Campagne d'évaluation du stock de palourdes du bassin d'Arcachon - Année 2006

Between 1980 and 1986, some Arcachon oyster farmers took up breeding the Japanese clam on the foreshore (Robert et Deltreil, 1990). This production was short lived. The steep fall in prices due to economic competition from Spain, Portugal, Italy and Tunisia, predator action such as the trigger fish (Popovsky et al., 1999), added to the significant amount of mortality that occurred in the summer and autumn of 1992 that affected two-thirds of the total stock, led to clam farming being abandoned at the end of the 1980s. In the Arcachon basin, the Japanese clam (Ruditapes philippinarum) population, now natural to the area, has found favourable conditions for development and is expanding rapidly there. This species has been supplanting the local species (European clam, Ruditapes decussates) since the beginning of the 1990s (Auby, 1993). Out of a sector of 24km2, Bertignac et al. (2001) has shown that 94% of the size and more than 97% of the biomass are occupied by the Japanese clam in 2000. Out of an area of 46km2, Caill-Milly et al. (2003) estimated these sizes1 at, respectively, 98% and 99%. This development generates significant fishing economic activity (mainly by shellfish gatherers). The first official catch statistics date from 1992. They highlight first of all an increase in yields up to 1997 (455 tons declared), then a reduction, which sets it at about 300 tons declared in 2002 and finally a rise in the take these last few years (figure 1). In 2004 (latest available data), the total declared landings rose to 520 tons. They only apply to professional fishermen and coast-fishing shellfish farmers (CPP). In addition, oyster farmers' catches are estimated by Bobo (1996) at 25 tons in 1995. For leisure fishing, we do not have any information available. For fear of overexploitation, measures for managing the resource were taken starting in 1996. These measures consisted of establishing licenses (36 in 1996, 70 issued in 2006) and closing certain zones to fishing (prefectorial order of the Aquitaine region no. 170 of 30 June 2000, of 28 June 2002, of 13 December 2002 and of 22 March 2006). Within this context, two clam stock assessment campaigns were undertaken by IFREMER (2000 and 2003). The first covered more than a third of the distribution area (24km2), the second, all of the distribution area that could be investigated with a Hamon basket (46km2). These two campaigns mainly highlighted: - the predominance of the Japanese clam in relation to other species (98% of the total size and 99% of the biomass); - a high total biomass, since it is estimated at 8,095 tons (+/- 1,046 tons) at the beginning of July 2003. For the clam, the Arcachon basin is therefore at the head of exploited French zones having the advantage of a stock assessment ahead of the Gulf of Morbihan and the Thau pond; - an equally high exploitable biomass, assessed at 2,730 tons in 2003, of which 2,210 tons are accessible because they are located outside the three reserve zones that year. The exploitable portion in biomass is akin to those obtained at the time of the last assessments conducted in the Gulf of Morbihan. - a size distribution characterised by lower lengths than those observed at other sites, a small proportion of juveniles and a large proportion of individuals of sizes between 25 and 37mm2. These items are merely a snapshot and are insufficient for guiding management. In the absence of any explanation for the existence of these imbalances, it was therefore decided in parallel with these studies to launch investigations on the variability of clam's growth intra-basin. Financed by the Common Fund of Aquitaine/Euskadi and the Regional Council of Gironde over 3 years, they should make it possible to estimate the parameters of population dynamics for the species on two study sites: Arcachon basin and Mundaka estuary in Spain. The principle collaborators of this operation3 are IFREMER (LRHA Bidart and LER Arcachon), the LOB of Arcachon (University of Bordeaux I), the LMA (University of Pau and Pays de l'Adour), the Cellule Cereca/Adera and AZTI. It is within this context that a third campaign was organised at the request of the CLPM of Arcachon, who brought the case. The participants were IFREMER, the CLPM and the Cellule/Adera. The professionals formed strong partnerships, particularly through the study's steering committee. They also participated actively in the samplings. This report furnishes the results of the 2006 clam stock assessment. For the information on the biology of the species, see the report of the 2003 campaign (Caill-Milly et al., 2003).