To split or not to split: Assessment of Georges Bank sea scallops in the presence of marine protected areas

Marine protected areas (MPAs) may create challenges for stock assessments because most models are based on the assumption that fishing mortality is uniform in space. Using both actual data and simulations, we explored two approaches to the stock assessment of Georges Bank Atlantic sea scallops (Placopecten magellanicus), where fishery closures were implemented in December 1994. One approach modeled the stock in “aggregate”, using domed commercial selectivity functions for the time periods when the MPAs were closed to scallop fishing. In the second “split” approach, separate models were used for the scallops inside (closed areas) and outside (open areas) the MPAs. The aggregate model converged only in 17% of the simulated runs, compared with 93% convergence for the open and closed runs using the split approach. With actual data, and in those simulations where both methods converged, the two approaches gave similar results, although biomass estimates in the most recent years from the aggregate model tended to be biased low. The closed area model, and to a lesser extent the aggregate model, estimated natural mortality M fairly precisely, but open area model estimates of M were poorly defined. Retrospective patterns were reduced using the split approach and when natural mortality was estimated. We conclude that the split assessment approach is better for sea scallops, but it may be best to use both approaches for comparative purposes.