Diet and trophic niche overlap of native and nonnative fishes in the Gila River, USA: implications for native fish conservation

The upper Gila River basin is one of the few unimpounded drainage basins west of the Continental Divide, and as such is a stronghold for endemic fishes in the region. Nevertheless, multiple nonindigenous fishes potentially threaten the persistence of native fishes, and little is known of the trophic ecology of either native or nonnative fishes in this system. Gut contents and stable isotopes (C and N) were used to identify trophic relationships, trophic niche overlap and evaluate potential interactions among native and nonnative fishes. Both native and nonnative fishes fed across multiple trophic levels. In general, adult native suckers had lower N signatures and consumed more algae and detritus than smaller native fish, including juvenile suckers. Adult nonnative smallmouth bass (Micropterus dolomieu), yellow bullhead (Ameiurus natalis) and two species of trout preyed on small-bodied fishes and predaceous aquatic invertebrates leading to significantly higher trophic positions than small and large-bodied native fishes. Thus, the presence of these nonnative fishes extended community food-chain lengths by foraging at higher trophic levels. Although predation on juvenile native fishes might threaten persistence of native fishes, the high degree of omnivory suggests that impacts of nonnative predators may be lessened and dependent on environmental variability. T. J. Pilger, K. B. Gido, D. L. Propst Division of Biology, Kansas State University, Manhattan, KS, USA, Conservation Services Division, New Mexico Department of Game and Fish, Santa Fe, NM, USA