The Pacific oyster is a delicious shellfish, but it has also become a dominant species in Dutch coastal waters, where it occurs locally in massive numbers. IMARES is conducting a study into the influence of this exotic species on the ecosystem.
Photo: Researchers on foot determine the density of an oyster beds at low tide in the Oosterschelde
The exotic Pacific oyster was introduced to the Oosterschelde as an alternative cultured species for the native flat oyster, which was suffering from disease. The new species thrived and expanded rapidly into the waters of the Delta and the Wadden Sea.
Oysters filter seawater in order to feed themselves with algae. Because they can filter a relatively large volume of water per hour, Pacific oysters have become a major food competitor for other shellfish. In the Wadden sea, the Pacific oyster has expanded greatly on existing mussel beds, but oyster beds have also developed at other locations, for example in areas with large amounts of shell material or on old cockle beds. The oysters and mussels can apparently coexist perfectly well in mixed beds. In the Oosterschelde, oyster beds actually enable mussels to settle on the tidal flats. Many other species seek protection or food in oyster beds.
Since the introduction of the Pacific oyster in the 1960s, the species has only been used commercially for oyster farming. In 2010, on behalf of the Ministry of Economic Affairs, IMARES began a study to determine if the wild oyster beds could also be exploited commercially, but with manual harvesting only. The aim of the study is to indicate whether harvesting wild oysters is economically feasible and whether it causes damage to natural habitats.
In the Delta region and the Wadden Sea, IMARES is conducting a survey on the tidal flats and in the channels to determine the biomass and area of the oyster population. The deep channels are surveyed with specially developed apparatus, while the tidal flats are surveyed on foot at low tide. In this way, the total quantity of oysters and their distribution can be determined. The survey will not only help monitor the manual harvesting (the oyster 'fishery'), but also the development of this exotic species and the possible consequences for the ecosystem.