Assessing the production and the impact of cultivated oysters in the Thau lagoon (Mediterranee, France) with a population dynamics model

The Thau lagoon (France) has been studied for many years because of its eco logical interest related to economic activities: shellfish cultivation, tou rism, and industry. The standing stock of cultivated filter feeders is arou nd 20 000 t and consists of two main species, the Japanese oyster Crassostr ea gigas and the Mediterranean mussel Mytilus galloprovincialis. To predict changes in the standing stock and the annual production, a mathematical mo del of the population dynamics was defined. It was based on the continuous equation of the density as a function of the mortality rate, the individual growth rate, and the interindividual variability. The daily growth rate wa s derived from field surveys and depended on the phytoplankton concentratio n and individual weight. The model also took into account the rearing strat egy of the producers defined by the timetable of seeding and harvesting, ob tained by an inquiry among the producers and used to simulate real cases of standing stock changes. The model was also used to assess the potential im pact on the environment through the computation of the food consumption, wh ich was compared with the residence time and the primary production.