The threat to wild salmon from sea lice could be worse than feared as a new study shows the parasite is building up resistance to the chemical used in fish farming pens to tackle it.

Campaigners also believe emamectin benzoate – known as Slice – is leaking out and causing environmental damage and want the toxic compound banned.

However, despite these concerns, the Scottish Government last year extended its use in Scottish waters to 2028. Now the Irish government has sounded a warning that Slice is becoming less effective in killing the lice that infest fish farms.

New research shows young wild salmon passing coastal fish farms on their migratory routes are increasingly falling prey to lice coming out of pens.

The report by the Inland Fisheries Ireland agency is based on almost 20 years of tagging wild salmon.

It concludes: ‘Results of the present analysis provide clear evidence of significantly reduced return of adult salmon linked to salmon lice infestation from salmon farms. Data also suggests the effects of lice from salmon farms on wild stocks are underestimated because of growing resistance to Slice.’

Slice can damage human DNA, and the Scottish Environment Protection Agency (Sepa) has found it leaking from fish farm pens and harming sea life. 

Campaign group WildFish Scotland said the latest study’s conclusions are ‘exactly what we have been saying for years’.

Interim director Andrew Graham-Stewart added: ‘Successive Scottish Governments have enabled the salmon farming industry to expand rapidly without meaningful safeguards to protect the environment and wild salmon. 

It is permitted to use a host of highly toxic chemicals, including Slice, for the treatment of parasites and diseases.

‘However, regulatory change has seen substantial watering down and delay, in response to heavy lobbying of Scottish ministers by the major salmon farming companies in Scotland.

‘Make no mistake, this industry, as it is currently run, is driving many wild Atlantic salmon sub-populations inexorably towards extinction.’

The ruling allowing the industry to continue using Slice came shortly after industry body Salmon Scotland treated Rural Affairs Secretary Mairi Gougeon and her husband Baptiste to hospitality worth £1,500 as guests of chief executive Tavish Scott at Scotland’s Six Nations rugby clash against France in February last year.

There is no note of what was discussed but the Scottish Government has repeatedly stated that the Slice decision was not Ms Gougeon’s to make and it was not discussed at Murrayfield.

The industry is worth £760 million to the Scottish economy and employs 2,500 people, but in 2023 a total of 17.4 million fish died prematurely in captivity amid enduring concerns over animal welfare.

A Scottish Government spokesman said: ‘To protect wild fish, Sepa introduced a new framework to manage the risk of sea lice from fish farms in February 2024.’

Salmon Scotland said: ‘Fish farmers use Slice as a fully-licensed product, approved by vets and regulators.’