Longterm Dynamics in Atlantic Surfclams: The Role of Bottom Water Temperature

The Atlantic surfclam (Spisula solidissima) fishery in the Middle Atlantic Bight (MAB) continental shelf is one of the most valuable single species US commercial fisheries.  Since 1997, populations from southern inshore regions of the clam’s range have experienced significant mortality events, which have coincided with a general warming of bottom water temperatures (reaching 21-24˚C in September) along the MAB.  The potential linkages between warming bottom temperatures and increased surfclam mortality and/or reduced growth are being addressed using a model that simulates the growth of post-settlement surfclam populations at specific locations on the MAB shelf.  External forcing for the growth model is provided by a 50-year simulation (1958-2007) of bottom water temperature obtained from an implementation of the Regional Ocean Modeling System (ROMS) for the MAB continental shelf.  The simulations show that surfclams experience high mortality (mortality > 0.20) during years when bottom water temperatures remain 1-2C above average conditions for an extended time at each location.  Further, simulations show that the magnitude of mortality is moderated by the existing population structure such that older, larger populations experience higher mortality than smaller, younger populations.  These results provide a basis for analysing the mechanisms responsible for long-term changes in surfclam abundance distribution along the MAB.