SPATIAL AND TEMPORAL VARIABILITY OF ATLANTIC SURFCLAM (SPISULA SOLIDISSIMA, DILLWYN 1817) POPULATION DEMOGRAPHICS ALONG THE MIDDLE ATLANTIC BIGHT

Trends in Atlantic surfclam (Spisula solidissima) population demographic parameters were analyzed using age and length observations obtained from NOAA stock surveys conducted from the 1980s to 2010s in six regions distributed along the Middle Atlantic Bight (MAB) continental shelf. Atlantic surfclam asymptotic length and specific growth rate were estimated for each survey region and decade using the von Bertalanffy growth function. Specific mortality rates were estimated using a linearized negative exponential relationship and the maximum Atlantic surfclam age observed in each survey region for each decade. The estimated Atlantic surfclam mean length in the southern regions of the MAB decreased from 127 to 103 mm, about a 19% decrease, over the four decades. The mean length remained stable at about 130-135 mm in the central survey regions in contrast to the mean length of the Georges Bank Atlantic surfclam population which showed an increase from 101 to 135 mm over the four decades. The asymptotic length estimated for the southernmost survey region declined by 20% and remained relatively constant for the other survey regions over the four decades. Estimated specific growth rates remained unchanged for most of the survey regions, with the overall regional mean decreasing from about 0.25 y(-1) in the 1980s to 0.20 y(-1) by the 2010s. Specific mortality rates were not significantly different across the survey regions, although the maximum age estimated for the population in the southern survey region decreased by about 36%. This study provides a quantitative assessment of past and ongoing changes in MAB Atlantic surfclam population demographics that can be used to develop projections of responses to stresses imposed by climate change and commercial fishing.