Conceptual Model of Energy Allocation in Walleye Pollock (Theragra chalcogramma) from Larvae to Age-1

Walleye pollock (Theragra chalcogramma) support the largest commercial fishery in the United States and are an ecologically important component of the eastern Bering Sea (EBS) ecosystem. Multiple forcing mechanisms during the early life stages of pollock result in variable recruitment. Relating the seasonal progression of energy content and allocation to the distribution and abundance of pollock allows for detection of spatial and temporal trends in pollock condition and provides critical information for the prediction of overwinter survival and recruitment to age-1. Larval, juvenile, and age-1 pollock were collected during 13 cruises in the EBS from May to September 2008-2010. Fish condition was determined through quantification of energy density (kJ/g) and proximate composition (% lipid, % protein, % moisture) with variation in energy density driven by variability in percent lipid. A conceptual model of energy allocation was developed using a four-parameter logistic model and fitting a spline smoother to age-0 energy density and % lipid data to describe seasonal changes in energy content and determine the period for increased energy storage. Energy densities were low during early larval development (age-0) in 2008-2010 (17.1 ± 1.8 kJ/g dry mass [mean ± SD]), indicating energy allocation to growth. Following completion of larval development (~25mm standard length [SL]), lipid acquisition rates increased leading to higher energy densities in fall (23.1 ± 1.1 kJ/g dry mass) as energy was allocated to storage for overwinter survival. The period of increased energy storage, determined from the estimated inflection point of the logistic model fit to all years, occurred in late summer (July 20 ± 7 days based on energy density; July 20 ± 5 days based on % lipid). A physiologically important shift occurs after larval development as walleye pollock begin allocating energy to storage for overwinter survival. The timing of this transition may affect the fall condition of pollock, which is increasingly recognized as a predictor of overwinter survival and recruitment to age-1. We hypothesize fish condition is dependent on oceanographic conditions and prey availability during a short critical period in late summer for lipid storage.