Etude et modélisation de la dynamique des populations de bivalves en élevage (Crassostrea gigas et Mytilus galloprovincialis) dans le bassin de Thau (Méditerranée, France) et des ascidies solitaires associées

The total standing stock of bivalves cultured in Thau Lagoon (France) that is mainly constituted of the Pacific oyster, Crassostrea gigas, and the Mediterranean mussel, Mytilus galloprovincialis, vary between 14,000 and 20,000 tons within a year. These variations were attributed to the rearing cycle of molluscs in the lagoon but also to their growth performances. Oysters cultured in the Thau Lagoon presented the best growth rate ever reported whereas trophic conditions are low : mean concentrations in organic particulate matter and total chlorophyll a averaged 1 mg.l-1 and 1.2 µg.l-1, respectively. The development of a growth model shows that temporal pattern (i.e. seasonality) of bivalve growth is explained by water temperature and spatial pattern is governed by concentration in organic particulate matter. Quantitative and qualitative variations of bivalve standing stocks are simulated and the marketable production is estimated with a deterministic population dynamics model. This model takes into account rearing strategies of farmers by including for instance seeding and harvesting timetables. Rearing strategies were studied through interviews of oyster and mussel farmers. A technical diagnostic of the production system used in Thau Lagoon and a typology of shellfish farms were also realised. The population dynamics model was calibrated on standing stock and distribution data recorded in 2000 and 2001. The marketable production of oysters was assessed at ca. 17,900 tons between March 2000 and March 2001, implying a ratio of 1.73 between production and biomass (P/B). For mussels, the production was estimated at 5,400 tons at equilibrium for a P/B ratio of 1.38. Cultured bivalves support large quantities of epibiota, among which solitary ascidians. Their biomass varied between 1,200 and 3,600 tons between 2000 and 2001. A growth model that was developed for the species Ascidiella aspersa (more abundant species observed in 2000-2001) and a stochastic model to represent the recruitment of ascidians on oysters were linked with the oyster population dynamics model to derive ascidian standing stock and production over time. The marketable production was assessed at 2,900 tons only for ascidians associated to « collées » oysters (i.e. corresponding to one culture method used in the lagoon). Population dynamics of filter-feeders were also coupled with ecophysiological equations to evaluate their relative impact on the ecosystem of Thau Lagoon. The higher impact comes from oysters at the scale of the basin but mussels have a high impact at the local scale of rearing structures.