North American Journal of Fisheries Management 17:1158-1172, 1997

─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 plop 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. The abundance and distribution of westslope cutthroat trout Oncorhynchus clarki lewisi have declined dramatically throughout the subspecies' historical range, which included the upper Columbia, Missouri, and South Saskatchewan river basins, as well as disjunct, isolated populations in the John Day drainage of Oregon and in the Lake Chelan, Methow, Entiat, Yakima, and Wenatchee 1 Corresponding author: [email protected] 2 The Unit is jointly sponsored by Montana State University, the Montana Department of Fish, Wildlife, and Parks, and the U.S. Geological Survey. drainages of Washington (Liknes and Graham 1988; Behnke 1992; McIntyre and Rieman 1995). Behnke (1992) stated that the original distribution of westslope cutthroat trout within the upper Missouri River basin is not known with certainty and suggested that their native range included "the upper Missouri basin (main river and all tributaries) downstream to about Fort Benton, Montana, about 60 km below Great Falls, as well as headwaters of the Judith, Milk, and Marias rivers, which join the Missouri downstream from Fort Benton" (Figure 1). Hanzel (1959) suggested that their original range extended down to the Musselshell River. Factors identified as leading to declines of