Trophic Ecology of Resident Chinook Salmon in Puget Sound

Approximately 30% of Chinook Salmon remain resident in Puget Sound rather than spend most of their marine life in the open ocean. Despite representing a significant fraction of the overall population, little is known about the life history or trophic relationships of resident Chinook salmon in Puget Sound. An earlier study estimated that cannibalism by resident Chinook might impose 10-50% mortality on the incoming cohort of subyearling Chinook, but these estimates are highly sensitive to assumptions about temporal changes in Chinook diet from May through July. I examined the monthly change in diet composition among size classes of resident Chinook during the May-July growing period in 2018 and 2019 in order to determine their role in the food web as prey, competitor or predator. Diets were examined for 226 Chinook ranging from 130 mm to 724 mm fork length (FL). The diet sample results indicate zooplankton are the predominant prey of Chinook smaller than 300 mm FL. Crab larvae contribute the most to the diet of the <300 mm FL size class in May and June, while amphipods are most prevalent in July diet samples. The majority of diet for Chinook larger than 300 mm FL is herring in 2018, while in 2019 crab larvae or amphipods largely replace prey fish. Chinook body condition remained constant between years despite a change in diet between 2018 and 2019. Resident Chinook are able to consume prey fishes up to 50% of their own fork length. Despite the lack of a size constraint, no cannibalism on subyearling Chinook or predation on other salmon was evident.