Growth variability in a metapopulation: The case of the southern geoduck (Panopea abbreviata)

Oceanographic conditions modulate the growth of bivalves. In Northern Patagonia, Argentina, seasonal thermohaline fronts define 3 oceanographic domains in the San Matías Gulf (SMG) and the San José Gulf (SJG), which create different environmental conditions for growth. We studied the variability of the growth pattern in a metapopulation of the geoduck clam Panopea abbreviata within and between oceanographic domains. P. abbreviata forms spatially disjunct subpopulations in the SMG and the SJG. Growth patterns were determined using size-at-age data from counts of inner shell layers for two subpopulations in each oceanographic domain. We used a multimodel inference approach to describe the growth pattern, fitting five candidate growth models (von Bertalanffy, logistic, Gompertz, Richards, and Schnute-Richards). Growth was fast during the first 8-10 years and slowed in later years. This pattern was consistent with the pattern described in studies for other Panopea species. Larger individuals of P. abbreviata were more abundant in mud-sandy places (Punta Conos and Playa Fracasso, SJG's eastern coast) than in locations with sand sediment (Punta Colorada and Puerto Lobos, SMG's western coast). Multimodel inference is a robust way to estimate growth parameters for this species. We found differences among locations, independent of the domains. The results suggest that local environmental conditions are greater growth modulators than general oceanographic conditions. The differences in growth observed among locations should be considered when developing biological reference points specific for each location.