Stable Isotopes of Carbon and Nitrogen Indicate Differences in Marine Ecology between Wild and Hatchery-Produced Steelhead

Abstract Hatchery-produced anadromous salmonids often differ from their wild conspecifics in behavior, growth, and survival after release, but our understanding of their comparative ecology at sea is very limited. We used stable isotopes of carbon and nitrogen to test the null hypothesis that hatchery-produced steelhead Oncorhynchus mykiss from Forks Creek in southwestern Washington would be similar in trophic level and marine distribution to wild fish from the same river. Analysis of scale samples from 30 fish of each type that had migrated to sea in 2001 and 2002 (120 fish in all) revealed significant effects of year and origin on both C and N. The values for δ15N were consistent with a higher trophic level for the wild fish (mean = 10.82, SD = 0.57) than for the hatchery fish (mean = 10.51, SD = 0.59), and the δ13C values were consistent with more reliance on nearshore sources of C by wild fish (wild fish mean = −16.52, SD = 0.30; hatchery fish mean = −16.73, SD = 0.31). The wild fish showed no relatio...