Restoration of oyster production in France by introduction of non-native species: History and overview

Before 1850, when oyster farming started in France, Ostrea edulis, the local flat oyster, had been harvested for several centuries. The Portuguese cupped oyster, Crassostrea angulata, was introduced into the Gironde estuary in 1868 when a ship from Portugal discarded an entire shipment of oysters considered too old. Populations of C. angulata developed on the French coast, sharing space with O. edulis until 1970. A viral infection (iridovi-rus) caused the total loss of C. angulata between 1971 and 1973. Decreased production of O. edulis from 20,000 mt in 1970 to 10,000 mt in 1973 was attributed to the protozoan parasite, Mar-teilia refringens. In 1979, appearance of another protozoan parasite, Bonamia ostreae, decimated production of O. edulis, and today there are only 2,000 mt per year. The Pacific oyster, Crassostrea gigas, was first introduced to France for experimental studies in the Marennes-Oleron and Ar-cachon basins, in 1966 and 1969; growth performance was excellent. During 1971 to 1974, following the collapse of C. angulata, more than 130 mt of Pacific oysters were brought to Arachon. Since 1982, successful natural reproduction and spat collection eliminated need for further importations to sustain C. gigas production. C. gigas resists endemic parasites in French growing areas; however, massive summer mortalities have occurred recently. Research is underway to understand the causes of these mortalities; intermediate results implicate a weak status of oysters during gametogenesis when the temperature is high. Changes in cultural practices, selection of resistant strains and development of sterile (and fast growing) triploid oysters are believed to be eventual solutions to mitigate summer mortalities.