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Study Finds Sea Lice do not Play a Significant Role in Salmon Mortality

01 February 2013

IRELAND - Sea lice infestation is unlikely to be a significant factor influencing the conservation status of salmon stocks according to a research paper published in the latest edition of the prestigious Journal of Fish Diseases.

This definitive research, involving more than 350,000 fish, released into eight different rivers in 28 separate experiments was carried out over a nine year period by the Marine Institute and NUIG Galway to investigate the impact of sea lice on the marine mortality of Irish salmon smolts and assesses the extent of sea lice-induced mortality in Irish Atlantic salmon stocks.

In this long-term study, one group of salmon smolts were treated with a commercial agent which protects them against sea lice infestation for eight weeks after going to sea. The return rates of control or unprotected mirror groups of fish were compared with the ‘protected’ fish to see if they suffered any additional sea lice induced mortality following release into the sea. The research also took account of the results of a similar but much smaller study carried out by Inland Fisheries Ireland.

Because the Marine Institute study involved the repeated release of hundreds of thousands of fish over the course of a decade across eight locations in Ireland, its results are highly accurate and very reliable. It found the level of marine mortality attributable to sea lice infestation to be very small – approximately one per cent in absolute terms.

“At these levels, it is unlikely to influence the conservation status of stocks and is not a significant driver of marine mortality,” the article states. The paper also offers an explanation as to why some researchers in this area have reached different conclusions and demonstrates serious flaws in the experimental design employed by these research groups.

The article concludes by noting the strong and significant trend in increasing marine mortality up to 2008 and finds that “there is no evidence to suggest that this trend is influenced by sea lice infestation levels of outwardly migrating smolts as treated and control fish are equally affected.” These findings agree closely with the outcome of a similar long term study carried out in Norway.

Welcoming the publication and its findings Donal Maguire Director of Aquaculture Development Services of BIM stated: “This is confirmation of the validity of the approach that we have taken with regard to the development of this industry. The scare stories in relation to sea lice being a threat to wild salmon put out by the opponents of salmon farming have no basis in scientific fact. Ireland is uniquely blessed in having an environment naturally suited to salmon farming and all stakeholders should now unite to realise the opportunities this represents and to deliver the much needed economic and employment benefits to coastal communities around Ireland.”

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