Oyster Imports as a Vector for the Introduction of Alien Species into Northern and Western European Coastal Waters

Abstract

In western and northern Europe there have been deliberate introductions of European flat oyster (Ostrea edulis), American oyster (Crassostrea virginica), Pacific oyster (C. gigas, including the so-called Portuguese oyster ‘C. angulata’), New Zealand oyster (Tiostrea lurida), hard clam (Mercenaria mercenaria), and Manila clam (Tapes philippinarum). Between about 1870 and 1939 tens of millions of Crassostrea virginica were introduced from the Atlantic coast of North America. However, C. virginica has been unable to establish itself in Europe. For 5 other species it is very likely that they have been introduced with American oysters. Between 1964 and about 1980 C. gigas was imported on a large scale from Japan and the Pacific coast of Canada and the USA. It has established itself in Europe permanently. C gigas brought its own parasites and the imports were accompanied by the import of more than 20 species of animals. Most of these observed imports failed, however, and only about 5–6 species seem to have established themselves in European waters. As a vector for the introduction of exotic species into the North Sea area, oyster imports are slightly more important than transport on ship’s hulls, and clearly more important than introductions through ballast water. In the Dutch Oosterschelde estuary Japanese oysters interfere with the recreational use of the estuary because of their razor-sharp shells. They also seem to have changed the ecological conditions in the estuary: coinciding with the increase of the oysters, mussels and cockles decrease, as does the oystercatcher (Haematopus ostralegus). It is not yet clear if this is a causal relationship. In the Wadden Sea near the island of Sylt, C. gigas established itself as an epibiont on mussel beds, and seems to be at the verge of transforming mussel beds into oyster reefs.