Prey availability and feeding ecology of juvenile Chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon in the northern California Current ecosystem, based on stomach content and stable isotope analyses

To understand the factors that regulate early marine survival of juvenile Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon, it is necessary to characterize their prey availability, selectivity, and dietary niche widths. Currently, no sampling protocol exists that directly measures the salmonid prey field and quantifies dietary niche width via stable isotope analysis (SIA). The main goals of this study were to quantify the dietary niche widths of juvenile salmon and to compare environmental prey community compositions to juvenile salmon diets. Juvenile Chinook and coho salmon and their prey were collected in the northern California Current ecosystem via herring and Nordic trawls during 9–15 June 2010. The catch per unit effort of the salmonid prey field was compared to juvenile salmon stomach contents, and a Manly–Chesson index was used to quantify prey selectivity. Salmon and their prey were measured for δ13C and δ15N values and salmon were analyzed via Stable Isotope Bayesian Ellipses in R. Results indicated that there was no significant difference between juvenile Chinook and coho salmon diets and that they principally consumed rockfish juveniles (Sebastes spp.) and Cancer megalopae. Juvenile salmon consumed somewhat different prey items compared to both herring and Nordic nets and these nets sampled different assemblages of similar prey communities. Though juvenile salmon were in isotopic disequilibrium with their marine prey, both species occupied similar isotopic niche widths. These results demonstrate that SIA provides complementary information to stomach content analysis.