Pacific oysters, Crassostrea gigas, established in Scotland

A report to Scottish Natural Heritage on European flat oysters, Ostrea edulis, surviving in the Firth of Forth was investigated. No live O. edulis were found, but eight feral Pacific oysters, Crassostrea gigas, were found attached to eulittoral boulders and bedrock.
The ages of the Pacific oysters indicated an initial settlement no later than 2007 and at least two subsequent settlements, which may have been from recruitment in situ, or from repeated introductions of larvae.
The mode of introduction is unknown. There is no current aquaculture of Pacific oysters anywhere on the east coast of Scotland, but there were attempts at cultivation in the Firth of Forth which ceased more than 20 years ago. In addition, the Firth is a busy port complex and popular recreational boating area, so larvae may have been released in ballast water or have been produced by adults attached to a vessel's hull.
This is the first detailed account of adult C. gigas established on natural substrata in Scotland and raises issues in relation to biosecurity and the implementation of environmental legislation and commitments.