78 subpopulations and >39,000 mature individuals. Although there are many threats to Bull Trout, especially due to its habitat requirements, many of these are localized within each region, and there is much heterogeneity in threats across this large area. In some areas, subpopulations of Bull Trout persist in relatively pristine and/or inaccessible areas throughout the province.

Classification of Threats adopted from IUCN-CMP, Salafsky et al. (2008).