Stable isotope signatures of native and nonnative fishes in Upper ColoradoRiver backwaters and ponds.

Naturally-occurring stable isotopes of carbon (C-13) and nitrogen (N-15) we re analyzed to address questions about trophic interactions among native an d normative fishes in the upper Colorado River basin, and to begin to evalu ate the discreteness of floodplain pond fish assemblages. Specifically, 2 h ypotheses were evaluated: 1) can stable isotope analysis be used to establi sh trophic relationships among native and nonnative fishes, and 2) can stab le isotope signatures be used as a naturally-occurring marker to identify, river fishes that have migrated from floodplain ponds Nitrogen isotope rati os showed that at a particular location, either in ponds or backwaters, cen trarchids were usually the top predators in each system. In one backwater, delta N-15 and delta C-13 signatures of the fish assemblage ranged about 4 parts per thousand. The number of trophic levels represented was Limited to about 2, and variation in carbon sources appeared to be great. The native flannel-mouth sticker, Catostomus latipinnis, was most distinct with a rela tively low delta N-15 and high delta C-13 signature. Stable isotope signatu res appeared to have utility as a naturally occurring marker for tracking i nflux of normative fishes to the Colorado River from ponds. Whereas pond si gnatures 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 b ackwater sites. Stable isotope analysis appears to have promise for studyin g trophic relationships and movement patterns of native and normative fishe s in the upper Colorado River basin.