Pacific oysters invade mussel beds in the European Wadden Sea

Abstract

Near the island of Sylt in the North Sea, importedCrassostrea gigas are grown on oyster trestles. These oysters reproduced successfully, and strong spatfalls occurred in 1991 and 1994 on natural mussel beds. The wild oyster population comprised about one million in summer 1995. Resampling in 1996 revealed a survival of 66% in spite of a foregoing severe winter. Oysters growing on mussel beds attained a length of 20 to 50 mm in their 1st year and 50 to 80 mm in their 2nd. Some of the oysters were larger and presumably much older. Abundance was highest (up to 8 oysters m−2) on exposed mussel beds at low tide level, not covered by algae. It is expected thatC. gigas will become a permanent member of the biotic community in the Wadden Sea. It does not invade the vacated niche of the regionally extincted EuropeanOstrea edulis but lives as an epibiont on densily packed mussels.C. gigas here consitutes an r-selected species invading a crowded community in an undisturbed habitat. Through oyster imports, the culture site served as a gateway for 5 other, inadvertently introduced species into this part of the Wadden Sea.