Over the past five years there has been increasing concern around the spread of wild Pacific oysters in Marine Protected Areas. In Devon and Cornwall, the abundance of Pacific oyster Magallana gigas (formerly Crassostrea gigas) within intertidal Marine Protected Areas has led to some sites being reported as being in unfavourable condition. Natural England, in partnership with South Devon Area of Outstanding Natural Beauty Estuaries Partnership and Cornwall Wildlife Trust, with funding from the European Maritime and Fisheries Fund, undertook an investigation into monitoring and control measures for Pacific oysters within Marine Protected Areas.
Between 2017 and 2020, volunteers led by Cornwall Wildlife Trust and South Devon AONB Estuaries Partnership, conducted surveys around Cornwall and South Devon, to record the density of Pacific oyster populations and to test the effectiveness of culling as a method of controlling population expansion. The project did not investigate whether the oyster reefs extended into subtidal areas.
A number of hotspots were identified where Pacific oyster settlement is a cause for concern. The worst affected areas were sites within the Fal and Helford SAC, the Plymouth Sound and Estuaries SAC, the Fowey estuary (adjacent to the Upper Fowey and Pont Pill MCZ), Whitsand and Looe Bay MCZ and the Exe estuary SPA (and their underlying SSSIs). Pacific oyster reefs (areas of 100% cover) were already established in some of these sites, and new or developing reefs were recorded on intertidal seagrass within the Salcombe to Kingsbridge SSSI and at other areas including within St John’s Lake SSSI.
Targeted culling was carried out at a number of sites but overall have not been enough to reduce the growth of Pacific oyster populations. Culling was particularly difficult on sediment habitats and there were also concerns of how large quantities of oysters could be removed without causing damage to intertidal seagrass beds. Further work is recommended including, investigating how culling might be made more effective including preventing populations becoming established in at-risk areas with MPAs.
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