Bull trout (Salvelinus confluentus) COSEWIC assessment and status report 2012: chapter 8
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; BCMWLAP 2004; NT: Stewart et al. 2007a, b). A generic life cycle that applies to all Canadian Bull Trout DUs is outlined in Figure 7. The many facets of Bull Trout biology are discussed below, with geographical variations highlighted. The strongest variations are shown among the divergent life history patterns. A general north to south trend may also be evident, with the timing of habitat shifts strongly correlated to water temperature. There are no strong shifts from west to east across Canadian Bull Trout DUs, and the description given herein refers to all DUs unless specified otherwise.
Bull Trout’s diverse life history patterns can be summarized into four migratory types. A non-migratory stream resident form spends its entire life cycle in small rivers and streams and is often isolated by barriers either physical (e.g., waterfalls, dams; Latham 2002), physiological (e.g., unfavourably high temperatures; Rieman and McIntyre 1993; Rieman et al. 1997), or biological (e.g., 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 as older fish they migrate to other water bodies. 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 they feed, mature and overwinter 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 freshwater, a fourth anadromous form migrates between freshwater and the sea. It 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, female 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 reach sexual maturity between five and seven years of age, with the extreme range between three and eight years. Maximum age is unknown but ages up to 24 years have been recorded. The generation time has been estimated from the average age of spawners from seven Bull Trout populations in British Columbia displaying different life history strategies to be nearly 7 years (Pollard and Down 2001).
Although Bull Trout are iteroparous, there is strong evidence that they display alternate-year spawning or resting periods between consecutive spawning events (Pollard and Down 2001; Johnston and Post 2009). This reproductive strategy, which is often condition and survival dependent, may enable them to accrue sufficient energy for reproduction in colder, less productive systems (reviewed in Johnston and Post 2009). Spawning may occur at 2 to 3 year intervals in the Northwest Territories from all life history types (Mochnacz 2002; Mochnacz et al. submitted). This strategy can also show a density-dependent response; the proportion of Bull Trout spawning annually has shown a decline with increasing density as the population in Lower Kananaskis Lake has recovered (Johnston and Post 2009).
Like all char, Bull Trout spawn in the fall, from mid-August to late October. Except for stream-resident populations that spawn locally, this is preceded by a migration. Younger Bull Trout 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°C, while temperatures below about 5 °C suspend it. As a result, southern populations 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 salmonines. 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 (i.e. sediment deposition) and freezing. A dominant male usually accompanies each spawning female. They vigorously defend her from other satellite males who try to compete for fertilizations (Kitano et al. 1994; Baxter 1997). Some populations 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), 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 fish, fecundity (egg number) in Bull Trout depends on female body size; the larger fluvial and adfluvial females produce more eggs (typically 2000-5000+) than the smaller stream-resident females (<1000). Fertilized eggs incubate in the gravel overwinter before fry typically hatch from March onwards (at a total length of about 25mm). The incubation period is temperature dependent and can take anything from 35 days to more than 4 months.
Bull Trout are opportunistic foragers. 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 (Figure 8). 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.
Figure 8. Generalized food web for Bull Trout showing the direction of energy flow. .
Bold lines indicate major food pathways, in comparison to thinner lines; solid lines indicate demonstrated and dashed lines putative pathways. Sourced from Stewart et al. 2007b
Description of Figure 8
Schematic depicting a generalized food web for the Bull Trout. Lines connect four main predator groups with Bull Trout at three life stages and eight prey groups.
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 do, however, 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 when opportunity presents in the presence of other fish species. They are often the top aquatic predator where they live and some adfluvial populations are almost exclusively piscivorous. 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 fish, 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. The feeding habits of anadromous Bull Trout at sea are unknown.
Bull Trout’s pattern of high genetic differentiation between populations and low diversity within them suggests that populations experience limited gene flow. They are, therefore, likely to be locally adapted to their spatially heterogeneous environment (see ‘Population Spatial Structure and Variability’ section). This cautions against initiating artificial gene flow between populations (via stocking or hatchery production) as disruption of local adaptations would likely increase a population’s vulnerability to extinction. Hatchery production of Bull Trout from Arrow Lakes Reservoir, BC was stopped in 2000, partly over concern about genetic diversity losses (Hagen 2008).
While Bull Trout have many specific habitat requirements, including depth, velocity, substrate, and cover (see ‘Habitat’ section), 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). 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 2-4°C, with survival to hatching declining precipitously above 8°C. Groundwater inflows are important in providing stable temperature for egg development (Baxter and McPhail 1999).
Given a natural thermal gradient (8-15°C), juvenile Bull Trout select the coldest water available (Bonneau and Scarnecchia 1996). Similarly, adult Bull Trout are generally found in water below 18°C, but most commonly in temperatures less than about 12°C (Dunham et al. 2003). 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). This, coupled with life history attributes (including top aquatic predator and high site fidelity) that result in relatively low population densities (see ‘Population and Sizes’ section) 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). Nevertheless, the species is at risk from human activities and their impacts (Rieman and McIntyre 1993, 1995). This cold water specialist may be especially vulnerable to climate change (Rieman and McIntyre 1993; Rieman et al. 1997, 2007). Populations near its southern limit will be most susceptible, given that this limit is defined by temperature but the thermal and precipitation effects of global warming are likely to exacerbate fragmentation of Bull Trout populations throughout much of the range (Kelehar and Rahel 1996; Rahel et al. 1996)
Another notable aspect of the Bull Trout’s physiology is the ability of at least some populations to tolerate salt water (see ‘Dispersal and Migration’ section).
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 from between one to four years, migration at age two or older is most common. 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 populations, being partly dependent on the distance to be travelled, which varies widely (up to several hundred kilometers; Pillipow and Williamson 2004; Pillipow pers. comm. from Hagen and Decker 2011). It is also thought that its onset is triggered by a hierarchy of environmental cues, including changes in river discharge and water temperature. Migratory movements generally occur nocturnally and fluvial populations 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 (Muhlfield and Marotz 2005; Monnot et al. 2008). 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 (Fraser and Squamish drainages), as well as the north west of Washington (Skagit drainage and the Olympic Peninsula). 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 populations in the Squamish River and Pitt River, of which the latter is part of the Fraser drainage.
More recently, radio-telemetry and otolith chemistry have verified that anadromy is a primary life history form in some coastal USA Bull Trout (Brenkman and Corbett 2005; Brenkman et al. 2007). More than half of 82 adult Bull Trout radio-tagged on the west side of the Olympic Peninsula were anadromous, migrating to the uppermost portions of rivers to spawn before returning to sea to overwinter and forage (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). Adult dispersal among watersheds using coastal routes can occur; a fish tagged in the Squamish River was recovered in the lower Skagit River in Washington (a marine journey of about 150km) and radio-telemetry revealed dispersal between drainages along the west side of the Olympic Peninsula (Brenkman and Corbett 2005).
Interestingly, this life history feature 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 (some of) Genetic Lineage 1 populations suggests that anadromy in Bull Trout originated, or at least persisted, in the Chehalis Refugium, from where Bull Trout (and anadromous Dolly Varden) are thought to have post-glacially recolonized these localities (Haas and McPhail 2001).
Interspecific interactions strongly influence the local distribution and abundance of Bull Trout. Their distribution may be affected by the availability of prey species, competition for these or other resources, predation or parasitism, or other indirect interactions within their ecosystems. Research on interspecific interactions involving Bull Trout has predominantly focused on just one of these: their potential competition with other native and non-native salmoninae. In fact, it has concentrated on one particular interaction within each of these two categories.
Interspecific competition with other native salmonines is likely an important factor in excluding Bull Trout or regulating their coexistence. One example has received particular attention; 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 (Figure 9). 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).
Figure 9. The parapatric species distributions of Dolly Varden (stipled), Bull Trout (shaded), and their overlap (stipled-shaded) in western Canada
Sourced from Baxter et al. (1997). Thutade Lake area studied in that manuscript is highlighted.
In part of the two species’ southwestern range, this contact has probably been continuous for most of the last 100 000 years. As with Bull Trout, Dolly Varden is composed of two major mtDNA clades (sequence of ~570 base pairs of mtDNA across 207 Dolly Varden samples collected from 50 sites spanning its geographical range revealed haplotype divergence of 1.4-2.2%; Redenbach and Taylor 2002). While one clade encompasses the majority of its range, the second one has a much more limited distribution from Washington State at the southern limit of its range to the middle of Vancouver Island (Redenbach and Taylor 2002). This pattern likely reflects its refuges during the last glacial period in two areas; a northern refuge (Beringia Refuge) and a southern one (Chehalis Refuge), which it likely shared with Bull Trout (Redenbach and Taylor 2002). There is a genetic signature of historical introgression of Genetic Lineage 1 Bull Trout mtDNA into ‘southern’ Dolly Varden prior to the most recent glaciation; their mtDNA is paraphyletic, with the ‘southern’ Dolly Varden clade clustering within Bull Trout from Genetic Lineage 1, despite their being reciprocally monophyletic at two nuclear loci (Taylor et al. 2001; Redenbach and Taylor 2002).
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 spawnings (Baxter et al. 1997; Hagen and Taylor 2001; Redenbach and Taylor 2003).
Current patterns of sympatry and hybridization are, therefore, 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 (e.g., from 2 – 25%; Redenbach and Taylor 2003). There is, however, a qualitative suggestion that present day hybridization may be more extensive in central and northern coast populations than in those along the southern coast; despite being broadly sympatric in southwestern BC and northwestern Washington (e.g., Leary and Allendorf 1997), present day hybridization has only been detected here in the Skagit River (McPhail and Taylor 1995).
Interestingly, it has recently come to light that sympatry between Bull Trout and Dolly Varden extends to the northern most tip of Bull Trout’s known distribution, and the most southerly range of a northerly form of Dolly Varden in Northwest Territories: the Gayna River (Mochnacz et al. 2009, submitted; Figure 10). Although they co-occur in the same river system they are largely not syntopic, with the Bull Trout occupying downstream areas and the Dolly Varden isolated above barriers (Mochnacz et al. 2009, submitted). Not surprisingly then, sequencing of mitochondrial and nuclear genes has not uncovered any genetic evidence of hybridization (Mochnacz et al. submitted). If future surveys find Bull Trout’s range extends into areas immediately north of the Gayna River, instances of sympatry and hybridization may be uncovered.
Figure 10. Distribution of northern Bull Trout and Dolly Varden, et al. (submitted)
Figure is showing new records from Mochnacz et al. (submitted) and point distributions from known and uncertain literature records. General distributions follow drainage basins and known point distributions. Only partial drainages are shown. Sourced from Mochnacz.
Description of Figure 10
Map of the distribution of northern Bull Trout and Dolly Varden, showing new records from Mochnacz et al. (submitted) and point distributions from known and uncertain literature records. General distributions follow drainage basins and known point distributions. Only partial drainages are shown.
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 (40-90cm fork length), migratory or adfluvial, and piscivorous, whereas adult Dolly Varden are small (12-21cm 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 (e.g., 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 occur 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; Jamieson pers. comm. 2010), although there is also potential for strong competitive interactions (e.g., 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 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 one other native salmoninae, Lake Trout (Salvelinus namaycush), has received some attention. Lake Trout, whose adults are piscivorous like Bull Trout, occur over most of continental North America north of 45°N. They overlap with about forty percent 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 so 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 (>1500m) and Lake Trout in lower ones (<1500m; 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).
While ongoing hybridization with native Dolly Varden presents no risk to the integrity of Bull Trout populations, direct interactions (e.g., hybridization, competition) with several species of introduced salmonines may displace Bull Trout populations, and threaten to extirpate them from many habitats throughout broad areas of its range. In western North America, Rainbow Trout, Brown Trout and Brook Trout are the most widespread non-native salmonines (Fuller et al. 1999). In particular, Brook Trout is considered a substantial threat to Bull Trout populations (see ‘Threats and Limiting Factors’ section). 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; Rieman et al. 2006; Earle et al. 2007). This pattern of segregation is likely influenced by direct interactions. Brook Trout compete with Bull Trout for food and space (Nakano et al. 1998; Gunkel et al. 2002; McMahon et al. 2007). 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; McPhail 2007; Earle et al. 2007) will tend to compound this effect. Bull Trout occurrence has been negatively associated with the presence of Brook Trout (Rich et al. 2003), 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 (e.g., Dunham and Rieman 1999; Rich et al. 2003; Rieman et al. 2006; Earle et al. 2007; McMahon et al. 2007).
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 males that are partially sterile (Leary et al. 1993; Kanda et al. 2002), although some backcrosses identified by molecular analyses indicate that F1 reproduction does occur (Kanda et al. 2002; McPhail and Taylor 1995). 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.
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