The Danish Wadden Sea endangered by pacific oysters


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By Mads Sønnegaard Poulsen, August 24, 2014

Pacific oysters are an invasive species and it threatens to disturb the entire ecosystem in the Wadden Sea. Orbicon has been responsible for the risk assessment published by the Institute of Marine Research in Bergen.

If nothing is done, Pacific oysters can disturb living conditions for animals and plants in the Danish Wadden Sea, which is one of our most important natural resorts. Pacific oysters are an invasive species and it threatens to disturb the entire ecosystem in the Wadden Sea.

– Pacific oysters particularly attach themselves to common mussels, and if a large amount of common mussels disappears it will affect bird life, since common mussels constitute a large part of their food basis. The Wadden Sea is very important to e.g. migratory birds, which means consequences of spreading of the pacific oysters can be far-reaching, tells Per Dolmer from Orbicon, who recently published a report and risk assessment of the Pacific oysters’ spreading.

The report was drawn up in cooperation with Nordic experts from the Institute of Marine Research in Bergen, University of Gothenburg and Roskilde University, and the survey shows that the Wadden Sea is exposed to large ecological changes in the next few years. The combination of a bioinvasion of Pacific oysters and climate changes makes the area very vulnerable.

Spread all over Scandinavia

Pacific oysters are now discovered more and more places in Scandinavia. Scandinavian scientists have – with support from The Nordic Council of Ministers – studied the distribution and spreading of Pacific oysters. Furthermore, they have examined the possible negative consequences of the species’ invasion of different environments, and looked at the impact in correlation with the different scenarios for climate changes.

Orbicon has been responsible for the risk assessment, published by the Institute of Marine Research in Bergen. The spreading of the pacific oyster is problematic on several locations in Scandinavia, but in Denmark it is mostly The Wadden Sea, which is affected.

– Pacific oysters are fond of places with large water flow. It simply gives them more algae to nourish on. And places with large tidal difference are ideal habitats. This is among other things why the Wadden Sea is an ideal habitat for Pacific oysters in Denmark – unfortunately is also at the same time one of the most important and most vulnerable habitats, says Per Dolmer.

The report mentions a high risk of Pacific oysters developing into a bioinvasion, which will chance the ecological system. The risk assessment can be used to develop an administration strategy of the habitats where pacific oysters have settled or where the species might invade in the next decades.

Eat more oysters

It has now been established that the spreading can cause large impact on the ecological system, but what can be done about it?

– One way is simply by harvesting a lot of oysters in the Wadden Sea and keep the population down this way locally. After some time the harvested oysters’ quality will go up, and a commercial fishing industry is possible, which would create jobs and keep the population down at a level not destroying the ecosystem, Per Dolmer points out.

Per Dolmer is in the process of examining exactly this. In cooperation with the South-West Jutlandic Fishermen organisation, University of Copenhagen and Aquamind, he intends to make a development project with the purpose of harvesting Pacific oysters in the Danish Wadden Sea.

In the beginning of April 2014 Orbicon hosted a Nordic conference on surveillance plans on Pacific oysters and other invasive species. The Nordic Council of Ministers finances the project and Scandinavian research institutes participate.

Read the complete report on the Institute of Marine Research’s Homepage.

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