Infectious salmon anaemia (ISA) is a viral disease that affects Atlantic salmon but poses no threat to humans. First identified in Norway in 1985, it was detected in Canada in 1997 but was believed to be "exotic" in the EU until 1998 when it was discovered in a salmon farm at Loch Nevis.
A report published in Britain last year was unable to identify the origin of the outbreak, but narrowed it down to two possible causes - accidental introduction on a boat from infected areas in Norway, or introduction through fish in a wild reservoir.
Initially there was widespread slaughtering of fish in Scotland, particularly in the Shetlands, as it was believed that the disease could only be contained by removing and destroying all the fish from the infected site, disinfecting equipment and fallowing the farm for at least six months.
At the time, it posed a serious threat to an industry employing some 6,000 people with a turnover of £300 million in 1998. The condition spread to over 30 farms in Scotland, but many of these sites are now back in production after fallowing for the required period.
In 1991 ISA was classified as a List 1 disease, with control measures requiring the immediate withdrawal of fish, but the European Commission took a more pragmatic approach in 1998. This allowed for efficient control of the disease while also trying to safeguard the interests of fish farmers. Where a remote farm had low mortalities and few clinical signs of the condition, a cage-by-cage clearance programme was permitted. However, infected farms close to other fish farms and experiencing high mortalities were required to clear as soon as possible.
Dr Ken Whelan of the Marine Institute said: "Obviously we have be very careful in relation to wild fisheries, particularly when there has been an escape, but so far we have no clinical outbreak here and the Marine Institute, North-Western and Western Regional Fisheries Board are working together to remove any of the escaped rainbow trout from the Clew Bay farm that are detected."