Lotic systems in mountain regions have historically provided secure habitat for native fish populations because of their relative isolation from human settlement and lack of upstream disturbances. However, rivers of mountain ecoregions are currently experiencing heightened levels of disturbance due to the introduction of nonnative species impacting endemic fishes in these areas. We compared the fish assemblages and diets of mountain steppe fishes of the stocked rivers in Wyoming with rivers in northern Mongolia where stocking is absent. Using gut content analysis, we quantified the selectivity and diets of fishes collected in these systems. Nonnative species had more generalist diets with lower levels of selectivity than most native species, and native species had high levels of dietary specificity and selectivity. High abundances of nonnative species and high levels of dietary overlap in our Wyoming sites are a cause for concern for native Cutthroat Trout and overall system stability. In contrast, fish assemblages characterizing Mongolia mountain steppe rivers were composed of only native species with diverse diets and higher selectivity values, suggesting low probability for interspecific competition.

At each site, fishes were collected from reaches measuring 20 times the average wetted width of the stream (Patton et al. 2000). Fishes were first collected using one-pass backpack electrofishing (ETS model: ABP-2) supplemented with hook and line and seining following electrofishing in areas where water depth or conductivity may have impacted electrofishing, following American Fisheries Society standard collection protocols (Bonar et al. 2009). To assist in areas where electrofishing success is non-optimal due to low conductivity, supplemental hook and line and seines were used. All collected fishes were identified to species, weighed (g), and measured for standard length (mm). When available, up to 10 fish for each species at each site were randomly selected and sacrificed for gut analysis. Stomachs were removed and preserved in 10 % formalin for later analysis. For fishes lacking a true stomach, the anterior portion of the gut to the first bend was used as a proxy (Rybczynski et al. 2008). For all fishes, only the stomach or anterior portions of the gut were examined in an effort to minimize bias caused by digestibility of diet items (Sutela and Huusko 2000).