Juvenile Bristol Bay Sockeye Salmon Ecology

Abstract

Predicting annual returns of Bristol Bay sockeye salmon (Oncorhynchus nerka) has been difficult due to large, unexplained variations in return strength. Ocean conditions, particularly during the first few months after salmon leave freshwater, are believed to have a strong influence on their early marine growth and survival. Limited historical and present research suggests that sea temperature can affect juvenile Bristol Bay distribution. During years with cool spring sea temperatures, juvenile sockeye salmon are distributed nearshore along the Alaska Peninsula, whereas they are found further offshore during years with warm spring sea temperatures. Juvenile sockeye salmon are larger, in better condition, and have higher marine stage survival after the first year at sea when they are distributed further offshore than when they are distributed nearshore along the Alaska Peninsula. Juvenile sockeye salmon stomach contents also shift from primarily Pacific sand lance ( Ammodytes hexapterus) and euphausiids to age 0 walleye pollock ( Theragra chalcogramma) when their distribution changes from nearshore to further offshore. Annual averages of juvenile sockeye salmon growth rate potential (GRP) were generally lower among years and regions with cool spring sea temperatures. In addition, juvenile sockeye salmon GRP was generally higher in offshore regions than nearshore regions of the eastern Bering Sea shelf. A sensitivity analysis indicated that juvenile sockeye salmon GRP was more sensitive to changes in observed (August to September) sea surface temperatures during years when prey densities were lower. The results of the dissertation suggest that variability in early marine survival is primarily due to bottom-up control of the trophic structure of the eastern Bering Sea ecosystem.