Adult Chinook salmon diets delineate regions with distinct forage assemblages in the Salish Sea

Forage fish are an important link between zooplankton and higher trophic levels, including marine mammals and economically valuable predatory fish. However, forage fish are often difficult to assess using traditional fishery-independent surveys, resulting in major data gaps for both commercially important and non-exploited species. In the Salish Sea, there are many data gaps about the distribution and regional importance of forage fish and other forage species (e.g., juvenile Gadiformes, euphausiids, crustacean larvae). We used the diet composition of adult Chinook salmon (Oncorhynchus tshawytscha), a generalist predator, to examine the spatial structure of forage assemblages in the Canadian Salish Sea from 2017 - 2021. Stomach contents analysis of >1700 stomachs revealed that the importance of forage species such as Pacific herring (Clupea pallasii), northern anchovy (Engraulis mordax), and Pacific sand lance (Ammodytes personatus) varied spatially and seasonally. Cluster analysis of Chinook salmon diet composition identified regions of the Salish Sea with distinct forage assemblages. The spatial structure identified here has implications for efforts to model and protect the food web supporting Chinook salmon, their marine mammal predators, and other economically and ecologically important species in the Salish Sea. This study demonstrates the utility of predator diet sampling to provide important data on forage species at spatial and temporal scales difficult to achieve using traditional fishery-independent surveys.