Population dynamics of Corbicula fluminea (Bivalvia) in the Parana River Delta (Argentina)

Benthic individuals and drifting planktonic juveniles of Corbicula fluminea were sampled in the Lower Delta of the Parana River (Argentina) at monthly intervals between October 1995 and October 1996. Densities of settled darns above 1 mm, around 300-1000 ind. m(-2), varied little throughout the year. Below I mm benthic juveniles, on the other hand, showed a single conspicuous abundance peak in October-November (up to 1722 ind. m(-2)), and were practically absent during the rest of the year (overall annual mean: 1070 +/- 797 ind. m(-2)). Drifting juveniles showed one major peak in December 1995 (160 ind. m(-3)), and a minor one at the end of March (24 ind. (-3)). Length-frequency analyses of the monthly field data clearly indicate a highly structured population with a single well defined reproduction period centered on October-November, Population parameters derived from the seasonally oscillating version of von Bertalanffy's growth formula were as follows: maximum shell length: 32 mm (maximum observed shell length. 33 mm); growth constant: 0.65; growth seasonality (i.e.. span of summer-to-winter growth-rate difference): 0.7; winter point (i.e., time of the year when growth is minimum): 0.5 (June-July); estimated size range for one year-old individuals: 15.3-22.4 mm; for two years: 23.5-27 mm, and three years: 27.5-29.3 mm. Comparison with previous data confirm the influence of water temperature on the clam's reproduction and growth and furnish additional evidence suggesting that food availability may be as important fur recruitment as thermic regimes. Contrasts between traits of the population analyzed herein with those described previously from a nearby (ca. 10 km) site more influenced by industrial pollution point at the influence of water quality on Corbicula's recruitment and growth.