Oyster, Native, oysters (Caught at sea)

Ostrea edulis

Method of production — Caught at sea
Capture method — Dredge
Capture area — North East Atlantic (FAO 27)
Stock area — UK
Stock detail

All Areas


Picture of Oyster, Native, oysters (Caught at sea)

Sustainability rating five info

Sustainability overview

Oyster beds are generally privately owned and managed. Shellfish farming is an extensive, low-impact method of mariculture and high quality water standards are required for cultivation of shellfish for human consumption. Dredging can cause disruption to the seabed and has a higher associated bycatch than manual harvesting techniques, but is less suited to deeper water for practical reasons. Some growers may hand-gather their stock by diving or by net to enhance quality.

Biology

The native or flat oyster is a filter feeding, bivalve mollusc. They live on the seabed in relatively shallow coastal waters and estuaries (from the lower shore to 80 m). They prefer habitats sheltered from strong wave action which tend to be muddy, but require something hard for larval settlement - usually shells or stones. All native oysters start out as males, and throughout their lives change back and forth from male to female. In Britain, breeding normally takes place in the summer. It reaches maturity at about 3 years old. The average reproductive size for the oyster is about 5 cm. Oysters can reach a shell length of up to 11 cm, and occur in variable shapes. Native oysters have a rough shell that is yellow, pale green or brown in colour, sometimes with bluish, pink or purple markings. The two halves of a native oyster’ s shell are different shapes. The left shell is deeply concave and fixed to the substratum, the right being flat with rougher edges and sitting inside the left, acting as a lid. Inner surfaces of both valves or shells are smooth and usually pearly, white or bluish-grey, often with darker blue areas.The shell shape is a good way to distinguish native oysters from Pacific oysters, which were introduced to the UK in 1926, and which compete with the native oyster for space and food. Native oysters have rounder shells with smoother edges, while their Pacific relatives have a more elongated shell with deeply grooved edges. A single female oyster can produce 2 million eggs. Although usually up to about 11 cm long, native oysters can grow to more than 20 cm and can live as long as 20 years.

Stock information

Stock Area

UK

Stock information

Throughout much of Britain the native oyster is severely depleted in the wild. Over exploitation, pests, disease (introduced with non-native species), pollution (mainly Tributyltin (TBT)) and harsh winters have contributed to its demise. Today it is widely distributed across the UK but in much fewer numbers than in the past. Stock abundance was probably greatest in the 18th and 19th centuries, when there were large offshore oyster grounds in the southern North Sea and the Channel producing up to 100 times more than today’s 100-200 tonnes. During the 20th century its abundance declined significantly in European waters. The main UK stocks are now located in the rivers and flats bordering the Thames Estuary, the Solent, River Fal, the west coast of Scotland and Lough Foyle. Its wider European range extends from the Norwegian Sea to the Atlantic coast of Morocco, and into the Mediterranean and Black Sea. Stock recovery has been hampered by the outbreak of a disease called Bonamiasis which can cause mass mortalities of the native oyster, although resistant strains are being developed. There remains only one active native oyster fishery in Scotland (Loch Ryan) and a number of sites where they are cultivated (on a relatively small scale). There are also active commercial fisheries in Chichester Harbour, the Solent, the Wash and the Thames. Oyster beds off the Isle of Wight in the Solent in Hampshire, once the largest self-sustaining native oyster stock in Europe, have been temporarily closed since 2013 to help protect and recover diminishing stocks. A prohibition has also been issued under the Shellfish Bed Byelaw to protect and help recover the severely depleted oyster populations in the Blackwater, Crouch, Roch and Coln Estuary. Closures are reviewed annually. No wild native oyster harvest was reported in 2012.

Management

Fisheries are generally privately owned and managed. There is a national closed season (14 May to 4 August) to protect native oysters during the spawning season, though a dispensation exists for cultivated stocks. Native oyster is a UK Biodiversity Action Plan (BAP) Priority Species and is included on the Scottish Biodiversity List. The conservation importance of this species is also reflected in its inclusion on the OSPAR List of Threatened and/or Declining Species and Habitats (2003) in the North Sea and English Channel. The BAP aims to maintain and expand, where possible, its distribution in UK waters. Native oyster fisheries (public and private) are subject primarily to UK shellfisheries conservation legislation and are managed by a mixture of national legislation, for example, in Great Britain by the Sea Fisheries (Shellfish) Act 1967) and, in England and Wales, local Inshore Fishery Conservation Authority (IFCA), formerly Sea Fisheries Committees (SFC) bye-laws. Almost all naturally occurring oysters in Scotland belong to the Crown Estate, except where the rights have been specifically granted to others. Many of the principal oyster fisheries in England and Wales are managed through Regulating or Several Orders (the latter extinguish the public right to fish). There are also some private oyster fisheries based on historic rights. The EC Directive 95/70/EC, which forms part of the EU fish and shellfish health regime, sets Community-wide rules to prevent the introduction and spread of the most serious diseases affecting bivalve molluscs. This is implemented in Great Britain through the Fish Health Regulations 1997 (SI 1997 No. 1881). The use of TBT-based paints on vessels less than 25 m in length was banned in 1987 (Food and Environment Protection Act 1985, Part III). Oyster growers believe this ban is helping to reduce the adverse effects on oysters. The Shellfish Hygiene Directive (91/492/EEC), implemented through the Food Safety (Fishery Products and Live Shellfish) (Hygiene) Regulations 1998, requires that all production areas must be classified according to the degree to which samples of shellfish from those areas are contaminated by coliform bacteria. The classification is a public health measure and determines whether the shellfish can go directly for human consumption or need to be treated beforehand by relaying in cleaner water or by depuration. Shellfish are monitored for marine biotoxins so that if Diarrhetic Shellfish Poison (DSP) is detected or if Paralytic Shellfish Poison (PSP) exceeds the maximum permitted level considered safe for human consumption, affected fisheries can be closed.

Capture Information

Since most flat oyster culture is developed in subtidal areas and in an extensive manner to avoid disease problems, oysters are usually harvested with dredges, one per side of the vessel, about 3.5-4 m wide and 2 m deep with 3-5 cm teeth blades, operated by hydraulic or pneumatic winch. Dredge and ring size may be limited by local Inshore Fishery Conservation Authority (IFCA) bylaws. Only the offshore beds are dredged. In some areas bylaws protect the beds from over-exploitation by limiting harvesting to non-mechanical means, such as hand gathering or raking by hand and may be the only methods permitted. Where intertidal culture on trestles is carried out, farmers bring the bags back to the packing houses for sorting, grading and restocking. The River Fal oyster fishery is the last one in Europe using only sailing and rowing boats to harvest oysters. The oysters in the estuary have been harvested in this highly sustainable way for more than 500 years. There is a closed season for native oysters from 14 May to 4 August (1 May to 31 October in Sussex IFCA area) during their spawning season.

References

http://www.seafish.org/media/1009412/nativeoystercultivationandasc.pdf; http://www.naturalengland.org.uk/ourwork/marine/mpa/mcz/features/species/nativeoyster.aspx; http://webarchive.nationalarchives.gov.uk/20110303145213/http://ukbap.org.uk/UKPlans.aspx?ID=495#1; http://www.fao.org/fishery/culturedspecies/Ostrea_edulis/en; http://www.snh.gov.uk/protecting-scotlands-nature/species-action-framework/species-action-list/native-oyster/