The common oyster and the Japanese oyster in the Zeeland delta

The exotic Japanese oyster is burgeoning in the Zeeland delta and competing with other shellfish.

Trends

The culture of the native common oyster (also called the flat oyster) was hit badly by the severe winter of 1962/1963 and by disease. In the 1970s the cupped oyster (also called the Japanese oyster or Pacific oyster) was introduced in the Oosterschelde to stimulate a recovery in oyster culture. Unlike the flat oyster, the Japanese oyster is resistant to oyster disease.

In 1982 the Japanese oyster had a high reproduction rate and the species increased greatly. Currently both the native flat oyster and Japanese oyster are being cultured on more than 2000 ha of commercial oyster beds in the Oosterschelde and Grevelingen. "Wild" beds of Japanese oysters are also increasing rapidly: there were 50 ha in 1985, 300 ha in 1990 and 600 ha in 2002.

Impact on other species

The Japanese oysters are found in large numbers mainly in sheltered areas, where they claim a large amount of the space available and fytoplankton. In certain areas over 80% of the water that is filtered by shellfish is filtered by this species. As a result, the Japanese oyster is a threat to other shellfish, both because it competes for food and because during filtering it removes the larvae of other shellfish. The flat oyster has been declining sharply since 1974. Unlike the flat oyster, the Japanese oyster cannot be opened by birds and therefore is not a suitable source of food.

The Japanese oyster is also increasing in the Waddenzee (Wadden Sea), but its biomass there is still small.

Technical note

The trends in the Japanese oyster vis-à-vis the flat oyster are derived from the statistics for the numbers of shellfish offered at auction in the Netherlands. These figures exclude the wild stocks of the Japanese oyster, which are not commercially interesting.

References

• Didde, R. (2002). De Japanse oester, Oosterse gast in Hollandse wateren. Resource juni 2002: 6-8.
• Dijkema, R. (1997). Molluscan Fisheries and Culture in the Netherlands. U.S. Dep. Commer. NOAA Tech. Rep. NMFS, 129: 115-134.
• Productschap Vis (1996 en 1998). Statistisch overzicht 1996, 1997 en 1998. Rijswijk.
• Tydeman, P., H.L. Kleef en J. de Vlas (2002). Ontwikkeling van de Japanse oester in het Eems-Dollard estuarium in de periode 1998-2001. Werkdocument RIKZ/OS/2002.601x.