Distribution, size, and interannual, seasonal and diel food habits of northern Gulf of Alaska juvenile pink salmon, Oncorhynchus gorbuscha

An integral part of assessing the northern Gulf of Alaska (GOA) ecosystem is the analysis of the food habits and feeding patterns of abundant zooplanktivorous fish. Juvenile pink salmon Oncorhynchus gorbuscha are highly abundant zooplanktivores, and support valuable commercial fisheries as adults. We document variability in pink salmon distribution and size from summer to early fall, and present major trends in their food habits by summarizing interannual (August 1999–2001), seasonal (July–October 2001) and diel (August 2000, and July–September 2001) feeding patterns based on analysis of stomach contents of juvenile pink salmon collected along the Seward Line (GOA) and in Prince William Sound (PWS), Alaska. Diets of juvenile pink salmon were more diverse in 2001 compared to either 1999 or 2000. Small pteropods (Limacina helicina) composed the majority (>60%) of prey consumed in 1999 and 2000; whereas large copepods, euphausiids, and small pteropods composed the majority of prey in 2001. As juvenile pink salmon increased in size, they consumed increasingly larger prey from August to October 2001 in the GOA. The diet of GOA juvenile pink salmon was different and more diverse than the diet of fish caught in PWS. The dominant prey in PWS during July–October was hyperiid amphipods, whereas the primary prey in the GOA were larvaceans and euphausiids in July, then copepods plus small pteropods, amphipods, euphausiids, larval crabs, and shrimp in August. In September and October, diets in both PWS and GOA included high percentages of larger prey items, including fish, euphausiids, and large pteropods (Clio pyramidata). Diel comparisons of stomach contents showed pink salmon fed during daylight hours with stomach fullness increasing from dawn to a maximum fullness 8–12 h after sunrise, and declining thereafter. othesize that juvenile pink salmon in the northern GOA consumed distinct and varied prey from the suite of zooplankton available during summer months, July–September. These prey were not necessarily the planktonic species with the greatest biomass or highest abundance on the shelf. Pink salmon fed on prey that were highly visible during daylight hours in the neuston and upper 10 m of the water column. Further work will integrate food habits with fish condition and growth data, zooplankton abundance, and water temperature to develop foraging and bioenergetics models. These models will enable us to assess the habitat quality and growth conditions for juvenile pink salmon and contribute to the goal of understanding how pink salmon and, therefore, tertiary production are affected by changes in ocean conditions.