Status and Risk of Extinction for Westslope Cutthroat Trout in the Upper Missouri River Basin, Montana

Abstract

Westslope cutthroat trout Oncorhynchus clarki lewisi now occupy less than 5% of the subspecies' historical range within the upper Missouri River drainage in Montana. We assessed the risk of extinction for 144 known populations inhabiting streams within federally managed lands in the upper Missouri River basin using a Bayesian viability assessment procedure that estimates probability of persistence based on subjective evaluation of population survival and reproductive rates as influenced by environmental conditions. We first customized this model using estimates of demographic parameters from the literature and field data. Each population was classified into one of three risk groups based on their Bayesian probability of persistence over 100 years (p100). Most (71 %) of the 144 populations had a very high predicted risk of extinction (p100 ≤ 50%), 19% exhibited a high risk (50% < p100 ≤ 80%), and 10% had a moderate risk (80 < p100 ≤ 95%). Higher average predictions of p100 were consistently associated with populations inhabiting watersheds with lower levels of management activities. Analysis of variance and a matrix of information divergence measures indicated that livestock grazing, mineral development, angling, and the presence of nonnative fish had the greatest association with both estimated population parameters and persistence probabilities. Of 26 major subbasins within the upper Missouri River drainage, 16 support at least one known westslope cutthroat trout population on federal lands, and 14 of these 16 support at least one population with an estimated p100 value of 0.5 or greater. Results of our analysis have led to action by citizens of Montana, prompting state and federal managers to develop a conservation and restoration program for this subspecies in the upper Missouri River basin.

An analysis of spatial and environmental factors influencing hybridization between native westslope cutthroat trout (Oncorhynchus clarki lewisi) and introduced rainbow trout (O. mykiss) in the upper Kootenay River drainage, British Columbia

“…Many of the remaining non-hybridized populations throughout the range of WCT are restricted to isolated headwaters (Shepard et al 1997;Mayhood 1999;Hilderbrand and Kershner 2000), and non-hybridized populations in the upper Kootenay River drainage seem to be no exception. All five localities with no evidence of introgression with RBT are located on tributaries further upstream from the mainstem upper Kootenay River than their hybridized counterparts.…”

“…There are also disjunct populations in Washington State, Oregon and in the South Thompson River (Fraser River drainage), Columbia River and Kettle River in BC. There have been significant declines in WCT populations throughout their historic distribution due to habitat loss and degradation, overexploitation, competition and predation by non-native salmonids, and introgressive hybridization with introduced rainbow trout (RBT, O. mykiss) and Yellowstone cutthroat trout (YCT, O. clarki bouvieri; Allendorf and Leary 1988;Liknes and Graham 1988;Shepard et al 1997). WCT are currently blue-listed in BC (i.e., species of special concern) and under review for a federal listing under the Canadian Species at Risk Act (SARA).…”

Genetic and Phenotype [Phenotypic] Catalog of Native Resident Trout of the interior Columbia River Basin : FY-99 Report : Populations of the Pend Oreille, Kettle, and Sanpoil River Basins of Colville National Forest.

“…Recently, Shepard et al (1997) estimated that westslope cutthroat trout of at least 90 percent genetic purity presently inhabit less than 3 percent of the former range within the upper Missouri River drainage, which has long been regarded as the heart or core of this subspecies' historic distribution (Behnke 1992). Of 16 upper Missouri subbasins still supporting at least one population, 14 contain populations at moderate or high estimated risk of extinction, and almost all of these remaining populations exist in isolation from one another in high elevation mountainous stream fragments less than 10 km long.…”

“…Of 16 upper Missouri subbasins still supporting at least one population, 14 contain populations at moderate or high estimated risk of extinction, and almost all of these remaining populations exist in isolation from one another in high elevation mountainous stream fragments less than 10 km long. Shepard et al (1997) identified existing and future land use activities and introduced species as the principal threats to these upper Missouri populations…”

“…If we apply the same 90 percent genetic purity criterion that Shepard et al (1997) used to the populations we examined in Colville National Forest streams, then 11 of our 15 cutthroat populations qualify, including our four A-purity populations and our seven B-purity populations. As noted earlier, one of our B-populations, Slate Creek, appears to have originated from hatchery introductions based on the findings of Shaklee and Young (2000), so we exclude it from the present discussion.…”