Trophic responses of juvenile Pacific salmon to warm and cool periods within inside marine waters of Southeast Alaska

Marine growth and survival of juvenile Pacific salmon (Oncorhynchus spp.) have been linked to marine temperatures and feeding conditions during their first few months at sea. Therefore, understanding what salmon consume under varying environmental conditions is important to understanding how their growth and survival are affected by climate change. Here, we examined how warm/cool-phase variation in water temperature, salinity, wind speed, and pycnocline depth influenced the diet composition and quality of four species of juvenile Pacific salmon in Icy Strait (Southeast Alaska) from 2013 to 2017. During the five-year period, water temperatures shifted from a cool phase in 2013 to warm in 2014-2016, then back to cool in 2017. Overall, the diet composition and prey diversity varied among zooplanktivorous species (pink salmon O. gorbuscha, chum salmon O. keta, and sockeye salmon O. nerka) and piscivorous (coho salmon O. kisutch) species, with the exception of 2015, when euphausiids were the dominant prey for all four species. The summer of 2015 was notable for its deep pycnocline although it was not the warmest year in the study. Zooplankton nutritional quality was below average in 2015, but lipid intake by juvenile salmon appeared to be supplemented by the availability of larger euphausiid prey. Across years for all species, diet composition was weakly correlated with a combination of water temperature, salinity, and wind (Pearson correlation = 0.216). We conclude that while the marine heat wave altered the Gulf of Alaska ecosystem, within the range of variability observed during the study period, juvenile salmon were able to meet their energetic demands by switching to alternative prey.