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Research 'Clears' Fish Farms of Sea Lice Charge
09/01/03
By JAMES REYNOLDS
SCIENTISTS have produced new evidence apparently disproving a link between the presence of fish farms with infestations of parasitic sea lice on wild fish.
Industry representatives said they were not surprised by the independent study, which they claimed revealed "other factors" had a greater importance in bringing about the decline of wild sea trout in Scotland.
The West Sutherland Fisheries Trust (WSFT) spent four years researching the effects of fish farms on wild sea trout in Loch Laxford, just north of Scourie, presenting their findings in the Bulletin of the European Association of Fish Pathologists.
Environmentalists have for the past 20 years claimed the rapid expansion of aquaculture - and a related increase in sea lice - is the primary reason for the drastic decline of wild salmon and trout in Scotland, although the industry claims the fall in numbers started before the introduction of fish farms.
Dr Shona Marshall, of the WSFT, carried out tests on wild sea trout post-smolts from 1998 to 2001 within the Laxford Bay area, which the fish swim through before returning to the River Laxford.
The site was chosen because the fish farm located in the bay, owned and run by Loch Duart Ltd, rotates production around three different lochs in the area, leaving the test area "fallow", or with no farming activity, in some years. This is done to give Laxford Bay a "rest" from the environmental impacts of aquaculture, and was judged to provide a "unique opportunity" to assess the relationship between fish farming and lice abundance on wild trout.
The results of the study show that, even when there was no fish-farming activity in the bay, sea lice abundance remained relatively constant on wild fish.
Dr Marshall said: "In 2000, when infestations within the farm could be expected to be at their highest as a result of the production cycle, significantly less lice were found on the cages than in the wild in April and May, altering in June such that the overall numbers of lice were similar in both situations.
"Prior to fallowing in 2001, substantially more lice were found in the cages than on the wild fish. Lice treatment by the farm at this time did not appear to have any effect on the number of lice in the wild, although a major impact was observed within the cages, again suggesting the relationship between the two populations is not direct."
The sea louse is a common marine parasite of trout and salmon throughout their life cycle. Background levels on wild fish are generally low, although this has not been the case within the aquaculture industry, where infestations of sea lice are estimated to cost Scottish fish farmers £15-£30 million a year in control measures and lost production.
Professor Christina Sommerville, an expert on fish parasites at Stirling University’s world-leading Institute of Aquaculture, said the study shows that there are no clear relationships between the infections of lice on farmed salmon and those on sea trout throughout the study period.
"It demonstrates that there are significant factors influencing the dynamics of the lice populations on the sea trout other than the fish-farming activity. Such influences might be related to natural phenomena, as well as those influenced directly by man."
Prof Sommerville added: "Interestingly, the data can be looked at from a different perspective, such as how infections on wild fish affect farmed fish. The sea trout appear to act as a reservoir of infection for the farm, maintaining populations during the fallow years."
Brian Simpson, the chief executive of the industry body, Scottish Quality Salmon, said the research did not surprise him, despite it running counter to "the views of some unqualified lobbyists on the issue".
He added: "Although it was confined to the Laxford Bay area, many of our member companies have believed these findings to be representative of their own area as well."
Dr Rebecca Boyd, environmental group WWF Scotland’s spokeswoman on aquaculture, described the research as "limited in scope" and said it had to be seen against a body of published evidence showing a link between sea lice populations and the presence of fish farms.
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