Food-limited growth and condition index in the eastern oyster, Crassostrea virginica (Gmelin 1791), and the bay scallop, Argopecten irradians irradians (Lamarck 1819)

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The growth response of the eastern oyster, Crassostrea virginica, and the bay scallop, Argopecten irradians irradians, to varying degrees of food limitation was evaluated. Under conditions of low current speed, dense assemblages of shellfish can rapidly deplete ambient food concentrations, resulting in measurable effects on growth and condition index. A flume study demonstrated significant growth and condition index responses to resource competition after reductions as small as 27% in relatively high ambient food concentrations (≃4.6 μg/l chlorophyll). Growth rates and condition index are linearly correlated with the average chlorophyll ration consumed. A field study demonstrated similar growth responses when the shellfish were cultured over a range of densities in a commercial aquaculture setting. By comparing the growth and condition index responses in the two experiments, we infer the degree of resource depletion occurring in the field from the correlations constructed in the flume study. Although physiological responses to food limitation will necessarily be site specific to varying combinations of temperature, current speed, and food concentration or quality, this work provides a unique opportunity to compare the growth response of oysters and scallops under a wide range of fold availability in both laboratory and commercial aquaculture settings. Doubling the stocking density from 2.5 to 5.0 kg of oysters per hag resulted in a 20% decrease in both the condition index and the growth rate (percent increase in weight). These observations may assist commercial growers determine optimal stocking density for their aquaculture grow-out systems. Natural food availability in Point Judith Pond, a classic salt wedge estuary, is highly variable on a daily basis and is related to the tidal exchange. The variation in food concentration superimposed on the tidal current oscillation leads to massive changes in food flux and the degree of local resource competition. Scallop and oyster clearance rates (milliliters per minute) were constant over a wide range of chlorophyll concentrations, suggesting that these species will filter natural seston at a near-constant rate despite fourfold tidal variations in food concentrations. Scallop clearance rates were reduced when chlorophyll concentrations were depleted to below 12% of the natural levels, suggesting a threshold feeding response.

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Journal of Shellfish Research