Macroinvertebrate prey availability and food web dynamics of nonnative trout in a Colorado River tributary, Grand Canyon

Nonnative fishes have been linked to the decline of native fishes and may affect aquatic food webs through direct and indirect pathways. These concerns have led to efforts to remove nonnative Brown and Rainbow Trout, which are abundant in tributaries of the Colorado River, to enhance native fish communities. We sampled fish, benthic, and drifting macroinvertebrates in November 2010, January 2011, June 2011, and September 2011 to assess resource availability and to evaluate the effects of nonnative Brown and Rainbow Trout in a tributary of the Colorado River in the Grand Canyon. We evaluated trout diets from stomach samples collected during macroinvertebrate sampling periods, and we estimated annual consumption with bioenergetics models. We used C-13 and N-15 stable isotopes to examine potential diet overlap between native and nonnative fishes. Contributions to benthic biomass varied among megalopterans (16-35%), trichopterans (19-28%), and ephemeropterans (9-32%), whereas ephemeropterans dominated biomass (44-64%) in drift samples. Ephemeropterans were dominant in diets of small (<150 mm total length [TL]) trout, whereas Corydalus and native fish dominated diets of large (>150 mm TL) Brown Trout, and Corydalus and algae dominated diets of large Rainbow Trout. Annual resource consumption was 6x higher for large trout than small trout. Stable isotopes suggested diet overlap between native and nonnative fishes. Large nonnative trout occupied the highest trophic positions. Our results suggest that suppression of nonnative trout may have a positive effect on native fishes via reduced predation and resource competition.