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The Manila clam population in Arcachon Bay (SW France): Can it be kept sustainable?
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The venerid clam Ruditapes philippinarum is the most prominent suspension-feeding bivalve inhabiting muddy intertidal seagrass beds in Arcachon Bay (SW France). It is exploited by fishermen, and Arcachon Bay ranks number one in France in terms of production and total biomass of this species. Previous studies revealed a decrease in the standing stock of R. philippinarum since 2003 and unbalanced length-frequency distributions with a lack of juveniles and of adults >40 mm. Consequently, the population dynamics of this bivalve were studied at four intertidal sites and one oceanic site in Arcachon Bay. As clam size structure did not allow classical dynamics computations. field monitoring was coupled with field experiments (tagging-recapture) over two years. Monitoring of condition index and gonadal maturation stages highlighted a high variability in spawning number and intensity between sites. Recruitment events in the exploited area varied spatially but with uniformly low values. Von Bertalanffy Growth Function (VBGF) parameters (K, L-infinity) were determined using Appeldoorn and ELEFAN methods. In the exploited sites in the inner lagoon, K was relatively high (mean = 0.72 yr(-1)) but L was low (mean = 41.1 mm) resulting in a moderate growth performance index (Phi' = 2.99). Growth parameters were not correlated with immersion time and L-infinity was different between sites. Comparison of mortality coefficients (Z) between cage experiments and field monitoring suggested that fishing accounts for 65-75% of total adult mortality. Low recruitment a low growth rate and a normal mortality rate led to low somatic production (4.1 and 8.7 g Shell-Free Dry Weight (SFDW) m(-2) yr(-1)) and an annual P/B ratio from 0.44 to 0.92 yr(-1). Under current conditions, the possibility of a sustainable population in Arcachon Bay will strongly depend on recruitment success and fishing management.
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