Influences of Size, Condition, and Diet on Winter Mortality of Juvenile Pacific Herring in Prince William Sound

Overwinter survival of juvenile Pacific herring (Clupea pallasii) is an important factor influencing recruitment. Temperate fishes generally survive food scarcity in winter by metabolizing their stored lipids. Favorable winter feeding conditions may further reduce risks of starvation and predation mortality. Variation in zooplankton prey quantity and quality likely influence herring survival, and models of herring survival may be improved by incorporating winter diet data.

For this study, a component of the Prince William Sound (PWS) Herring Research and Monitoring program, we investigated seasonal trends in YOY herring condition and diet in Simpson Bay, Alaska, from autumn 2011 through summer 2012. Juvenile herring lipid stores peaked in November, then declined and remained low until summer. Few small herring were captured in March, suggesting size-dependent mortality. Small fish had exhausted their lipid stores by March and were reliant on diet energy for their survival. However, estimated diet energy content in March appeared insufficient to meet daily metabolic needs. These findings suggest that small juvenile herring were unlikely to survive winter, due to low lipid stores and limited diet energy intake in March. Quantifying winter survival in the population will require representative sampling of juvenile herring size, condition and diet energy.