In 2000, the northern quahog (= hard clams) Mercenaria mercenaria population was surveyed in Raritan Bay with the purpose of determining sustainable harvest levels. To complement this population survey, we determined the size-at-age structure and experimentally determined mortality rate and size specific growth of adult clams. Clams of a range of sizes, obtained from the sampling program, were measured, cleaned, and aged by counting growth rings in sectioned shells. Experimental plots were established in the low intertidal zone at two sites in the Raritan/Sandy Hook Bay system. Marked clams of five sizes were planted in three seasons and harvested quarterly. Experimental estimates of mortality and survival were based on collected live and dead individuals and are thus conservative because they do not address the numbers missing. Some of the clams from both sites were removed from the area by predators. Estimated mortality for individuals >25 mm by instantaneous rate yielded a mean of 0.0176. Integrating the size specific information with the size-frequency distribution from field survey yielded an average instantaneous mortality rate of 0.0187. Growth, based on the difference between the mean size planted and the mean size of the same size class retrieved was analyzed with a general ANOVA, and exhibited typical seasonal growth. The smallest size individuals grew faster than larger individuals. Survey data indicated an increasing clam population and increasing harvests. The survey mortality estimates, based on box counts, seem to overestimate losses. Our experimental work suggests adult mortality rates of nearly 2%, but loss of individuals from the plots made computation of exact mortality rates difficult, and 2% probably underestimates adult natural mortality rates. The results indicate that current levels of fishing mortality are sustainable with 3% natural adult mortality, but a natural adult mortality rate just above 5% would reduce the population growth to near zero. This information is important, because there has been interest in establishing additional depuration facilities to take advantage of the clam population and put more people to work. To sustain current levels of harvest, it will be essential to increase population level monitoring activities over time to assure the population is not being over harvested because of slight changes in recruitment or mortality rates.
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Vol. 28 • No. 2