Differences in individual growth rate among scallop ( Chlamys tehuelcha (d'Orb.)) populations from San José Gulf (Argentina)

Differences in mean individual growth rate between populations of Chlamys tehuelcha (d'Orb.) from San José Gulf (Argentina) have been reported. These differences may reflect an environmental gradient in food availability. The objectives of this study were (a) to document statistically differences in scallop growth between several grounds of San José Gulf and (b) to discuss the consistency of the pattern with two alternative hypotheses: (H1) a gradient in food availability and (H2) density dependence. Two types of data were used to estimate growth rate: growth rings (five grounds) and the monthly average size (during 3 years) of closely monitored strong cohorts (three grounds). The biomass and density of all scallop year classes, temperature and chlorophyll a were recorded monthly in the three grounds monitored. Carrying capacity (calculated annual production of bivalve meat, given net annual phytoplankton production) and annual production of scallop meat were estimated in the three grounds where cohorts were monitored. Statistically significant differences in individual growth rate were detected between populations. Growth was greater in the N and SW, and less in the SE of the Gulf. Environmental data were consistent with the food availability hypothesis. Growth rate was not correlated with mean scallop biomass (annual or 3 year averages). Estimated carrying capacity was lower in the SE than in the SW and N of the Gulf. Measured annual production of scallop meat and calculated carrying capacity were similar at each locality, independent of biomass. Intrapopulational variation in growth due to density dependence is very small compared to interpopulational differences due to food availability.