The Atlantic surfclam, a biomass dominant of the benthic ecosystem of the Middle Atlantic Bight (MAB), supports a lucrative fishery on the continental shelf of the U.S. east coast. Federal stock surveys, ongoing since the 1980s, have provided extensive demographic information for Atlantic surfclams, which show regional variability in population characteristics along the MAB. Population mortality has been identified as a process contributing to the observed population variability. To investigate mortality effects a non-linear age-dependent mortality equation was constructed and fit to the observed Atlantic surfclam age frequency distributions for the entire MAB population and to populations in three sub-regions that represent the southern, middle, and northern regions of the MAB. Age-dependent mortality rates were then obtained for populations that represent the entire MAB and the three sub-regions. The resulting age-dependent mortality rates, and demographic parameters (growth rate and asymptotic length) calculated for each sub-region using the von Bertalanffy growth function, were input to a population dynamics model to assess trends in Atlantic surfclam density along the MAB.
The age frequency pattern for the three sub-regions showed a rapid decrease of survivors at young ages, a small constant decrease at mid ages, and another rapid decrease at old ages. This pattern was represented by two mortality peaks. A small mortality peak with a maximum at about an age of 5 years was obtained for each sub-region. A larger mortality peak was obtained in each sub-region, with a maximum that moved from 20 to 25 years old for all but the middle sub-region, which reached a maximum at 30 years of age. Demographic parameters calculated for the sub-regions show an increasing south-to-north gradient in asymptotic length and growth rate of 7.5% and 31.4%, respectively.
Simulations showed that the mean density of the Atlantic surfclam population reached lower equilibrium densities for age-dependent mortality relative to constant mortality. Similar results were obtained for the mean densities per size classes. Age-dependent mortality rates produced a gradual decrease of survivors throughout all size classes; whereas constant mortality rates resulted in an initial decrease of densities that remained constant until the last size class where densities dropped close to zero. Comparison of simulated and observed Atlantic surfclam densities was done using root mean square error and bias was estimated with mean percentage error. Both statistical indices showed values close to zero for simulations that used age-dependent mortality rates. These results suggest that Atlantic surfclam populations experience age-dependent mortality and inclusion of this morality is needed to accurately represent population densities and trends on the MAB continental shelf.