Impacts of nonnative Brown Trout on Yellowstone Cutthroat Trout in a tributary stream

Nonnative trout are a considerable threat to native salmonids, yet our understanding of the mechanisms behind interspecific interactions remains limited. We evaluated the impacts of nonnative Brown Trout Salmo salar on a population of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in Montana. We contrasted diets, growth, and survival of Yellowstone Cutthroat Trout occurring in allopatry (i.e., where no Brown Trout were present) with individuals sympatric (i.e., co‐occurring) with nonnative Brown Trout. We assessed summer and autumn diets using gastric lavage methods and survival and growth using mark–recapture analyses. Overlap in diets at sites where Yellowstone Cutthroat Trout were sympatric with Brown Trout was high during July (Horn's index: H = 0.94) and October (H = 0.83). In the presence of Brown Trout, Yellowstone Cutthroat Trout growth rates were significantly lower for juvenile (<175 mm) length and adult (≥175 mm) length and mass than in allopatric sites. Allopatric Yellowstone Cutthroat Trout survival was greater across size‐classes; the most pronounced difference was in the age‐2 size‐class (125–175 mm). Together, these results in concert with observed changes in length‐frequency data, indicating a considerable lack of Yellowstone Cutthroat Trout recruitment where they are sympatric with Brown Trout, suggest the negative implications of Brown Trout are notable.