Disruption of an Atlantic Cod Spawning Aggregation Resulting from the Opening of a Directed Gill-Net Fishery

Atlantic cod Gadus morhua exhibit multiyear homing to discrete spawning grounds, where they aggregate in dense schools. Within an aggregation, a series of complex mating behaviors takes place before mate selection and successful spawning. Disruption of these behaviors has been suggested as a cause of diminished reproductive success and poor recruitment in some stocks. An area known to support a spawning aggregation in Massachusetts Bay was closed to both commercial and recreational fishing for the months of May and June 2009. During the closure period, 10 Atlantic cod were captured, tagged with acoustic transmitters, and released back to the aggregation. Four stationary acoustic receivers were deployed in the area to record transmissions from the tagged fish. Overlapping detection ranges of the receivers allowed for the reconstruction of fine-scale movements of the tagged fish over several days. The tagged cod showed a consistent pattern of aggregation prior to the fishery, characterized by limited movement and similar space use. With the opening of the fishery, the aggregation behavior was disrupted, resulting in increased horizontal and vertical movements and dissimilar space use among individuals. Half of the tagged fish appeared to have been caught in gill nets within 9 h of the opening, while the remainder left the area within 18 h. Even though the receivers were maintained for 9 d after the opening, none of the tagged fish that left the area returned. These results indicate that the spawning aggregation was completely dispersed by the onset of the fishery. Managers hoping to protect spawning aggregations should be aware that the effects of fishing on a spawning aggregation go beyond the removals from the spawning stock.