Salmon farmers have been accused of playing dirty by using fish caught in the wild to clean lice from Scottish fish farms. Marine conservation experts say that shipping tonnes of English-caught wrasse a year – to tackle lice infestations in salmon pens north of the border – is endangering natural stocks. English anglers have also warned wrasse is becoming harder and harder to find in local waters.

However, salmon farmers have rejected the charge. They say the use of wrasse as a “cleaner” fish is part of a long-term plan to replace chemicals – which are currently administered to pens to control lice infestations – with sustainable, biological controls.

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Fishermen remain concerned, nevertheless. “Wrasse play a role in keeping the marine ecosystem in balance,” said David Mitchell, of the Angling Trust. “We simply do not know what will be the consequence of removing so many of them from our coastal waters.”

More than 170,000 tonnes of salmon a year are grown in Scotland at more than 200 marine farms. However, production is affected by lice infestations that cause lesions and secondary infections in the fish. Chemicals can control this but pollute water around the farms. Another solution is provided by wrasse which feed on marine insects. Many species – such as ballan and goldsinny wrasse – will eat lice that infect larger fish. As a result wild wrasse are being caught in pots and shipped to Scotland to tackle sea-lice infestations. One wrasse for every 25 salmon is used.

But this exploitation of wrasse is raising concerns. “We are very worried that a large local fishery has developed rapidly over the past couple of years – with large numbers of wrasse being taken from local waters – without proper management or any indication of its sustainability,” said Samuel Stone, of the Marine Conservation Society. “It is a real concern.”

Wrasse are killed and discarded after they have finished cleaning lice. That is causing real problems of depletion Mark Lloyd, Angling Trust chief executive

A similar line is taken by the Angling Trust, which is particularly concerned that wrasse are killed after they have completed their lice-devouring activities. Wrasse caught by anglers are usually put back in the sea and the Angling Trust said it was receiving more and more reports from anglers who had found very few wrasse left in their local waters, particularly around south-west England.

“Wrasse are very popular and many young people take up angling as a hobby after fishing for them,” said Mark Lloyd, chief executive of the Angling Trust. “They put the wrasse back in the water because they are not particularly appetising. By contrast, those that are being shipped north are killed and discarded after they have done their work cleaning lice and that is causing real problems of depletion. It is also a waste of protein.”

The fear that wild wrasse populations are shrinking badly is backed by researchers in Norway, where wrasse-catching to supply fish farming has also soared in recent years. According to a report in New Scientist, annual wrasse catches have risen from 2m to 22m in less than a decade to supply Norwegian salmon farms with cleaner fish. However, this was matched by considerable depletion of wrasse stocks where fishing took place.

Conservationists and anglers are now calling for a number of measures to be introduced to tackle the issue. In particular, they want careful monitoring of wrasse numbers to be introduced and strict limits imposed on catches.

However, the danger posed to wrasse stocks was dismissed by Scott Landsburgh, chief executive of the Scottish Salmon Producers’ Organisation. “The fishermen who supply us with wrasse do not take away juveniles or brood stock, so at the end of the day stock should always recover. And we simply do not have any hard figures that show serious depletion in going on.”

He said that the industry – which employs around 2,200 people – was also moving towards wrasse sustainability. “We have set up farms for both wrasse and lumpfish, which also eat lice, and the aim is that we will produce our own cleaner fish from our own farms in a few years,” he told the Observer.

It remains to be seen how successful this will be. Some marine conservation experts have questioned the potential usefulness of wrasse raised in farms as cleaner fish compared with those taken from the wild. Landsburgh remained confident. “We are spending a great deal of money on this. I am not worried.”