Sainsbury’s has been accused of breaking a promise not to sell salmon from farms infested with lice in breach of guideline limits.

The angling group, Salmon and Trout Conservation Scotland (S&TCS), has attacked the supermarket chain for continuing to market caged salmon from the north west coast of Scotland that could harm wild fish.

Salmon farms have long had a problem with sea lice, tiny parasites that eat fish alive. Lice can multiply in cages holding many thousands of salmon, and then spread to passing wild salmon and kill them.

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S&TCS complained a year ago that Sainsbury’s salmon came from fish farms that failed to meet industry guidelines on lice infestation. The group was then assured that the problem was being dealt with.

But S&TCS sayslice levels on salmon farms at Loch Duich, Loch Alsh and Ardintoul near the Isle of Skye breached industry good practice code in the last three months of 2015. In December average levels reached nearly seven adult female lice per fish, compared to a recommended threshold of one.

The salmon farms are run by the Norwegian seafood multinational, Marine Harvest, and supply fresh salmon fillets and smoked salmon to Sainsbury’s. According to their packaging, they are “responsibly sourced”.

S&TCS director Andrew Graham-Stewart accused Sainsbury's of “paying lip service” to the protection of wild fish. "If Sainsbury's is serious about tackling this issue, the obvious solution is for it to refuse to accept farmed salmon from those farms that fail to keep sea lice numbers within agreed limits.”

He added: “West coast salmon farms have long been releasing huge numbers of juvenile sea lice into the surrounding sea loch environments with serious implications for local wild salmon and sea trout populations.

“All supermarkets have a huge part to play in protecting wild salmon and sea trout by making it crystal clear to the salmon farmers that they will not buy and then sell on to their customers any farmed fish from those Scottish farms that cannot or will not control their sea-lice numbers.”

Bill Whyte, chairman of the Wester Ross Area Salmon Fishery Board, warned that billions of lice were spread by salmon farms. This was a “scourge on wild salmon and sea trout stocks,” he said.

“Supermarkets have a duty to insist that their suppliers operate without posing a major threat to the wider environment.”

Sainsbury’s stressed that sea lice occurred naturally, and did not pose a threat to human health. A spokeswoman said: “We remain committed to addressing the concerns raised by S&TCS and working with our supplier to continue to make good progress.”

Marine Harvest insisted it was “one hundred per cent” committed to reducing sea lice. Company manager Steve Bracken said he was "very pleased with the development of new technologies which physically remove the sea lice from salmon using increased flow or temperature of the water”.

“We have never suggested this would eradicate sea lice totally overnight - indeed sea lice will always be naturally present in the environment. What we are doing is developing and introducing a range of long term solutions which will take some time.”