The relative importance of sub-populations to the Gulf of Maine stock of Atlantic cod

Abstract

Abstract Discriminating among sympatric sub-populations is critical not only for the investigation of fish population structure, but also for effective fishery management. The Gulf of Maine stock of Atlantic cod contains two genetically distinct sub-populations whose spawning grounds overlap in space, but not season. Lack of a practical tool for discriminating between these spring and winter spawners has thus far prevented addressing metapopulation structure in stock assessments and fishery management plans. To address this issue, we developed a simple approach to discriminate between the sympatric sub-populations that relies on internal otolith structures. Using a logistic regression model fit to training data (n = 577) and evaluated through cross-validation, we correctly classified individuals with 81% accuracy and predicted the overall sub-population mixture within 1%. Applying the model to a broader population dataset (n = 1642) revealed that spring-spawned cod are more concentrated within closed fishing areas and therefore experience a lower mortality rate. However, despite dominating older age classes and comprising a large fraction of the spawning stock, these spring cod contribute little to recruitment. This apparent sub-population difference in the stock–recruitment relationship has important consequences for assessment models and the success of management measures designed to rebuild the stock.