Seasonal changes in the diet composition and prey selection of walleye pollock (Theragra chalcogramma) in the northern Gulf of Alaska

Walleye pollock, Theragra chalcogramma, are one of the dominant fishes in the Gulf of Alaska (GOA) ecosystem, yet relatively little information is available on the diet and prey selectivity of pollock in the northern GOA. Stomachs of midwater trawled adult pollock were collected in April, August and November 2003 in the northern GOA. Euphausiids were the dominant prey in April, averaging 59% by number and 70% by weight. Euphausiids persisted as the dominant prey in August, averaging 84% by number and 95% by weight. Decapods were the dominant prey item in November. This was primarily due to the shrimp Pandalus borealis, which averaged 68% by number and 53% by weight. Stomach contents were also compared with available prey fields using chi-square-based resource selection statistics. This analysis revealed significant differences between the environmental and dietary distributions of prey types. Additional analysis found significant selection for euphausiids over all other categories of available zooplankton in both spring and summer. Amongst euphausiids, there was significant selection for Thysanoessa inermis in spring and T. spinifera in summer.