POPULATION DYNAMICS OF THE YELLOW CLAM MESODESMA MACTROIDES FROM A HIGH-ENERGY, TEMPERATE BEACH IN NORTHERN ARGENTINA

HERRMANN, M.; JOSE E. FERNANDEZ ALFAYA; LEPORE, M.L; PENCHASZADEH P.E.

PUERTO MADRYN

Congreso; VIII CONGRESO LATINOAMERICANO DE MALACOLOGIA; 2011

CENTRO NACIONAL PATAGONICO

The yellow clam Mesodesma mactroides Reeve, 1854 (Bivalvia: Mesodesmatidae) wasonce the most abundant intertidal species on the Atlantic coast of northern Argentina and animportant commercial resource in South America. This study of a population inhabiting theintertidal zone of the sheltered-dissipative sandy beach Santa Teresita documents the species population biology, including demographic structure, growth and production duringDecember 2004 and December 2006, and adumbrates the critical state of M. mactroides atpresent. A total of 3,015 yellow clams were collected and measured, whereas individualswere found with an anterior?posterior shell length between 2 and 64 mm. A von Bertalanffygrowth function with an asymptotic length (L∞) of 85 mm and a growth constant (K) of 0.47 year-1 was established from length?frequency distributions. The longevity of the species is estimated at approximately 6 years, and instantaneous mortality rate was about three times higher than 40 years ago. Besides, this study confirmed that the overall growth performance index (OGP) is habitat-specific and can be used to group M. mactroides and M. donacium from different areas into temperate and upwelling species. Furthermore, OGP is inversely correlated with the latitudinal distribution of Mesodesma populations. The intertidal biomass ranged between 0.06 and 0.07 g AFDM m-2 year-1. Individual production was observed to be highest at 47 mm length (0.35 g AFDM m-2 year-1), and annual production ranged between 0.12 and 0.19 g AFDM m-2 year-1, resulting in productivity values (P/B) between 1.84 and 2.93. The comparison of the results of the present study with those of growth studies conducted on M. mactroides 40 years ago revealed the following considerable differences in the population structure of M. mactroides, indicating the conservation status of this intertidal bivalve as endangered: (1) present growth rates are faster, but that the maximum length attained has decreased, (2) the numbers of individuals per square metre were many times higher in the past than in the present, (3) bivalves from the present work never reached the ?commercial size? of 60 mm and (4) 40 years ago, the population of M. mactroides was composed of up to three cohorts, whereas in this study, there was only one single cohort visible.