Estuary-Dependent Growth Differences in Farmed Northern Quahog (Mercenaria mercenaria) in New Jersey, with Implications for the Aquaculture Industry

The northern quahog (hard clam), Mercenaria mercenaria, is an economically important shellfish species in New Jersey, yet farmers in some parts of the state recently have experienced reduced growth in cultivated clams. To identify possible reasons for this slowed growth, clams from two farms were compared, where one farm had been experiencing reduced growth and the other had not observed changes. Growth differences were examined between the two areas, both within a single season and as shown by yearly marks on shells collected at both sites. Clam condition was also measured across the growing season, as well as environmental conditions (bottom temperature and current speed) at each site. Clams in the unaffected area reached market size 1 y earlier than those in the area with reduced growth. Analysis of yearly rings showed that growth at the unaffected farm was also higher than in wild clams near either farm, implying that husbandry practices may contribute to enhanced growth. This was supported by clam growth measurements at standardized stocking density on both farms, which showed a smaller but still significant difference in seasonal growth between farms. Bottom water temperature was not different between sites, but current speed differed seasonally. Other important factors affecting growth may include food availability and carrying capacity. The data show that overstocking most likely contributes to the reduced growth. It is hoped that this information can help farmers make informed decisions about farm management when growth issues are observed.