Sea lice killing 'large numbers' of salmon
Large numbers of free ranging salmon are being killed by parasitic sea lice in European waters every year, an international study has suggested.
The research involved the release of 280,000 tagged salmon smolts into 10 rivers in Ireland and Norway.
Sea lice were responsible for 39% of deaths among the young fish, according to the study's newly-published results.
Scientists from University of St Andrews' Scottish Oceans Institute worked on the research.
Also involved were the Department of Zoology at the University of Otago in New Zealand; Atlantic Veterinary College at the University of Prince Edward Island in Canada; Inland Fisheries in Ireland; the Institute of Marine Research in Norway; and the Norwegian Institute for Nature Research.
The scientists said natural mortality of wild salmon during their ocean migration could be as high as 90-95%, with deaths caused by a variety of factors including sea lice.
The creatures are a naturally occurring parasite.
The pest also affects the salmon farming industry. Its efforts to control sea lice include the introduction of ballan wrasse, a so-called "cleaner-fish" that prey on the pest, to a farm in the Western Isles.
Prof Christopher Todd, of the Scottish Oceans Institute, said it was important that the industry continued to give control measures high priority.
Angling organisations the Association of Salmon Fishery Boards (ASFB) and Rivers and Fisheries Trusts of Scotland (Rafts) said the study confirmed that, in some circumstances, fish farming was having "highly significant impact" on the health of wild salmon.
Fish farmers said there were many possible causes for the loss of wild fish at sea, including seals, fishing and a lack of food.
The sea lice research team analysed the rates of survival of salmon when they returned to freshwater as mature adults.
The data analysed included 24 trials carried out between 1996 and 2008 which involved the 280,000 individually tagged smolts that were released into rivers.
In each trial half the fish were treated chemically before their release to protect them from sea lice infection during their first one to two months at sea.
The remainder in each trial were untreated control fish.
A proportion of each group were then recovered as adults on their return to coastal waters a year later.
By comparing the tags recaptured from both the treated and control groups in each trial, the researchers showed that sea lice were responsible for an average 39% of the total mortality losses of salmon at sea.
Prof Martin Krkosek, of the University of Otago and who led the study, said the research was similar to clinical studies in medicine but with fish instead of human patients.
He added: "Usually we think of food, climate, predators and fishing as the major drivers of fish abundance, but we have learned that parasites are taking a very large share of the catch."
In a joint statement, the ASFB and Rafts welcomed the research.
The statement added: "In some circumstances and in some locations, sea lice arising from farmed fish can have a highly significant impact on wild salmon.
"Indeed, over the last 20 years there has been no substantive evidence to counter the hypothesis that sea lice arising from aquaculture cages harm wild salmon."
Scott Landsburgh, chief executive of Scottish Salmon Producers' Organisation, said suggestions that the primary origin of sea lice on wild fish came from farms did not "stand up", adding that the parasite was endemic in the marine environment.
He added: "The consensus amongst the scientific community is that the overwhelming majority of wild salmon, 95% of total populations according to this paper and higher according to other published papers, die at sea - this has nothing to do with salmon farming.
"Major international organisations and projects are in total agreement that the largest problems for the survival of wild salmon are the conditions at sea - this is where further research should be undertaken."