Westslope cutthroat trout COSEWIC assessment and status report: chapter 2
Executive Summary
Species information
The cutthroat trout, Oncorhynchus clarkii (formerly Salmo clarkii), is a polytypic species of salmonid native to western North America. Two subspecies occur naturally in Canada: the coastal cutthroat trout (O. c. clarkii) and the westslope cutthroat trout (O. c. lewisi). Cutthroat trout are highly variable in terms of phenotypic traits and life history characteristics. The most conspicuous character distinguishing cutthroat from similar species is the presence of bright orange-red slashes beneath the lower jaw.
Based on genetics and range disjunction, two Designatable Units are recognized, viz an Alberta population and a BC population.
Distribution
The distribution of westslope cutthroat trout straddles the Continental Divide and includes drainages in Montana, Idaho, Washington, Oregon, and Wyoming in the United States. In Canada, they are restricted to southeastern British Columbia (primarily the Upper Kootenay and Upper Columbia drainages) and southwestern Alberta (primarily the South Saskatchewan drainage). Globally, their range has become extremely fragmented (in high elevation, isolated headwater areas) and the heart of their distribution now centres on the upper Kootenay River drainage in southeastern BC.
Habitat
Westslope cutthroat trout have strict habitat requirements during various life history stages, requiring cold clean water and varied forms of cover (i.e., undercut banks, pool-riffle habitat, and riparian vegetation) to maintain their numbers. They inhabit large rivers and lakes in BC, as well as many small mountain streams. In Alberta, they are now largely restricted to the upper reaches of mainstem rivers and the extreme headwaters of a few major tributaries. They tend to inhabit cooler, less productive streams than other closely related species.
Biology
Populations are usually small but show strongly developed natal philopatry and well-defined population structure. Habitat degradation may make populations especially susceptible to displacement and/or hybridization with introduced species (rainbow trout, other cutthroat trout subspecies). As such, populations in degraded habitats are more likely to decline, and their high degree of demographic independence suggests that losses are not likely to be offset by immigration from nearby sources over the short term.
Population sizes and trends
Little quantitative data exists on westslope cutthroat trout population trends in Canada. Population sizes are generally expected to be smaller than for other freshwater salmonids (typically 1 – 10% within even the largest systems). The number of adult spawners supporting population growth is usually quite small, typically 100 or less per stream. While some populations are likely stable, all available information suggests that many populations are depressed relative to historic levels, and numerous local extirpations have occurred.
Limiting factors and threats
The greatest threats to westslope cutthroat trout are the anthropogenic manipulation and degradation of the environment in which it lives. Forestry, hydroelectric development, mining, urbanization and agriculture have all contributed to the loss and degradation of stream habitat in the range of cutthroat within both Alberta and British Columbia. Introgressive hybridization is widespread (particularly in Alberta), and further introduction of non-native species could affect the genetic integrity of the remaining populations. The number and distribution of pure populations have steadily declined in response to the cumulative effects of habitat loss and degradation, overexploitation, and detrimental interactions with introduced species (i.e., competition, predation, hybridization).
Special significance of the species
Cutthroat trout are a unique and important component of Canada’s freshwater fish fauna and are often the only native trout throughout much of their Canadian range. As such, cutthroat trout likely play an important role in structuring many north temperate aquatic ecosystems. Because of their strict habitat requirements, cutthroat are viewed as an indicator species of general ecosystem health. Westslope cutthroat trout are a popular freshwater sport fish in western Canada, second only to rainbow trout/ steelhead in terms of angler interest.
Existing protection and other status designations
Cutthroat trout habitat is protected under both provincial and federal legislation and as a popular sports fish, populations are subject to provincial recreational harvest and National Park regulations. Compliance with habitat protection, harvest and National Park regulations, however, has been lacking in the past. Currently both subspecies are provincially blue-listed as ‘vulnerable’ in BC. In Alberta, no populations are formally listed and the status of westslope cutthroat trout as a subspecies has yet to be formally assessed. Populations in the United States have been petitioned for listing under the Endangered Species Act but were deemed not to currently require that level of formal protection. Globally, westslope cutthroat trout are ranked by the Nature Conservancy as ‘vulnerable to extirpation or extinction’ (G4T3).
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
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 it 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.
Environment Environnement
Canada Canada
Canadian Wildlife Service canadien
Service de la faune
The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.