Seasonal shift in energy allocation from somatic growth to lipid storage and the link between pre-winter condition and overwintering potential in juvenile Pacific cod

Warming temperatures in the Gulf of Alaska have been linked to recruitment failure in Pacific cod (Gadus macrocephalus), but the mechanisms and timing of mortality events for juveniles are unclear. To date, limited research has focused on overwintering success, and the knowledge of juvenile ecology and physiology is based entirely on summer observations. Here, we investigate the changes in body condition, diet composition, and tissue-specific fatty acid (FA) storage for age-0 Pacific cod in Kodiak, Alaska, from February to December during 2018 and 2020. We observed protracted nearshore residency from June to December. Cod body condition (Kdry) and predicted weight at length were lowest in October, November, and December. Although not different interannually, diet composition varied seasonally, which corresponded to an increase in cod length. A range of condition metrics (HSIwet, FA concentration in liver tissue, and the % of whole body FAs stored in the liver) began to increase in September. Cod prioritized growth during the summer, while in the autumn and pre-winter they allocated more energy into lipid storage. We conclude that seasonal changes in tissue-specific FA storage and pre-winter fish conditions are important factors to consider for understanding overwintering potential of juvenile Pacific cod.