Temporal variability in the Pacific oyster spatfall along the French Coast: a new monitoring project

In France, spatfall of Pacific Oysters occurs traditionally in two shellfish area of the south west of France: the Marennes-Oleron bay and the Arcachon basin. The spat collected in both sites ensures approx. 70 % of the need of the French oysters production (the other part is produced in hatcheries). But, since ten years, recruitment have become highly variable especially in Arcachon basin. For instance, in this site, spatfall (assessed at the end of the reproductive season) can vary between 50 oysters per collector to more than 20 000 oysters per collector (Figure 1). This huge variation arise questions from oysters producers and the French National Committee for Shellfish Culture want to get some explanations of the phenomenon. It is well known that recruitment of bivalves shows large variability in time and space, depending directly on environmental factors such as temperature, phytoplankton or salinity or more indirectly on climatic conditions. For Pacific oysters, previous regional studies are available in Marennes oleron and Arcachon bays and show clearly that the reproduction of this species is highly dependent on temperature, phytoplankton (quality, quantity) especially in spring for gametogenesis and in summer for spawning and larvae survival. But these studies were only regional and this idea is now to approach the phenomenon on a larger scale. In the other side, there is now evidence of the ecological impacts of recent climate change in many environments and for many species including some European bivalves (e.g. Macoma balthica, Philippart et al., 2003) and a similar hypothesis can also be formulated for the Pacific oyster in France, since the reproduction ability of this species seems to move to the north of France. New sites are now investigated for spat collecting by oysters producers (e.g. Rade de Brest). In that context, a national monitoring project is currently starting at Ifremer in 2008...