Ecology and energetics of early life stages of walleye pollock in the eastern Bering Sea: the role of spatial variability across climatic regimes

Abstract

Understanding mechanisms behind variability in early life survival of marine fishes can improve predictive capabilities for recruitment success under changing climate conditions. Ecosystem changes in response to climate variability in the eastern Bering Sea affect commercial species including walleye pollock (Theragra chalcogramma), which represent an ecologically important component of the ecosystem and support the largest commercial fishery in the United States. The goal of my dissertation was to better understand spatial and temporal dynamics in the ecology of early life stages of walleye pollock in the eastern Bering Sea through: (1) an examination of shifts in larval fish community composition in response to environmental variability across both warm and cold conditions; (2) a quantification of the seasonal progression in energy content of age-0 walleye pollock which provides critical information for predicting overwinter survival and recruitment to age-1 because age-0 walleye pollock rely on sufficient energy reserves to survive their first winter; and (3) a modeling approach to better understand the role of prey quality, prey composition, and water temperature on spatial and temporal patterns of juvenile walleye pollock growth with implications for year-class survival and recruitment success. In the community analysis, I identified a strong cross-shelf gradient delineating slope and shelf assemblages, an influence of water masses from the Gulf of Alaska on species composition, and the importance of nearshore areas for larval fish. Species assemblages differed between warm and cold periods, and larval abundances, including that of walleye pollock, were generally greater in warm years. I identified different energy allocation strategies indicating that distinct ontogenetic stages face different survival constraints. Larval walleye pollock favored allocation to somatic growth, presumably to escape size-dependent predation, while juveniles allocated energy to lipid storage in late summer. Finally, I provide evidence that a spatial mismatch between juvenile walleye pollock and growth 'hot spots' in 2005 contributed to poor recruitment while a higher degree of overlap in 2010 resulted in improved recruitment. I highlight the importance of climate-driven spatial patterns in community structure, prey dynamics, and environmental conditions that influence the growth and survival of an important gadoid population in a sub-arctic marine ecosystem.