The level of chemicals used by fish farmers to treat sea lice infestations has risen dramatically, a BBC Scotland investigation has learned.
Scottish government figures showed that over the past five years, the industry used a broader range of chemicals and more of them.
Campaigners claim the figures are evidence the natural parasite is becoming resistant to the treatments.
Fish lice have been blamed for damaging salmon and sea trout stocks.
According to the government’s figures, the use of chemicals used to fight the parasite increased significantly between 2005 and 2009.
Andrew Wallace, from the Association of Salmon Boards, said that when young fish migrate from rivers to the sea they can be susceptible to naturally occurring sea lice.
He added: “Now in normal circumstances there aren’t that many lice around, and the lice that are around originate from existing wild fish populations.
“But if you have a million farmed fish in a cage on the migratory route of those fish, then suddenly you’re encountering an entirely different scale of problem.
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