Using the Diet Composition of Adult Chinook Salmon to Understand the Regional Structure of Salish Sea Forage Communities

ABSTRACT Small pelagic fish and other forage species are patchily distributed over space and time, resulting in variable foraging conditions experienced by their predators. The high‐resolution data necessary to understand the spatiotemporal structure of forage communities are challenging to collect with expensive fishery‐independent surveys, meaning that other, complementary approaches are needed. To fill this gap, we partnered with recreational anglers to sample the stomach contents of more than 2500 adult Chinook salmon ( Oncorhynchus tshawytscha ) throughout the Canadian Salish Sea in both summer and winter from 2017 to 2022. Previous studies found that the diet composition of adult Chinook salmon correlated well with conventional indices of forage species abundance, meaning that Chinook salmon diets can be used to understand the availability and distribution of their prey. Cluster analysis of Chinook salmon diet composition identified regions with distinct forage communities in the Salish Sea. Regions were distinguished from one another by the elevated importance of only one or two forage species. Pacific herring ( Clupea pallasii ) were key prey in all regions and throughout the year, while the importance of other forage species including Pacific sand lance ( Ammodytes personatus ), northern anchovy ( Engraulis mordax ), myctophids, and mysids was spatially and (or) seasonally restricted. The regional structure of Salish Sea forage communities was associated with static and dynamic oceanographic processes, such as bathymetric features, tidal mixing, and freshwater input. Predator diets can provide novel insights into the ecology of their prey, supplementing fishery‐independent surveys to support ecosystem approaches to fisheries management.