Native oyster

What action is going on for this species?

The following will give you information on what work has been taking place through the Species Action Framework:

• Updates on recent action 
• Action up to start of SAF, April 2007 

Species background

The native or common oyster (Ostrea edulis) is a marine bivalve mollusc with a rough, irregularly shaped shell up to about 10 cm across.

Why is this on the Species Action List?

The native oyster meets criterion 4 of the Species Action Framework as a threatened species of socio-economic value requiring action to ensure sustainable use.

It declined greatly as a commercially harvested species in the 19th and early 20th centuries around our waters.  Its distribution in the wild is now very restricted with a stronghold on the Scottish west coast, but threats remain, such that it potentially meets criterion 1a as well.

There is also sufficient knowledge of the species to apply effective targeted action.  Native oyster is a potentially valuable resource that could be sustainably harvested if population status is improved, unlawful fishing is ended and there is a robust management framework.  Furthermore, both management of 'wild' stock/habitat and cultivation have the potential to contribute positively to recovery of the species. 

Native oyster is a UKBAP 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).

Habitat, distribution and abundance

Native oysters live on the seabed in relatively shallow coastal waters and estuaries (from the lower shore to 80m). T hey prefer habitats sheltered from strong wave action which tend to be muddy but require something hard for larval settlement - usually shells or stones.  Native oysters are found (or have been known to occur) at locations around the whole of UK from the Thames estuary to the Northern Isles.  In Scotland they now mainly occur in scattered populations fringing sea lochs on the west and north coasts.

The native oyster may live up to 15 years but about six years is more usual.  It is an active filter feeder taking plankton and particulate matter from the water column.  Native oysters have an unusual life history as an alternating hermaphrodite species that may change sex successively throughout its life.

General ecology

The native oyster may live up to 15 years but about six years is more usual.  It is an active filter feeder taking plankton and particulate matter from the water column.  Native oysters have an unusual life history as an alternating hermaphrodite species that may change sex successively throughout its life.

History of decline, contributory factors and current threats

The native oyster has been harvested or cultivated for centuries and once supported a prolific fishery in several parts of Scotland, perhaps the best known example being in the Firth of Forth.  I

n the 13th century the native oyster fishery of the Forth covered over 129 km2 and was one of the most commercially important in Scotland.  At peak production the fishery was producing 30 million oysters per annum and the superior reputation of the oysters was so widespread that they were transported to Glasgow, western Scotland, England and the Continent for consumption and on-growing.

Various attempts at managing the fishery (close seasons, minimum size for market, ban on exportation for on-growing) failed and under extreme pressure from harvesting and illegal poaching the fishery began to collapse by the 1870s and eventually ceased entirely in 1920.  Surveys of the Firth of Forth carried out in 1957 reported that native oysters were not only commercially extinct but were biologically extinct since no living oysters were found.  A similar pattern of decline was recorded elsewhere in the UK and continental Europe.

There remains only one active oyster fishery in Scotland (Loch Ryan) and a number of sites where they are cultivated (on a relatively small scale).

At various points in the past the decline in stocks resulted in the import of oysters to Scotland from overseas, to establish or replenish fisheries.  Such movements are a potential route for the introduction of non-native species including some which are pests and diseases of oysters (e.g. the slipper limpet) although Scotland has remained free of most of these. 

Current issues and threats to the native oyster also include illegal harvesting in Argyll and the first Scottish case of the oyster disease Bonamia, which was found in native oysters in Loch Sunart in July 2006.

The 'Species Lead' at Scottish Natural Heritage

David.Donnan@snh.gov.uk   Tel 01738 444177