Studying an Inshore Spawning Group of Atlantic Cod: An At-Sea Laboratory

After decades of overfishing, the Gulf of Maine stock of Atlantic cod has declined in abundance and spawning diversity. In 2009, the Commonwealth of Massachusetts implemented a fishery closure to prevent the overexploitation of a cod spawning aggregation by recreational and commercial fishermen. The closure provided a unique observational opportunity to investigate the spawning dynamics of cod.  We used an acoustic tag positioning system to investigate the fine-scale 3-D movements of cod tagged with acoustic transmitters. Hydroacoustic surveys were also used to estimate the biomass of the spawning aggregation to assess its relative stock contribution. Spawning occurred over a protracted period in Apr-Jul, with cod exhibiting extremely fine-scale, multiyear fidelity to the spawning site. Distinct diel and sex-related behavioral differences were identified, as well as the disruption of spawning activity in response to fishing. Movements away from the spawning site were investigated using recaptured data storage tags and t-bar anchor tags. Results of this project have been used to refine the boundaries and timing of this spawning closure area, and to provide managers with advice on the implementation of additional closures in the Gulf of Maine. Additionally, our results are expected to provide insight into cod population structure for consideration in future stock assessments and fishery management strategies.