Low Diversity of High Quality Food Items May Amplify the Potential for Competition Between Native and Non-Native Fishes in a Large Regulated River

Regulation of large rivers is often accompanied by declines in native fishes, and though these declines are often linked to altered habitat, they are frequently associated with changes in the availability of food resources and the invasion of non-native fishes.  Interactions with nonnative fishes are increasingly implicated in losses of native fish fauna. The impacts of predation by non-natives are well documented, but competition with non-natives for food is less understood.  To evaluate the potential for such exploitative competition, we quantified rates of production, the trophic basis of production (TBP), rates of consumption, and coefficients of competition for all native and non-native fishes at 6 sites in a regulated reach of the Colorado River, Grand Canyon.  We sampled the diets of fishes seasonally during 2007 and 2008, incorporated assimilation efficiencies (AE), net production efficiencies, and secondary production data to estimate TBP, consumption (demand) rates, and coefficients of potential competition.  Native and non-native fishes overlapped spatially at 5 of 6 sites and overlap was greatest at the confluence of the Little Colorado River, a stronghold for endangered humpback chub (Gila cypha).  Two invertebrate taxa (Simuliidae and Chironomidae) fueled 54±6% and 47±5% of total fish assemblage production across all sites during 2007 and 2008, respectively.  Approximately one third of fish production was fueled by materials assigned low AE (<0.2) during both years.  Coefficient analyses suggest the potential for strong interspecific competition between native and non-native fishes in several cases.  Such competition appears likely, particularly for the low diversity of high quality invertebrate prey, and may contribute to the decline of native fishes in this and other large regulated rivers.  Given the dynamic nature of invertebrate availability, trophic generalism among extant native fishes is not surprising and may buffer competitive interactions if food resources are limiting.