Since the 1960s Pacific oysters(Crassostrea gigas) were imported to the North Sea coasts for aquaculture. The species was introduced to replenish the stock of the indigenous European oysterOstrea edulis which was strongly diminished in the past by overexploitation, diseases and severe winters. The introduction of this non-native species was regarded as uncritical due to the low water temperatures of the North Sea which would not have promoted natural reproduction and settlement. Nevertheless, strong spatfall of culturedC. gigas was recorded in several years in the Oosterschelde estuary (The Netherlands). From these spatfalls wild populations developed in the vicinity of the culture plots. Since the early 1980s Pacific oysters were also observed in the Wadden Sea area near Texel. The pelagic oyster larvae can bridge the distance between the Oosterschelde estuary and Texel by drifting with the residual currents which, at westerly winds, may transport them about 4 to 5 NM per day. Extreme spatfalls from culture plots were also recorded in the backbarrier tidal flats of the North Frisian Islands Sylt and Romo in 1991, 1994 and 1998. Settlement occurred predominantly on intertidalMytilus beds. The extended wild populations resisted two severe winters which were characterized by a drift-ice coverage lasting several weeks. In the backbarrier tidal flats of the East Frisian Islands the first specimens ofC. gigas were found in August 1998 onMytilus beds. In contrast to the wild oyster populations from the Oosterschelde and Sylt, the settlement ofC. gigas in the study area occurred far away from culture plots. In the following years, further records indicate a continued introduction of oyster larvae. This implies that the distribution gap ofC. gigas along the Wadden Sea coast is now closed.
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