A series of emails, released under the Freedom of Information Act, and reported by investigative news site, The Ferret, in March suggest the Scottish government appears to be smoothing the path for CleanTreat – a system that uses the neonicotinoid insecticide imidacloprid to rid farmed salmon of sea lice, to be accepted for regulatory approval in Scottish fish farms.

The insecticide is one of three nicotine-based, or neonicotinoid, chemicals banned by the European Union in 2018 for agricultural use on crops, a decision upheld this month by the European court of justice, which rejected an appeal by the Bayer chemical multinational. The ban does not apply to rivers or the sea. US government scientists have described the insecticide imidacloprid as an “environmental hazard” that can be “very toxic to aquatic life with long-lasting effects”.

The Guardian now reports that Scottish government appears ready to approve the banned insecticide for use in Scottish salmon farms, according to internal documents seen by the Guardian, as MEPs warn of its potentially “devastating” impact on aquatic life.

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Efforts to rid fish farms of blood-sucking aquatic lice by using chemical pesticides have ended up giving the pests increasing resistance, leading to widespread infestations in the north-eastern Atlantic Ocean.

In natural conditions, lice numbers drop over winter as trout return to fresh water and salmon disperse into the cold ocean waters, but intense fish farming has provided the parasites with abundant, year-round hosts which allow them not only to thrive in winter, but also to mutate rapidly to avoid being impacted by pesticides.

Researchers looked for specific genes associated with resistance to organophosphates and pyrethroids commonly used to control salmon lice in the past two decades, and were able to map out how quickly lice populations mutated – usually within a few years – to survive pesticide treatment.  They found that more than 50 per cent of the lice sampled around fish-farming regions were resistant to both chemicals, and in some areas they had a hard time finding any lice that hadn’t mutated towards resistance to at least one chemical.

They were also able to infer how the mutations spread – mainly, cropping up in farms a few years after treatment and then leaking out little by little across the ocean. High numbers of resistant lice in fish farms have apparently ridden currents across farm barriers, keeping a steady flow of parasites into wild fish populations that then carry the genes far and wide.  Click here.