Effect of region on the food-related benefits to age-0 walleye pollock (Theragra chalcogramma) in association with midwater habitat characteristics in the Gulf of Alaska

Abstract

Abstract Wilson, M. T., Mier, K. L., and Jump, C. M. 2013. Effect of region on the food-related benefits to age-0 walleye pollock (Theragra chalcogramma) in association with midwater habitat characteristics in the Gulf of Alaska – ICES Journal of Marine Science, 70: . In the western Gulf of Alaska (GOA), we compared age-0 juvenile walleye pollock body weight, food habits, and expected growth rates between two regions during 3 years. The benefit to rearing off Kodiak Island vs. in the Semidi Islands vicinity increased among years with body length because larger fish consumed more euphausiids, and euphausiids were apparently more available in the Kodiak region. This explains previously observed regional variation in body size, condition, and growth rates within the GOA population and points to potential importance of the Kodiak region as a walleye pollock nursery. During September 2005, 2007, and 2009, Kodiak midwater was cooler and saltier than Semidi midwater, presumably due to a relatively narrow continental shelf and greater oceanic influence. Zooplankton composition differed between regions with the greater euphausiid population density off Kodiak being most relevant to walleye pollock food habits. As found historically, age-0 walleye pollock abundance was lower in the Kodiak region than in the Semidi region. However, the Kodiak fish were larger in both length and length-specific weight, and had fuller stomachs than Semidi fish, although differences were not significant in all years. Regional differences were most pronounced during the relatively warm year 2005 when fish were particularly large. Fish dietary mass was dominated by euphausiids, especially for Kodiak fish ≥55 mm standard length. Bioenergetics model output suggests that the greater euphausiid dietary proportion translates into faster growth of the Kodiak fish. The midwater habitat off Kodiak Island might therefore support above-average production of recruits to the GOA adult population due to better condition and likely survival of walleye pollock juveniles, especially when fish are large and thus able to more fully exploit euphausiids, but confirmation of above-average production of recruits depends on our ability to determine spatial structure in juvenile-adult population connectivity.