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

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 delta N-15 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 delta C-13 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 relationship between body size and isotope values, but among the hatchery fish larger individuals tended to have lower delta N-15 values and more negative delta C-13 values. The wild fish were, on average, larger than the hatchery fish but had higher delta N-15 values and less negative delta C-13 values. These patterns are the opposite of what would be expected from a difference in growth rate, suggesting that the differences arose from ecological processes rather than a size or growth effect. Interactions between trophic level and location prevent a determination of which features of the marine ecology differed between wild and hatchery fish, but either factor could result from differences in the genetic background and rearing history of wild and hatchery fish and could contribute to differences in their performance (growth and survival) at sea.