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The Pacific oyster is spreading

The Pacific oyster, Crassostrea gigas, is spreading in Scandinavian coastal areas – which effects will it cause on the ecosystems?

The Pacific oyster is spreading in Scandinavia. Scientists from the Norwegian Institute of Marine Research, Gothenburg University in Sweden, The Danish Technical University, Norwegian Institute for Water Research and ORBICON have – with funding from the Nordic Council of Ministers – mapped the spreading and present a risk assessment on the effect on the spreading in different habitats and climate scenarios. This risk assessment evaluates the impact on the ecosystem of the invasive Pacific oyster, Crassostrea gigas, as function of climate change on four different Scandinavian habitats:

  • Low energy rock
  • Littoral sand and mud
  • Littoral biogenic reefs
  • Sub-littoral sediments

The risk assessment is based on a review of existing scientific literature and expert judgments, and the data was evaluated on a workshop in April 2012 with the participation of 14 experts on C. gigas in Scandinavia.  Long-term climate scenarios are adapted from IPCC (Scenario AIB and A2) and a short term climate scenario is adapted from a Nordic analysis on climate change. Four different habitat types are included in the assessment. The habitat types represent important habitats for C. gigas in Scandinavia. The risk assessment is based on an analysis of the present scientific knowledge of the distribution patterns of C. gigas in Scandinavia, including the change in density over time. It is concluded that at the habitat types Low energy rock, Littoral sand and mud and Sub-littoral sediments in low energy areas there are a limited to moderate risk that a bio-invasion of C. gigas will develop. For Biogenic reefs and Sub-littoral sediments in high energy areas there is a moderate to high risk for a bio-invasion.

The information in this risk assessment may be used to develop site-specific strategies for conservation in areas with a present or an expected presence of C. gigas in the next decades. Different strategies can be implemented. One strategy is to accept the presence of the species in the ecosystem and the impact on coastal habitats. An alternative strategy for conservation is a control of density and dispersal of C. gigas in smaller or larger areas.

Mitigation of invasive species like C. gigas should prioritize a control strategy in Scandinavian coastal waters targeting areas with the highest risk for a bio-invasion. In high energy areas with high tidal amplitude and/or narrow straits with high current speed, a high risk for an invasion of C. gigas is identified on Littoral biogenic reefs and on Sub-littoral sediments. The density of C. gigas on especially Littoral biogenic reefs has been reduced after the last cold winters due to impact from drifting ice, and the current low density may be an excellent starting point for the implementation of a control strategy.

The easy access and the ability to visually inspect littoral sites may allow an implementation of a strategy for removing oysters from smaller areas in order to protect areas with undisturbed biodiversity and habitat structure. Cooperation with commercial fisheries may allow a control strategy for a larger area, whereas cooperation with local groups of volunteers may allow a control strategy of a smaller area by handpicking of the species. The risk assessment found a limited to moderate risk that a bioinvasion of C. gigas will develop in low energy habitats like Low energy rock, Littoral sand and mud and Sub-littoral sediment. The optimal strategy in these areas may include a monitoring program, which allow an early implementation of a control program in order to detect if a local population of C. gigas change to an expansive phase of invasion and calls for an effort to reduce density.


Facts about The Pacific oyster

Latin / scientific nameCrassostrea gigas
Family: Ostreidae
Life span: More than 20 years, more than 25 cm shell height and maximum weight more than 1,5 kilos
Habitat: attached to hard substrate, usually on rocks, gravel or mussel shells from the littoral zone, down to a few meters depth
Spawning: during summer, dependent on warm water. Fertilization in the water, larvae are planktonic for several weeks until they settle. 
Food: Filter phytoplankton, bacteria, other microbes and detritus from the water

The Pacific oyster


Torjan Bodvin
907 95 847
Stein Mortensen
55 23 85 89

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