Pacific oyster

Crassostrea gigas

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Location

All areas

Production method

Culture, suspension

Rating summary

Pacific oysters do not require any commercial feed sources as they extract their nutrient requirements from the surrounding water. There are some impacts from farms including sediment changes and shading, which can alter the local environment. There are concerns about the escape and spread of Pacific oysters around UK coasts and the resultant competition, with native species such as oysters, mussels and seagrass beds of most concern. Although regulations are in place to limit environmental impacts, disease spread and control species introduction, Defra are yet to declare a position on whether Pacific oysters are invasive or naturalised. Therefore, regulations are not clear, appropriate or effective when considering ecosystem impacts.This assessment is for uncertified oysters.Rating last updated June 2022.

How we worked out this Rating

No external feed is supplied to farmed oysters.Farmed oysters do not require any commercial feed sources as they get all their nutrient requirements from the surrounding water. They feed by filtering mainly microscopic algae (plankton), but also some organic detritus in sea water.

Although Pacific oysters are farmed in high density the net impacts on the surrounding environment are considered negligible. However, for this non-native species there are concerns that escapes from farms can result in wider ecological impacts.Overall, Pacific oyster aquaculture has some environmental impacts. The escape and spread of Pacific oysters around UK coasts and the resultant competition with native species such as oysters, mussels and seagrass beds is of most concern. Oysters are found in marine and brackish water and therefore culture is entirely sea-based and does not deplete freshwater supplies. Habitat concerns include alteration of hydrodynamics and current velocities, reduced flow rates, altered sedimentation and shading. The increased sediment from farms is considered balanced out by the filtering of organic materials by oysters. Raised oyster farms do provide temporary habitats for associated species such as invertebrates and fishes. However, these are regularly disturbed and ultimately removed limiting these benefits to robust and mobile species.Recent oyster culture generally does not entail the application of chemicals (i.e. antibiotics, pesticides, herbicides, fertilizers) to control fouling and predators or to prevent disease. Furthermore, the water in which chemicals would be used generally is not released to the marine environment. There is no evidence that discharges from oyster culture cause or contribute to cumulative impacts beyond the immediate vicinity of the farm. Furthermore, oyster farming may provide increased benefits through their extractive nature when cultured with other species.Juveniles are supplied by hatcheries. There are some concerns that Pacific oysters are an incidental carrier of a serious disease for native oysters (Bonamia ostreae) though this does not appear to be an issue currently. Disease outbreaks for Pacific oysters can and do occur, although they do not threaten regional level populations. Wild Pacific oysters are now widespread in the southern UK and reported elsewhere including Scotland and Norway. There are concerns that the species are dominating coastal environments and competing with native species and altering coastal habitats. A variety of oyster predators exist among oyster farms, including echinoderms, snails, crabs, fishes and seabirds. A variety of methods are employed to reduce predation but there are no direct negative impacts on predatory species.

Animal welfare is not applicable for shellfish as it is not covered by EU regulations on welfare. Humane slaughter has been carried out by RSPCA definitions.

Although regulations are in place to limit environmental impacts and control species introduction, Defra are yet to declare a position on whether Pacific oysters are invasive or naturalised. Therefore, regulations are not clear, appropriate or effective when considering ecosystem impacts.Aquaculture policy in the UK is a devolved matter, with the separate administrations of Wales, England, Northern Ireland and Scotland responsible for its collective oversight. Marine plans are either in development of have now been published.In the UK, regulations regarding the environmental impacts of aquaculture are in place. This includes the Habitats Directive 92/43/EEC and the Birds Directive 2009/147/EC, which form the cornerstones of the EU’s nature conservation policy and protect valuable habitats and species. There is also regulation in place to cover the use of land and water resources, discharges including effluents and their impacts, disease management and biosecurity.However, while regulation exists on species introduction, Defra are yet to declare a position on whether Pacific oysters are invasive or naturalised. Therefore, regulations are not clear, appropriate or effective when considering ecosystem impacts. This assessment is for uncertified oysters.

References

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