Population dynamics of the Argentinean surf clamsDonax hanleyanus and Mesodesma mactroidesfrom open-Atlantic beaches off Argentina

The surf clams Mesodesma mactroides Reeve, 1854 and Donax hanleyanus Philippi, 1847 are the two dominating species in macrobenthic communities of sandy beaches off northern Argentina, with the latter now surpassing M. mactroides populations in abundance and biomass. Before stock decimation caused by exploitation (during the 1940s and 1950s) and mass mortality events (1995, 1999 and 2007) M. mactroides was the prominent primary consumer in the intertidal ecosystem and an important economic resource in Argentina.Since D. hanleyanus was not commercially fished and not affected by mass mortality events, it took over as the dominant species, but did never reach the former abundance of M. mactroides. Currently abundance and biomass of bothsurf clams are a multiple smaller than those of forty years ago, indicating the conservation status of D. hanleyanus and M. mactroides as endangered.Therefore the aim of this study is to analyse the population dynamics (population structure, growth and reproductive biology) of D. hanleyanus and M. mactroides, and to compare the results with historical data in order to detect possible differences within surf clam populations forty years ago and at present.In conclusion, although the commercial fishery of M. mactroides is closed since more than 50 years and extractions are still forbidden and D. hanleyanusis not used as a natural living resource yet, the strong decline of surf clam abundance and biomass within the last four decades implies that populations of the Argentinean D. hanleyanus and M. mactroides are in unstable conditions,due certainly to the increasing anthropogenic impact.