Bull trout (Salvelinus confluentus) COSEWIC assessment and status report 2012: chapter 3

COSEWIC
Executive Summary

Bull Trout
Salvelinus confluentus

South Coast British Columbia populations
Western Arctic populations
Upper Yukon Watershed populations
Saskatchewan - Nelson Rivers populations
Pacific populations

Wildlife Species Description and Significance

Bull Trout is a large char. This salmonid derives its name from its large head and jaws. Bull Trout are olive-green to blue-grey in colour and pale round spots on their flanks and back distinguish them from most other similar-looking salmonids. It is difficult to visually distinguish them 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, this sportfish is highly sensitive to habitat changes. Bull Trout are, therefore, 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 are recognized; Genetic Lineage 1 (Southcoast BC populations) and Genetic Lineage 2 (Western Arctic, Yukon, Saskatchewan-Nelson and Pacific populations).

Distribution

Bull Trout is native to western Canada and the U.S. 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 western 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 USA 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

This cold water species’ very strict habitat requirements vary across life history stages. In order to maintain their numbers, Bull Trout require habitat that is cold, clean, complex and connected. 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

Bull Trout are voracious predators that eat other fish when given the opportunity. They exhibit considerable diversity in life history traits, including four migratory 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. Each type breeds in headwater or tributary streams at higher elevations but habitat occupied at other times varies. The first three forms are common throughout the Canadian range but the anadromous populations are restricted to the southwestern portion of British Columbia.

Population Sizes and Trends

Typically comprising less than 5% of total catch from broad faunal surveys, adult Bull Trout populations are expected to be smaller than most other freshwater salmonids. A substantial body of qualitative and quantitative data estimates both historical and current Bull Trout population sizes from Alberta, and to a lesser extent British Columbia. However, long-term data sets quantifying Bull Trout abundance are rare, and much of our current knowledge of population trends relies on qualitative expert opinion. In recent decades, Bull Trout populations have experienced declines in abundance across parts of their range, particularly in the USA and Alberta. The full range of life histories is also being lost from some populations. This historical pattern of decline is mirrored in the short-term declining trend of 57% of Alberta Bull Trout populations whereas 29% are stable and 8% increasing. Less is known about Bull Trout populations from the remainder of its Canadian range, although their general trend is considered to be stable to diminishing in British Columbia, and there is no evidence of decline of Bull Trout in Yukon or the Northwest Territories. In both Alberta and British Columbia, some populations appear to be recovering from historical threats.

Threats and Limiting Factors

Bull Trout’s specific habitat requirements are their most significant natural limiting factor. The most serious threats to Bull Trout, however, are from human disturbance. The greatest threat is habitat loss through degradation and fragmentation. Commercial forestry, hydroelectric, oil, gas and mining development, agriculture, urbanization, and their associated road development, and climate change may all contribute to this. Interactions with other species strongly influence the local distribution and abundance of Bull Trout. Habitat degradation may exacerbate Bull Trout’s susceptibility to displacement and/or hybridization, leaving Bull Trout vulnerable to invasion by non-native species, such as Brook Trout. Misidentification with other char and trout species increases fishing pressure on this species that is vulnerable to overharvesting.

Protection, Status, and Ranks

Bull Trout habitat is protected under both provincial and federal legislation. As a sportfish, populations are subject to National Park and provincial fishing regulations that incorporate a variety of measures to protect fish stocks. Currently, Bull Trout is ‘blue-listed’ as a Species of Special Concern in British Columbia and has also been identified as such in Alberta. The General Status of Species in Canada lists Bull Trout as Sensitive nationally (N3), in British Columbia the interior lineage is listed as S3S4. It is listed as S3 in Alberta and Yukon. It is listed as May Be At Risk (S2) in the Northwest Territories. Populations in the USA are listed as Threatened under the Endangered Species Act. Its Global Heritage Status rank and its listing under the IUCN Red List of Threatened Species is Vulnerable (G3).

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