Potential Impacts of Climate Change on the Trophic Ecology and Production of Juvenile Chinook Salmon in the California Current

Juvenile Chinook salmon (Oncorhynchus tshawytscha) are the dominant salmon species caught off Oregon and Washington in coastal surveys and a key nexus in the food web of the California Current.  From sampling beginning in 1980, we analyzed the effects of changing ocean conditions (ocean warming, variable El Niño intensity, increased deoxygenation and acidification of coastal waters) on the diet composition and feeding intensity of juvenile salmon.  Larval and juvenile fish prey comprised the majority of the diet by weight for both yearling and subyearling stages, although a different diet community is consumed during warm vs. cool ocean conditions. The northern anchovy and juvenile rockfishes were dominant fish species consumed especially in warmer years.  Euphausiids and crab larvae were more important during cool summers.  Feeding intensity and caloric density of individual meals is significantly higher and proportion of empty stomachs lower during warm years.  Bioenergetic modeling simulations suggest that more food will be necessary during warming ocean conditions but expected changes in prey production may limit the potential for maximum growth and survival.  Increased acidification of coastal waters may have led to a decrease of pteropods as prey since the 1980s, but these prey are relatively minor components of the diets.