MORE than one-third of deaths in some Atlantic salmon are caused by sea lice found largely in stocks of farmed fish, new research has claimed.

An unexpectedly large number of free-ranging salmon are being killed by the parasitic lice in European waters every year, a major international study involving experts in Scotland has found.

The study, published today in the biological research journal Proceedings of the Royal Society B and partly carried out by scientists at St Andrews University, is claimed to be the first evidence of the full impact of lice on salmon mortality levels.

The aquaculture industry has been urged to protect the health of its own stocks in order to reduce the threat to wild fish.

Professor Christopher Todd, of the Scottish Oceans Institute at St Andrews, was part of an international group which found sea lice to be responsible for 39% of the deaths among salmon in the north-east Atlantic.

He said: "For the first time we can effectively place a reliable value on the predicted mortality loss of free-ranging salmon subject to infection from this parasite. This high percent mortality attributable to sea lice was unexpected.

"The salmon aquaculture industry has long placed a high priority on controlling sea lice on their captive salmon – but these results do emphasise the need for the industry to not only maintain the health of their own stocks, but also to minimise the risk of cross-infection of wild fish."

Sea lice are natural parasites of wild salmon and also present the salmon aquaculture industry with major challenges as the parasite can debilitate or kill the salmon host.

Producers insist 95% of salmon will die naturally at sea during their ocean migration, but over the past 20 years controversy has surrounded the contribution of sea lice parasites to mortality rates.