Oncorhynchus lewisi

Oncorhynchus clarkii lewisi has been elevated to species status. Until a full, modern systematic revision can be completed, an interim classification of the Oncorhynchus clarkii species complex by Markle (2018) recognizes four taxa as distinct, clarkii, lewisi, henshawi, and virginalis. This treatment has been accepted by the American Fisheries Society (Page et al. 2023). Under this arrangement, there are no subspecies of Oncorhynchus lewisi, but COSEWIC (2005a) recognizes two populations. Subspecies alpestris is considered a synonym of lewisi (Markle 2018, Behnke 1992).

Extensive introductions of Yellowstone cutthroat trout have been made in the range of westslope cutthroat trout, and "hybridization" has resulted. In Glacier National Park, hybridization occurred in previously barren lakes into which both subspecies were introduced but did not occur where Yellowstone cutthroats were introduced into areas with native westslope cutthroat populations (Yellowstone cutthroats did not survive) (Behnke 1992). Widespread hybridization with introduced rainbow trout has occurred (but not where cutthroat trout and rainbow trout evolved in sympatry) (McIntyre and Rieman 1995).

Range much reduced but still widespread in British Columbia, Alberta, Washington, Oregon, Idaho, Montana, and Wyoming; currently occupies approximately 54,000 stream-kilometers; many protected and appropriately managed populations; major threat is genetic introgression from introduced exotic fishes.

This widespread species is found in montane river basins of northwestern United States and southwestern Canada (Young et al. 2018).

West of the Continental Divide, this species is believed to be native to several major drainages of the Columbia River basin, including the upper Kootenai River drainage from its headwaters in British Columbia, through northwest Montana, and into northern Idaho; the Clark Fork River drainage of Montana and Idaho downstream to the falls on the Pend Oreille River near the Washington-British Columbia border; the Spokane River above Spokane Falls and into Idaho's Coeur d'Alene and St. Joe River drainages; and the Salmon and Clearwater River drainages of Idaho's Snake River basin (USFWS 2003). The native distribution also includes disjunct areas draining the east slope of the Cascade Mountains in Washington (Methow River and Lake Chelan drainages, and perhaps the Wenatchee and Entiat river drainages), the John Day River drainage in northeastern Oregon, and the headwaters of the Kootenai River and several other disjunct regions in British Columbia (USFWS 2003).

East of the Continental Divide, the native distribution is believed to include the headwaters of the South Saskatchewan River drainage (United States and Canada); the entire Missouri River drainage upstream from Fort Benton, Montana, and extending into northwest Wyoming; and the headwaters of the Judith, Milk, and Marias rivers, which join the Missouri River downstream from Fort Benton (USFWS 2003).

This species is represented by numerous robust populations, including several hundred "conservation" populations (USFWS 1999, 2000, 2003; Shepard et al. 2005; May 2009).

Hybridization with nonnative rainbow trout or their hybrid progeny and descendants, both of which have established self-sustaining populations in many areas in the range westslope cutthroat trout, remains the greatest threat to westslope cutthroat trout (USFWS 2003). The available empirical evidence and speculations of many fishery scientists indicate that introgression of rainbow trout genes will continue to move upstream into many stream reaches presently inhabited by westslope cutthroat trout, although there may be limits to that upstream spread set by environmental factors and the superior fitness of extant westslope cutthroat trout populations in their native habitats. The eventual extent that such hybridization moves upstream may be stream-specific and impossible to predict (USFWS 2003). However, numerous nonintrogressed westslope cutthroat trout populations are distributed in secure habitats throughout the subspecies' historical range. USFWS (2003) considered slightly introgressed westslope cutthroat trout populations, with low amounts of genetic introgression detectable only by molecular genetic methods, to be a potentially important and valued component of the overall westslope cutthroat trout subspecies. USFWS (2003) concluded that westslope cutthroat trout are not threatened by introgressive hybridization. Genetic analyses found no evidence of genetic introgression in 768 samples (58 percent of samples tested) (the numbers of individuals tested per sample were variable and sample sites were not randomly selected) (Shepard et al. 2005).

Impacts of introduced kokanee, lake trout, and brook trout have eliminated populations in some areas (e.g., kokanee may outcompete cutthroat for zooplankton, lake trout is an effective predator on cutthroat). Some westslope cutthroat trout populations have persisted despite the presence of large kokanee populations (see McIntyre and Rieman 1995). Lake whitefish and non-native mysid shrimp also evidently have caused cutthroat declines through competitive interactions. However, USFWS (2000, 2003) concluded that extant headwater populations of westslope cutthroat trout are relatively secure from colonization by non-native fishes (and from adverse effects of human activities).

Stocked, hatchery-reared steelhead that do not migrate to the ocean (residual steelhead) sometimes migrate over 12 km upstream from their release point and may move into areas occupied by westslope cutthroat trout (McMichael and Pearsons 2001). Locally, residual steelhead could pose a threat through ecological interactions.

This species has been negatively affected by loss/degradation of habitat from logging, road construction, mining, and grazing (Spahr et al. 1991), which may result in sedimentation and increased water temperature . Habitat loss has been a primary cause of depressed populations in Idaho (McIntyre and Rieman 1995). These fishes are sensitive to pollution and generally to siltation of streams (some populations may persist despite abundant sediment). Dams, irrigation diversions, and other migration barriers have negatively affected habitat and probably have interfered with metapopulation dynamics (McIntyre and Rieman 1995). Populations have become increasingly fragmented. However, many populations exist in streams that are not affected by these factors (USFWS 2003).

Westslope cutthroat trout are sensitive to fishing pressure (McIntyre and Rieman 1995); restricted or catch-and-release fishing has been needed to maintain wild populations (Spahr et al. 1991). Climate warming would eliminate some habitat.

Overall, USFWS (2000, 2003) concluded that the magnitude and imminence of existing threats are small.