Spread of the Pacific Oyster in the Wadden Sea. Causes and consequences of a successful invasion

Conclusions and Recommendations from the Trilateral Workshop on Pacific Oyster invasion in the Wadden Sea (22 March 2007). Interreg IIIB - HARBASINS Project

Introduction and scope of Work

The recent spread of the Pacific oyster Crassostrea gigas in the Wadden Sea is one of the most spectacular biological invasions in this ecosystem and has attracted the attention of the public as well as of the scientific community. Pacific oysters have most probably been brought to European waters since ships are sailing between the continents. They have been cultivated in various places for decades and were seemingly not adapted to prosper in the Wadden Sea. Brought out in cultures in France and the Netherlands to substitute the European Oyster which became extinct in many places, it was reported to spread soon in warm summers and form stable populations outside the cultures since the seventies which are increasing until today. Pacific oysters are found in the Wadden Sea since the eighties, where cultivation has been licensed in the Lister Deep between the island of Sylt and the mainland since 1986. As commonly reported from other biological invasions, the development in the Wadden Sea was rather slow over many years and some colonised sites were abandoned again. However, following a period of rather warm summers, Pacific Oysters have drastically increased, overgrown many native blue mussel Mytilus edulis beds and are suddenly an abundant and dominating species on the tidal flats. By forming extensive and stable reefs on the tidal flats, Pacific oysters create new habitats which may be suitable for other species living in the mussel bed community. The sudden and fast spread of this invader has raised concern about the ecological impacts. Will blue mussel beds be displaced by Pacific oyster reefs, and if so, what will be the consequences for the benthic communities, how will mussel eating birds be affected? As Pacific oysters mainly settled into native mussel beds they are also considered as a problem for mussel fisheries. On the other hand, it is not quite clear how to deal with an invader in the Wadden Sea ecosystem, a habitat protected in all parts. May fishing or even removal of Pacific oyster reefs be licensed within the Wadden Sea National Parks?

For the first time a comprehensive data compilation of the entire Wadden Sea was carried out in the framework of the TMAP to document the spread of the Pacific Oyster with special emphasis on the recent strong increase. The report aims to provide support for reporting and assessment under the EU Water Framework Directive and Habitats Directive and to support necessary updates of the Trilateral Monitoring and Assessment Program.