To Split or Not to Split? Assessment of Georges Bank Sea Scallops in the Presence of MPAs

MPAs create challenges for stock assessments because most models are based on the assumption that fishing mortality risk is uniform in space. Here we explore two approaches to the stock assessment of Georges Bank Atlantic sea scallops (Placopecten magellanicus), where fishery closures were implemented in December 1994, that differ in biological realism and model complexity. One approach is to model the stock in aggregate, using domed commercial selectivity functions for the periods when the MPAs were fully closed to scallop fishing. The second approach is to construct separate models for the populations inside and outside the MPAs. The latter requires splitting landings inside and outside the MPAs prior to the MPA’s establishment, and also requires  assumptions regarding the level of illegal landings within the MPAs after their establishment.  Both these procedures carry considerable uncertainty. Results from the two analytic approaches are similar, in part due to the priors imposed on survey catchability. However, the split models allow for examination of the responses to the MPAs and other measures both inside and outside the MPAs. Additionally, an estimate of natural mortality can be obtained from the model of the MPAs. While arguments can be made for each of the two approaches, exploring both can give more confidence in the results.