Distribution of Atlantic Surfclam (Spisula solidissima) Population Demographic Characteristics Along the Middle Atlantic Bight

The distribution of Atlantic surfclams (Spisula solidissima) along the continental shelf of the Middle Atlantic Bight (MAB) has been routinely surveyed since the 1980s by the National Marine Fisheries Service. The surfcalm length and age data obtained from the surveys were binned by decade and used to estimate latitudinal variability in growth rate, maximum length, and mortality rate. These parameter estimates show north-south differences since the 1980s, with the largest change associated with the surfclam mortality rate in the southern portion of the distribution. The maximum length of surfclams increased towards the northern end of the range, with an 87% decrease at the southern end of its range from the 1980s to the early 2000s. These demographic parameters were input to a surfclam population dynamics model to assess trends in surfclam biomass distribution along the MAB continental shelf from 1980 to the 2000s. Simulations indicate that an increase in mortality of about 7%, with no change in growth rate, results in a 16% reduction in surfcalm reproductive biomass. This level of mortality change is supported by the trends obtained from the stock survey analysis. Maintenance of contemporary surfclam densities, with this level of mortality, requires unrealistic increases in growth rates. This has important consequences for surfcalm populations because larger (older) surfclams with slower growth rates contribute to the major fraction of the reproductive biomass. An implication of these results is that environmental conditions that affect surfclam mortality rates, such as warming bottom temperatures, have a disproportionate effect on larger surfclams.