Introductions and developments of oysters in the North Sea area: a review

To replenish the exploited native stocks of Ostrea edulis, imports from almost all European coasts have arrived in the North Sea, particularly in the Dutch Oosterschelde estuary. The American oyster Crassostrea virginica and the Portuguese oyster C. angulata have also been imported into the North Sea several times. However, only the introductions of various genetic strains of the Pacific oyster C. gigas have been of lasting success. Spat from British Columbia (Canada) was first imported to the Oosterschelde in 1964. First spatfalls in this area took place in the warm summers of 1975 and 1976. Further larval outbursts occurred in 1982 and 1989, and good settlements took place on culture plots as well as along the dikes of the Oosterschelde. Since 1977 no more cupped oysters have been imported from overseas. The population maintained itself and was able to spread in a northern direction along the Dutch North Sea coast. In Britain, hatchery-produced C. gigas were transferred to several sites, in the 1970s including the British North Sea coast. Here, occurrence in the wild seems to be rather limited up to now. Into the German Wadden Sea, C. gigas larvae and spat from a Scottish hatchery have been introduced since 1971, as were medium-sized oysters from a variety of European sources in the 1980s. Strong spatfalls on intertidal mussel beds in the northern German Wadden Sea occured in 1991 and 1994. For the introductions of C. gigas along the west European coasts, precautionary measures to minimize unintentional transfers of associated organisms, as recommended by the ICES Code of Practice in 1994, came too late.