The native or flat oyster (Ostrea edulis L.) is a sessile, filter-feeding, bivalve mollusc. It is associated with highly productive estuarine and shallow coastal water habitats with sediments ranging from mud to gravel. Ostrea edulis is widely distributed around the British Isles, the North Sea, Mediterranean and Black Sea. Along with other oyster species, it is also cultivated in North America, Australasia and Japan. 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.
Native oyster fisheries are subject primarily to UK shellfisheries conservation legislation; the species is not named in any national or international nature conservation legislation or conventions.
Current factors causing loss or decline
The dramatic reduction in stock abundance seen in the middle of the last century is attributed mainly to over-exploitation following the increased demand that accompanied improved rail transport.
The American oyster drill Urosalpinx cinerea and the slipper limpet Crepidula fornicata were introduced with Crassostrea virginica from North America around 1900. Urosalpinx is a predator alongside indigenous species such as crabs, starfish, dog whelks, shell boring worms and sponges. Crepidula is a filter feeder that deposits pseudofaeces and creates `mussel mud`. This mud degrades the grounds and hinders recruitment, but dead Crepidula shell provides culch upon which oyster settle.
Severe winters, such as those experienced in 1947 and 1963, caused high mortalities in the UK, particularly on the east coast where stock levels have not recovered to the pre-1963 levels.
The parasitic protozoan Bonamia ostreae has caused massive mortalities in France, from whence it was introduced, and in the Netherlands, Spain, Iceland and England. Another protozoan parasite, Marteilia refringens, has also been found in French stocks but hitherto it has not affected UK stocks.
TBT (tri-butyl tin) anti-fouling paints used on ships and leisure craft in the early 1980s caused stunted growth and probably affected reproductive capacity.
There are many other factors that affect oyster stock abundance, most contributing to the high variability of recruitment: temperature, food supply, hydrodynamic containment in a favourable environment, anthropogenic effects (eg coastal development, waste disposal). Also spawning stock density or biomass may be too low in many areas to ensure synchronous spawning or sufficient larval production for successful settlement.
Native oyster fisheries in the UK are managed by a mixture of national legislation (eg in Great Britain by the Sea Fisheries (Shellfish) Act 1967) and, in England and Wales, local 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. 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.
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.
Action plan objectives and targets
Maintain the existing geographical distribution of the native oyster within UK inshore waters.
Expand the existing geographical distribution of the native oyster within UK inshore waters, where biologically feasible.
Maintain the existing abundance of the native oyster within UK inshore waters.
Increase the abundance of the native oyster within UK inshore waters, where biologically feasible.
Proposed actions with lead agencies
Policy and legislation
Assess whether the existing EU Directives and UK legislation provide sufficient controls to minimise the risk of introducing new diseases and pests into the UK. (ACTION: DANI, MAFF, NAW, SE)
Recognising that EU legislation only covers disease controls, consider re-establishing pest controls equivalent to the Molluscan Shellfish (Control of Deposit) order 1974 which could aid pest control by prohibiting the movement of shellfish. If considered necessary to prevent recontamination or the introduction of alien species, UK fisheries ministers should encourage new controls on the use of seaweed and other natural products used as packing for live transport. (ACTION: DANI, DETR, MAFF, NAW, SE)
Site safeguard and management
Oyster grounds, and hence oyster abundance, require suitable surfaces for spat settlement. Slipper limpets have degraded some and made them difficult to re-establish. Consider whether appropriate mechanisms are available to encourage oyster farmers to carry out an environmental impact assessment and, if appropriate and feasible, to rework derelict areas to increase both oyster distribution and abundance and benthos diversity. (ACTION: DANI, MAFF, NAW, SE)
Integrate oyster habitat safeguards into Marine Nature Reserves, Special Areas of Conservation and estuary management plans where relevant to the site’s conservation objectives. (ACTION: All relevant authorities)
Species management and protection
Define clearer, tighter objectives, and apply specialist advice, in managing the UK regulated fisheries. (ACTION: Carrick District Council, DANI, MAFF, NAW, NIO, SE, Southern SFC)
Maintain the existing stock abundance in the main self-regenerating fisheries. (ACTION: DANI, MAFF, SE, SFCs, NAW)
Ensure adequate recruitment to maintain stock abundance. Target to be defined following a review (see 5.5.1). (ACTION: DANI, MAFF, NAW, SE, SFCs)
Endeavour to stop the spread of the introduced pests Urosalpinx cinerea and Crepidula fornicata beyond their existing distribution. (ACTION: DANI, MAFF, NAW, SE, SFCs)
Control stock density to reduce the risk of transmission of disease. (ACTION: DANI, MAFF, NAW, SE, SFCs)
Endeavour to prevent the introduction of the oyster disease marteiliosis, limit the spread of bonamiosis. (ACTION: DANI, MAFF, NAW, SE, SFCs)
Maintain genetic variability. Target to be defined (see 5.5.5). (ACTION: DANI, MAFF, NAW, SE, SFCs)
Produce guidance notes and a code of practice on habitat restoration and species protection. (ACTION: CCW, DANI, EHS, NE, MAFF, NAW, SE, SNH)
Future Research and Monitoring
Review the evidence of a relationship between spawning stock biomass and recruitment, and define safe biological reference points. (ACTION: DANI, NE, MAFF, NAW, SE)
Provide managers of several and regulated fisheries with guidelines and code of practice for habitat protection, stock management and species protection. (ACTION: DANI, MAFF, NAW, SE)
Continue and extend surveys of all wild stocks and fisheries to establish stock biomass, distribution and spatfall variability including assessments of any recovery in areas previously contaminated by TBT. (ACTION: DANI, MAFF, NAW, SE, SFCs)
Assess and report on the implications for genetic variability and biodiversity of using hatchery brood stock to produce seed for stock replenishment. (ACTION: CCW, DANI, EHS, NE, MAFF, NAW, SE, SNH)
Communications and Publicity
To raise awareness and provide information about the Biodiversity Action Plan, write articles on progress with the plans for appropriate trade journals (eg Fishing News, Fish Farming International) explaining the action plan. (ACTION: CCW, DANI, NE, MAFF, NAW, SE, SNH)
Links with other action plans