Evidence for Metapopulation Structuring of Atlantic Cod in US Waters

The Gulf of Maine and Georges Bank stocks of Atlantic cod (Gadus morhua) struggle to rebuild following considerable declines in abundance. The lack of understanding of cod population structure is an important factor contributing to delays in rebuilding. We present evidence for an emerging paradigm that the United States' cod resource can be described as a metapopulation comprised of multiple subpopulations and many finer-scale spawning components. Mechanisms contributing to the formation and maintenance of the observed metapopulation structure include temporal variation in spawning (i.e., spring and winter subpopulations), spawning site fidelity, adult connectivity, and larval transport dynamics. Declines in abundance were accompanied by the serial depletion of historical spawning components, which are often vulnerable to overexploitation given cod spawning dynamics (i.e., dense, predictable aggregations close to shore). The resulting decline in spawning diversity and the lack of recolonization of abandoned spawning sites have contributed to delays in rebuilding by reducing stock productivity and stability. Fishery managers have begun to implement fine-scale spawning closures to prevent the extirpation of persistent spawning components and support rebuilding. Incorporating the observed metapopulation structure into future stock assessment models and fishery management plans is expected to prevent continued declines and help achieve rebuilding objectives.