Oysters

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. In countries where natural spatfall is poor, or in the case of introduced oysters, spat can be hatchery reared.

Pacific oyster is the most widespread farmed oyster species globally. Through intentional and accidental introduction the species has become established in the wild in many regions outside of its natural range. 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.

Pacific oysters are not believed to impact on native oyster populations¹; 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}. 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 functioning⁴. Although eradication of naturalised Pacific oyster beds is still advocated by some⁵, this has proven unsuccessful⁶ and it is not considered practical to restrict farming of Pacific oysters once established.

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 farms⁷. However, there is some doubt on the effectiveness of this as an approach^8,9. Triploid oyster seed may also be difficult to obtain from hatcheries and more costly for on-growers to purchase.

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. This could potentially promote their incidence and spread and affect both farmed and native oyster species.

The introduction of non-native bivalve species for aquaculture purposes is now highly regulated helping to reduce the introduction of diseases and pests. Internationally the “Code of Practice on the Introductions and Transfers of Marine Organisms 2005”¹² has been adopted by many countries, whilst in the EU international shellfish trade has been regulated for many years¹³. Upcoming European legislation will further prioritise the prevention and control of biological invasions¹⁴.

Biosecurity measures are important to mitigate oyster diseases that can affect oysters and could lead to high levels of mortality (e.g. Bonamiosis and Oyster herpesvirus)¹⁵, and regulatory measures aim to limit imports only from countries where no outbreak of disease occurs¹⁶. Key elements of biosecurity include; practical and appropriate legislative controls, adequate diagnostic and detection methods for infectious diseases, disinfection and pathogen eradication methods, reliable high quality sources of stock, and best management practices^{17, 18}. It is also critical that oyster hatcheries implement strict biosecurity plans to help prevent transfer of disease into, within and from their facilities.

Transfers of spat from hatcheries to on-growing areas, and the relaying of oysters between sites, must be carried out in ways that minimise the risk of disease transfer¹⁵, and the monitoring of oyster populations as well as parasite occurrence/levels is important. Management measures include regulation (e.g. lease conditions and permit requirements) but also the use of voluntary agreements, Codes of Good Practice and certification.

Also important are designations to protect sensitive marine habitats. For example in the UK these include Marine Protected Areas (MPAs), Special Areas of Conservation (SACs), Special Protected Areas (SPAs), and intertidal areas identified as Sites of Special Scientific Interest (SSSIs)¹⁹.