Winter Diets of Pacific Salmon in the North Pacific

The winter feeding ecology of Pacific salmon and Steelhead trout in the North Pacific Ocean was investigated, with a focus on species-specific diets and interannual and spatial variability in diet composition. We used results from the 2022 International Year of the Salmon Pan-Pacific Expedition to examine diet composition across the North Pacific and compared our findings with earlier surveys conducted in 2009–2011 and 2019–2020. Stomach contents were dominated by prey items typically associated with that species' diet: primarily cnidarians for Chum salmon, cephalopods and fish for Coho and Chinook salmon, and euphausiids for Pink and Sockeye salmon. The diet of Steelhead trout, encountered in one region in 2022, was composed of cephalopods, fish, and euphausiids. Some significant interannual and regional variability was observed, particularly in the eastern North Pacific, where prey resources have been noted to be more limited. This suggests that prey partitioning and adaptability may influence interspecific competition. Chum and Sockeye salmon exhibited interannual variability in most regions where multiple years of surveys were conducted. Pink salmon had the most spatial variability in winter diet with differences detected in all regions. We found minimal evidence of diet shifts based on size, with some exceptions in Chum and Sockeye salmon. Overall, these findings contribute to a deeper understanding of the trophic dynamics and feeding strategies of Pacific salmon during the winter months in the open ocean, highlighting the potential for competition and the importance of fine-scale spatial analyses for future research on salmon ecology and conservation.