A Study of Surfclam Larval Transport and Survival in the Middle Atlantic Bight Using a Coupled Physical-Biological Model

The Atlantic Surfclam (Spisula solidissima) is one of the most commercially important species along the Northeast U.S. coast. According to the assessment summary report for the surfclam fishery from the 49th Northeast Regional Stock Assessment Workshop (2010), in the southern Delmarva (DMV) and New Jersey regions where the surfclam fishery has been heavily concentrated, recruitment of new surfclams to the fishable stock is low, and commercial catch rates and stock biomass have also declined in recent years. Commercial harvesting has already been shown to be unlikely as the cause of the recent period of poor recruitment. Other potential mechanisms of poor recruitment include: 1) insufficient settlement of larval recruits from upstream sources; 2) low survival rates of surfclam juveniles after larval settlement; and 3) a northward shift of surfclam predators into the DMV region, with consequent higher mortality of surfclam juveniles.  In this study, we couple a circulation model for the Middle Atlantic Bight and the Gulf of Maine to an individual-based model for surfclam larvae to study surfclam larval transport, connectivity and survival within and among different geographically distributed sub-populations over the period 1999 through the present.  We ask whether insights from the coupled modeling study and/or available observations support any of these three potential mechanisms.