Do Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi) and Sculpin (Cottus sp.) Compete in Tributaries of the Bitterroot River, MT?

Interspecific competition between fish species can have a large effect on population dynamics. Competition between species of high economic or conservation value is regularly examined, but non-game species are often overlooked. Previous studies from the Pacific Northwest region have demonstrated competition between several trout species and sculpin, but little is known about the strength of competition. In western MT, westslope cutthroat trout (Oncorhynchus clarkii lewisi) often live in the same streams as sculpin (Cottus sp.). We examined diet overlap for these species in sympatry and potential diet shifts for cutthroat trout in sympatry versus allopatry in the Bitterroot River basin, MT. We expected that sculpin are primarily feeding on aquatically-derived invertebrates. Therefore if there is competition, we expected diets of trout in the presence of sculpin to contain a greater proportion of terrestrial food sources than where they occur alone. In the summer of 2014, we collected preliminary data by lavaging westslope cutthroat trout and sculpin in two 100m reaches in each of four streams where they occur in sympatry and where cutthroat trout occur alone (due to a small barrier for sculpin). We identified invertebrates to the highest taxonomic level (Order or family) required to determine habitat of origin. We detected diet overlap between westslope cutthroat trout and sculpin highlighting the potential for competition. However, there was no significant difference in westslope cutthroat trout diet regardless of the presence of sculpin. Given the high variation in season, sculpin density, and habitat among streams, we plan on expanding our sample size to improve our inference strength. Understanding species interactions between westslope cutthroat trout and sculpin is a useful step for understanding complex food webs and how anthropogenic changes and wildfire may be influencing our food webs and ecosystems