The descriptive text, below the map, is from the Cornish Red Data Book (2009). The map on this web page depicts the organisms distribution and shows the records made pre-2000 and those made since.
Range & Status
The native or flat oyster, a sessile, filter-feeding bivalve mollusc is associated with highly productive estuarine and shallow coastal water habitats with sediments ranging from mud to gravel. This species is widely distributed around the British Isles. The main stocks are now located in the rivers and flats bordering the Thames Estuary, the Solent, the west coast of Scotland, Loch Foyle and the Fal-Helford Estuary complex.
Numerous factors have caused loss or decline of oyster stocks including introduction of the alien species American Oyster Drill Urosalpix cinerea and Slipper Limpet Crepidula fornicata with the oyster Cassostrea virginica from North America around 1900 and problems arising from the parasitic protozoan Bonamia ostrae . Tributyl tin (TBT) antifouling paints used on large ships in the nearby Falmouth Docks and extensively on leisure craft in the 1980s caused distorted growth and probably affected reproductive capacity. Other factors affecting the oyster stock abundance, mostly contributing to the high variability of recruitment include: temperature, food supply and anthropogenic effects (e.g. coastal development, waste disposal, mining, etc.). Finally, spawning stock density and/or biomass may be too low in some areas to ensure synchronous spawning or sufficient larval production for successful settlement. Any introduction or cultivation of Crassostrea gigas , the Pacific Oyster, would be monitored closely by the statutory authorities.
Native oyster fisheries in the UK, including that in the Fal Estuary, are managed by a mixture of national legislation e.g. the Sea Fisheries (Shellfish) Act 1967, and in England and Wales by local Sea Fisheries Committees (SFC) bye-laws. There is also a national closed season (14th May \endash 4th August) to protect native oysters during their spawning season. The EC Directive 95/70/EC sets rules to prevent the introduction and spread of most serious diseases affecting bivalve molluscs. This is implemented in Great Britain through the Fish Health Regulations 1997.
The use of TBT-based paints on vessels less than 25m was banned in 1987 (Food & Environment Protection Act 1985, Part III) and in 2008 was extended to much of the world' s ocean-going shipping under the International Maritime Organisation' s Anti-fouling System Convention' s ban. Its use was banned for any ship visiting an EU port. It is thought that this ban is helping to reduce the adverse effect on oysters, especially in the Fal-Helford Estuaries where there are very large numbers of leisure craft. Ostrea edulis is listed as an OPSAR species.
I.J. Bennallick, S. Board, C.N. French, P.A. Gainey, C. Neil, R. Parslow, A. Spalding and P.E. Tompsett. eds. 2009. Red Data Book for Cornwall and the Isles of Scilly. 2nd Edition.Croceago Press.
The Cornish Red Data Book Project was led by the Cornwall and Isles of Scilly Federation for Biological Recorders (CISFBR). The full text and species accounts (minus the maps) are available on the CISFBR website.