The introduction of non-native bivalve species for aquaculture purposes is now highly regulated helping to reduce the introduction of diseases and pests. This could potentially promote their incidence and spread and affect both farmed and native oyster species. Oyster relocation has in the past been the source of invasive non-native species (transported along with the oysters as ‘hitchhikers’) 10, 11, and the movement of oyster seed, stock and equipment could potentially introduce or transfer diseases and parasites. Triploid oyster seed may also be difficult to obtain from hatcheries and more costly for on-growers to purchase. However, there is some doubt on the effectiveness of this as an approach 8,9. The use of hatchery reared triploid (sterile) oysters for large aquaculture production of Pacific oysters has been seen as a method of preventing the release of spat from farms 7. Although eradication of naturalised Pacific oyster beds is still advocated by some 5, this has proven unsuccessful 6 and it is not considered practical to restrict farming of Pacific oysters once established. ![]() In addition, Pacific oysters in the Wadden Sea (part of the Dutch, German and Danish North Sea) co-exist with native mussels and form intertidal beds and “oyssel” reefs which are considered important for the Wadden’s community composition and ecological functioning 4. Pacific oysters are not believed to impact on native oyster populations 1 generally the lack of native European oysters seems to be due to a combination of over-exploitation, environmental conditions, and disease events, rather than competition from Pacific oysters 2, 3. They may then impact or compete with other bivalves, but relatively little is known about the ecological impact (either positive or negative) of Pacific oysters on native communities. Through intentional and accidental introduction the species has become established in the wild in many regions outside of its natural range. Pacific oyster is the most widespread farmed oyster species globally. In countries where natural spatfall is poor, or in the case of introduced oysters, spat can be hatchery reared. Most global oyster aquaculture does not rely on hatchery seed but on wild spat collection and this should be from abundant, well-regulated natural sources.
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