Salmon lice – genetic comparisons

Wednesday 8 September 2004

Researchers at the University of St Andrews have recommended that the international salmon farming industry should take a highly precautionary stance on parasite management.

The marine ecologists have spent the last 7 years studying the genetics of lice on salmonids around the Atlantic Ocean and publish their findings today in a leading fisheries journal.

Salmon lice, living on the skin of salmon and sea trout, are widely believed to be implicated in recent declines of wild sea trout and some salmon stocks in parts of Ireland, Scotland and Norway over the past 15 years.

The ecologists at St Andrews, in collaboration with Fisheries Research Services in Pitlochry and funded by the Natural Environment Research Council, set out to examine whether sea lice on farmed salmon differ from those on wild salmon and sea trout.

The scientists found no significant genetic differences between populations of salmon lice on farmed salmon and wild salmon and sea trout around the coasts of Scotland. The lack of genetic differences amongst lice populations is consistent with regular cross infection between farmed and wild hosts. In addition they found no differences between Scottish lice and lice sampled from farmed salmon in East Canada or wild sea trout in North Norway.

The scientists say that because of this finding, salmon lice throughout the North Atlantic should be treated as one huge population rather than separate infestations. While previous reports and environmentalists have suggested that salmon farms have spread the disease, the University of St Andrews researchers claim that the lack of genetic differences means that it is impossible to distinguish between farmed and wild lice, so the two types are freely intermingling.

Professor Todd explained: “If two populations are isolated from one another they will, over time and by chance alone, evolve to become genetically distinct. Our research has shown conclusively that sea lice throughout the North Atlantic ocean on farmed and wild salmonids comprise a single population. This important finding is consistent with the well-known long-distance oceanic migrations and intermixing of European and North American salmon on their ocean feeding grounds off Greenland, Iceland and Norway, together with high levels of cross- infection between wild salmonids and farmed salmon.”

Over the period 1998 to 2004 Professor Todd and his team also sampled 344 wild Scottish salmon netted in coastal waters and recorded annual average infestations of sea lice fluctuating between 17 (1998) and 32 (2000) lice per fish. The highest density recorded for a single fish was 114. The researchers conclude there is no evidence that average numbers of sea lice on wild salmon are increasing or decreasing over time. Regarding the wider significance of this research, published in the Canadian Journal of Fisheries & Aquatic Sciences, Professor Todd said:

“There are concerns in the salmon farming industry that genetic resistance to louse chemotherapeutants can evolve in sea lice. The high levels of gene flow amongst sea lice that we have recorded throughout the North Atlantic ocean suggest that any such resistance genes may spread extremely rapidly.

“It is not possible, from our data, to calculate the levels of infection from farmed-to-wild and from wild-to-farmed. But the very high numerical imbalance between farmed and wild salmon in Scottish waters, which probably exceeds 100 to 1, indicates the potential for a high level of salmon lice production and export from fish farms and this could pass to wild populations of both salmon and sea trout. Concerns relating to chemical resistance in sea lice and the inferred levels of farmed- to-wild infection would lead us to suggest that the management of sea lice by the international salmon aquaculture industries should be highly precautionary and from an ocean-wide perspective.”





CAPTION: Adult female sea lice (carrying eggstrings) attached in their typical position along the back of a wild Atlantic salmon caught in July 2004. The fish weighed 6lb, carried 52 sea lice, was healthy and in good condition: such densities of sea lice are not uncommon on wild adult salmon when caught in the sea. Within hours or days of the fish entering freshwater the parasites begin to drop off so anglers tend to see far fewer parasites, even on freshly-run salmon.


· The paper, “Population genetic differentiation of sea lice parasitic on Atlantic and Pacific salmonids: analysis of microsatellite DNA variation among wild and farmed hosts¿, appears in the Canadian Journal of Fisheries & Aquatic Sciences, vol. 61, July 2004. The paper is available online at: bin/rp/rp2_tocs_e?cjfas_cjfas7- 04_61

· European Commission funding for part of this work was provided by DG Fisheries.

· The Natural Environment Research Council (NERC) is one of the UK’s seven Research Councils. It uses a budget of about £300 million a year to fund and carry out impartial scientific research in the sciences of the environment. It is addressing some of the key questions facing mankind such as global warming, renewable energy and sustainable economic development. For more information contact Owen Gaffney at the NERC Press Office on 01793 442629 or mobile 07775 713203; email [email protected]

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View the latest University news at Contact Gayle Cook at the University of St. Andrews Press Office on 01334 467227 or mobile 07900 050 103; email [email protected]

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