Studies on the physiological basis for over winter mortality in juvenile aquacultured hard clams, Mercenaria mercenaria (Linnaeus, 1758)

Over-winter mortality of juvenile aquacultured hard clams is a significant problem for the Mid-Atlantic and Northeast aquaculturists. Although protecting seed from predators improves survival, significant mortalities still frequently exceed 50%. This is often attributed to the severe temperatures of winter; however, this suggestion has yet to be systematically investigated. We hypothesize that extended periods of low water temperatures (<5°C) will result in reduced hard clam pumping, and thus an increase in the use of energy stores for metabolism. This would then lead to an insult in physiological condition and cause mortality in the spring when water temperatures increase, food levels are low, and metabolic demand is high.;In this study, juvenile aquacultured hard clams (SL=10mm) were planted at two sites in Jamaica Bay, New York during the fall of each year between 2001-2004, to investigate the magnitude of over-winter mortality through the winter and spring. Measurements of temperature, chlorophyll-a and clam biochemical composition were conducted to identify any correlations with over-winter mortality. Concurrent controlled laboratory experiments (using recirculating aquaculture systems) studied the effect of varying temperatures and food regimes in relation to clam metabolic processes. The field data indicate that a mild winter results in negligible mortality. Similarly, a severe winter followed by a spring in which the rise in water temperature coincides with high food levels also results in low mortality. In contrast, significant mortality (up to 45% per sample) occurs in the spring following a severe winter, at a time when water temperatures are rising but food levels are low (<3μg L-1). During this period a rapid decline of carbohydrate content is observed, suggesting the use of energy reserves to maintain metabolic activity. Mortality is associated with carbohydrate levels below 10% of the tissue dry weight. Therefore, significant mortality of juvenile aquacultured hard clams occurs when phytoplankton abundance is low as water temperature is increasing during the spring. A seemingly unrelated mortality occurred in the laboratory experiments and appears to be associated with significant loss of endogenous protein reserves.