Quantifying Piscivory in Buffalo Bill Reservoir: Are the Wild Oncorhynchus Fisheries Sustainable?

Buffalo Bill Reservoir in northwest Wyoming, supports an entirely wild and self-sustaining trout fishery, with all stocking discontinued in 1995. Recent upsurge in Lake Trout (Salvelinus namaycush) abundance and an illicit introduction of Walleye (Sander vitreus) have put Yellowstone Cutthroat (Oncorhynchus clarkii bouvieri) and Rainbow Trout (Oncorhynchus mykiss) at risk of decline. Walleye have the potential to decimate populations of salmonids in western reservoirs. Field sampling for this investigation began in April 2012 to evaluate consumptive demand by Lake Trout and Walleye, then determine possible management actions to permit a sustainable fishery for both Yellowstone Cutthroat and Rainbow Trout. Our data show that the thermal conditions of Buffalo Bill Reservoir are sub-optimal for both predator species, and therefore growth rates are below average. Unfortunately, the Oncorhynchus spp. are a major prey source for both predators, Walleye have had successful spawning events since their introduction, and Walleye become piscivorous earlier than Lake Trout. But, the Oncorhynchus spp. grow rapidly in the reservoir, allowing some to outgrow predator gape limits. Using stable isotope and diet analyses coupled with growth and thermal data, we use bioenergetics models to determine predator consumptive demand and the prospects for persistence of the Oncorhynchus fishery.