First evidence of genetically distinct winter- and spring-spawning components of Pacific cod in the Gulf of Alaska Canadian Journal of Fisheries and Aquatic Sciences

Size and hatch date variation among 0-group Pacific cod in the Gulf of Alaska (GOA) has increased following a series of marine heatwaves (2014-16, nicknamed “The Blob”, and 2019) that negatively impacted the fishery. In this study, we used daily otolith increments, whole genome sequencing and fatty acid analyses (FA) to compare two size groups of age-0 cod co-occurring in sites near Kodiak Island in 2021. Size groups reflected genetically differentiated winter and spring spawning groups within the currently defined ‘Western GOA/Eastern Bering Sea’ complex. Winter-spawned juveniles were larger and in higher energetic condition (FA density) than younger, spring-spawned juveniles. Our results raise several immediate questions, including whether these genotypes have unique environmental adaptations (e.g., ‘ecotypes’), and whether spawning phenology underlies genetic subpopulation structure more broadly. Importantly, our findings demonstrate that phenotypic and genotypic alignment could mediate how environmental conditions differentially select for early- and late-spawning strategies, shaping population resilience under future climate variability.