Stable Isotope Signatures of Native and Nonnative Fishes in Upper Colorado River Backwaters and Ponds

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trophic relationships among native and nonnative fishes, and 2) can stable isotope signatures be used as a naturally-occurring marker to identify river fishes that have migrated from floodplain ponds? Nitrogen isotope ratios showed that at a particular location, either in ponds or backwaters, centrarchids were usually the top predators in each system. In one backwater, 815N and 8'3C signatures of the fish assemblage ranged about 4%o. The number of trophic levels represented was limited to about 2, and variation in carbon sources appeared to be great. The native flannelmouth sucker, Catostomus latipinnis, was most distinct with a relatively low 815N and high 813C signature. Stable isotope signatures appeared to have utility as a naturally occurring marker for tracking influx of nonnative fishes to the Colorado River from ponds. Whereas pond signatures were not consistently depleted or enriched relative to those from backwaters, mean stable isotope signatures were more different among ponds than among backwater sites. Further, within particular species, some ponds had carbon or nitrogen signatures that were quite different from those at backwater sites. Stable isotope analysis appears to have promise for studying trophic relationships and movement patterns of native and nonnative fishes in the upper Colorado River basin. RESUMEN-Is6topos estables de carb6n (13C) y nitr6geno (15N) que ocurren naturalmente fueron analizados para responder a preguntas acerca de interacciones tr6ficas entre pescados nativos