Small-scale life history variability suggests potential for spatial mismatches in Atlantic cod management units

A basic challenge to successful management and conflict resolution is to correctly identify the spatial scale at which strategies for harvesting are developed. For commercially exploited marine fish, distributional boundaries of many stocks are based on the premise that productivity is similar at spatial scales that represent a small fraction of an individual's potential migration distance. Within such confined geographic regions, differences among individual life history traits - primary determinants of population productivity - are assumed to be negligible. The empirical basis for these assumptions bears re-examination for some widely distributed species, such as Atlantic cod (Gadus morhua L.), for which there is evidence that life history structuring is evident at much finer scales than previously thought. Here, we focus on a unique system in the Norwegian Skagerrak, divided into inner and outer fjord habitats between which cod movement across distances of a few kilometres is limited and populations show signals of genetic differentiation. Based on back-calculated length-at-age data, cod in the inner fjord are systematically smaller at young ages (1-5 years) and are characterized by shorter asymptotic body lengths than cod in the outer fjord. Our work on growth and life history supports genetic analyses which indicate that productivity and resilience of widely distributed marine fish are likely to differ at considerably smaller spatial scales than those delineated by traditional stock management boundaries.