Conservation of the native oyster (Ostrea edulis), in Scotland

BACKGROUND
Native oyster (Ostrea edulis) populations in Scotland have declined significantly in abundance and distribution since the 19th century, mainly as a result of over-exploitation. Most of the remaining populations are thought to exist in west coast sea lochs. The native oyster is the subject of a UK Species Biodiversity Action Plan, the Native Oyster Species Action Plan (NOSAP), so there is a requirement to consider what conservation measures are appropriate.
This project aimed to develop advice on the conservation management of the native oyster in Scotland, based on an assessment of the current status of extant populations, reviews of the history of oyster exploitation in Scotland, and conservation and fisheries management practices in the UK and elsewhere. Sites throughout the west and north coasts of Scotland were surveyed for oysters and detailed population studies were carried out at three of these sites.
MAIN FINDINGS
• Native oysters occur mainly in small, scattered populations fringing sea lochs around the west and north coasts of Scotland, usually at low population density. The only managed fishery is in Loch Ryan, which appears to have a large, self-sustaining population. There is evidence of unlawful gathering of oysters on a wide scale having a severe impact on small populations.
• Population density and abundance at the Argyll sites were within the range of estimates for other British and European populations, but were much lower than in Loch Ryan at present and in the Firth of Forth historically.
• Native oysters were attached to all hard materials surveyed, but with an apparent preference for oyster shell and other shell types (‘cultch’). However, shell material was sparse and patchy. There was no evidence of competition for space with other sessile species surveyed.
• Detailed analysis of the small-scale distribution of oysters indicated a potential for limited reproductive success, owing to the distances between adult oysters. Sparseness and patchiness of oysters is probably exacerbated by a lack of cultch and by unlawful gathering.
• Genetic analyses indicated that past translocations of oysters have probably masked any former local differences, but more-widely separated populations can still be differentiated. A Skye population appeared very distinctive, but was represented by only a small sample, so further studies are desirable to ascertain whether this population is deserving of special conservation status.
A range of management measures were reviewed in relation to their applicability to conserving native oysters in Scotland. Low population densities and unlawful gathering are the two key issues to be addressed. Effective control of unlawful gathering is essential if remaining populations are to persist. Measures to increase broodstock density or habitat availability may also be required to sustain some populations.