Anomalous ocean conditions in 2015: impacts on spring Chinook salmon and their prey field Marine Ecology Progress Series

In the northern California Current, Columbia River Chinook salmon Oncorhynchus tshawytscha that return as adults in spring are primarily hatchery-produced, though they include natural-origin fish listed under the US Endangered Species Act. Anomalously warm ocean conditions persisted in the California Current during 2015 (>2.5°C above normal) through the winter period when fish prey resources of juvenile salmon develop and during spring as salmon enter the ocean. The biomass of ichthyoplankton in winter 2015 was the 4th highest of our 18 yr time-series (1998-2015), predicting good food conditions for salmon and high adult salmon returns several years later. The larval composition of 2015 ichthyoplankton included abnormally large amounts of the warm-water taxa northern anchovy Engraulis mordax and rockfish Sebastes spp. When the composition of ichthyoplankton is dominated by warm-water taxa in winter, we would predict poor returns of salmon. May diets of juvenile Chinook salmon collected in coastal waters reflected high proportions of juvenile rockfish, no evidence of northern anchovy, and most closely resembled those of other warm years. June diets also reflected a warm prey community being consumed, predicting poor returns of salmon. Chinook salmon had high percentages of empty stomachs and were small and thin in 2015, with fish weighing 17.6% less than the same-length fish in a cold year (2008). Lower condition of juvenile Chinook salmon related to decreased returns of adult salmon. Overall, all but one biological predictor (biomass of prey) suggests that the prospects for the 2015 ocean-entry smolts were not favorable for survival.