Status and risk of extinction for westslope cutthroat trout in the Upper River Basin, Montana
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 (p 100). Most (71 %) of the 144 populations had a very high predicted risk of extinction (p 100 = 50%), 19% exhibited a high risk (50% < p 100 = 80%), and 10% had a moderate risk (80 < p100 = 95%). Higher average predictions of p 100 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 p 100 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.