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Density-dependent processes may drive the population dynamics in marine species through intraspecific competition for food or space. We examined processes controlling pop ulation dynamics in the purple clam Amiantis purpurata population of Playa Villarino (north of San Matias Gulf, Argentina) at the southernmost limits of its distribution. Population structure at this location has been dominated for more than 2 decades by 3 consecutive cohorts settled in 1978−1980. We used data on mean size and mass, growth rate, area covered by residents, density and biomass collected at 2 sites between 1982 and 2003, and a survey performed in 1995 over the whole population, to explore density dependence. The spatial pattern showed that: (1) whereas density was heterogeneous with highest values at the western side of the ground, biomass was more uniform, and the relationship between biomass and density was non-linear, becoming asymptotic at a density of 200 clams m−2; (2) body size and density decreased over time; and (3) the substrate area used by clams was less than 35% of the available substrate. Temporal patterns showed that (1) density decreased with time but biomass was almost constant, and (2) growth curves were indicative of a density-dependent effect: individuals at low-density sites grew faster than individuals at high-density sites. Differences were strong between sites and slight (or absent) between cohorts. Natural mortality, which did not differ between cohorts, was estimated by the decrease in density over time as M = 0.126. We propose that the purple clam population is regulated by a compensatory effect whereby losses due to mortality are compensated by increased growth rate, and therefore enhancement of local biomass.

Growth compensation as a regulatory mechanism of purple clam Amiantis purpurata population dynamics in Patagonia