Fine-scale life-history structure in a highly mobile marine fish

Background: A highly mobile marine fish, Atlantic cod (Gadus morhua), inhabits southern Norwegian coastal habitats that offer limited potential for individual dispersal and migration. Questions: Do coastal populations of cod differ in life history? If so, is the variability spatially persistent and does it vary with time? What factors are responsible for life-history differences among the potential populations? Method: Use long-term, fisheries-independent survey data to measure and compare lifehistory metrics among nine regions along the southern Norwegian coast. Conduct maturity analyses, using generalized linear mixed-effect models, where the probability of being mature is a function of fixed effects (length, age, weight, sex, and growing degree days) and random effects (location and year class nested within location). Results: We detected that the probability of being mature has increased spatially (along an increasing longitudinal cline) and temporally (throughout a 30-year time-series). Neither of these trends could be fully explained by variation in sea temperature or population density. Conclusion: Life-history variability in a highly mobile marine fish can be evident, and temporally persistent, at spatial scales considerably smaller than those encompassed by established fisheries management units and species recovery strategies. Our findings provide empirically defensible justification for studies on the ecological factors and evolutionary mechanisms responsible for producing and maintaining this variability.