Temperature effects on spatiotemporal patterns of forage fish and crustaceans in Gulf of Alaska groundfish diets

Thesis (Master's)--University of Washington, 2023Recently, the North Pacific Ocean has experienced unprecedented and extreme marine heat waves (MHWs), which have caused cascading ecological effects within marine food webs. Examining past responses of groundfish feeding ecology to temperature shifts will help illuminate how they may respond in the future. In this study, we investigated the role of temperature in explaining spatiotemporal patterns in the diets of four groundfish predators in the Gulf of Alaska (GOA). The objectives of this research were to 1) characterize spatiotemporal patterns of groundfish predation on forage fish and crustacean prey species; and 2) evaluate effects of temperature on patterns of prey occurrence in predator diets. Prey species were identified for analysis based on hypothesized relationships with temperature, relevance to management, and empirical measures of relative importance in groundfish diets. We used 15 years (1990 - 2022) of groundfish stomach contents data to model prey occurrence as a function of temperature, year, location, depth, predator species, and predator length. Our model results showed seven relationships between temperature and prey occurrence in diets, including a negative effect for euphausiids and pandalids, and a positive effect for pagurids and tanner crab. Euphausiids in particular showed consistent negative trends in response to temperature in the diets of Pacific cod, walleye pollock, and arrowtooth flounder. Euphausiids are a high lipid, energy-rich food source for GOA predators including groundfish, whales, and seabirds. The complex and unprecedented effects of the GOA MHW demonstrate the need to better understand the predator-prey interactions in this system across multiple species and trophic levels. This work can help to inform development of prey abundance and biomass surveys and to supplement data-limited species or years