Large number of salmons in European waters are killed by parasitic lice every year, reveals a new study.

A team of international researchers involving experts from the University of St Andrews in UK, have found that sea lice caused 39 percent of salmon deaths in the Northeast Atlantic Ocean.

"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," Christopher Todd, of the Scottish Oceans Institute at St Andrews who was involved in the study, said in a statement.

"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 emphasize the need for the industry to not only maintain the health of their own stocks, but also to minimize the risk of cross-infection of wild fish," he said.

For their study, researchers analyzed the survival rates of salmon that were released in rivers when they were young and returned as adults to coastal waters a year later.

At least 280,000 molts (young salmon) were tagged and released in 10 rivers in Ireland and Norway. As many as 24 trials were carried out between 1996 and 2008, and in each trial half of the fish were treated to avoid being infected by sea lice for the first two months after their release. Rest of the fish in each trial was left untreated.

A section of each group was recovered as mature adults. When experts compared the tags from both treated and untreated salmon, they found that on an average 39 percent of the salmon deaths was caused by parasitic lice in sea.

"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," said Martin Krkosek, of the University of Otago, New Zealand, who led the study.

The findings of the study are published in the journal, Proceedings of the Royal Society B.