More than a million salmon died in just ten hours due to a toxic accumulation of hydrogen sulfide. Another 100,000 suffocated after being left without oxygen and tens of tonnes of formaldehyde dumped into Scottish lochs. This is the increasingly controversial picture that emerges from new investigations into the salmon industry in Scotland, which for years has been presented as a “premium” aquaculture model
According to The Guardian, the data was made public by a decision by the Information Commissioner’s Office (ICO), the British authority that supervises administrative transparency. The body forced the Animal and Plant Health Agency (APHA), the British government agency for animal and plant health, to disclose documents that until now had been classified to protect the commercial interests of the companies involved.
The case is raised Animal Equality UKwhich in recent weeks has released new data obtained through requests for access to official records and documents from the British and Scottish authorities.
Almost 50 tonnes of formaldehyde in Scottish lakes
According to the association, in 2025 alone over 48 thousand kilograms of formaldehyde would have been discharged directly into Scottish lakes from salmon farms. The substance is classified by International Agency for Research on Cancer as a known human carcinogen.
In fish farms, formaldehyde is mainly used as a disinfectant and treatment against fungal infections and parasites. But the quantity dispersed in the water increased by 40% compared to the previous year. Over the past five years, according to activists, almost 176,000 kilograms of the substance have ended up in Scottish aquatic ecosystems.
The main user would be Mowia Norwegian aquaculture giant, which alone would be responsible for 84% of the formaldehyde used in 2025. In particular, the Loch Lochy plant would have exceeded the authorized annual limit in a few days.
Salmon (and trout) dead from suffocation and toxic substances
Among the most serious cases is the one involving a land farm owned by the Norwegian company Mowi, Scotland’s largest salmon producer. According to one of the reports released, in 2021 over 100 thousand salmon would have suffocated to death after being left unattended by an operator, thus interrupting the supply of oxygen to the tanks.
At the same site and in the same month, a second accident reportedly caused the death of over a million fish in just ten hours, due to an accumulation of hydrogen sulphide, a highly toxic gas that can develop in aquaculture plants. Despite the magnitude of the event, APHA would not take any sanctioning measures.
Another story concerns a farm run by Bakkafrost, certified by the RSPCA. Here, in 2022, around 600 thousand salmon died from the same cause. A few months later, the problem would return on an even larger scale, causing the death of over 1.5 million specimens. Even in this case no coercive measures were taken.
The documents released also tell the story of the situation of a trout farm where, in 2023, around 70 thousand fish died without the site ever notifying the competent health inspectors of the losses. The approximately 7,800 surviving specimens were later killed because they were deemed “economically unsustainable”.
The health agency would have limited itself to suggesting that the company consult a veterinarian and sending a code of good practices via email.
The accusations from animal rights associations
According to Animal Equality UK, the system would have protected the interests of the salmon industry more than animal welfare or citizens’ right to information. The organization also released videos shot on a farm owned by the Scottish Sea Farms company, where blind salmon were seen, with open wounds, sea lice infestations and serious mutilations. The site is also reported to supply salmon to chains such as Marks & Spencer and Co-op.
For its part, Salmon Scotland – an association representing producers – rejects the accusations, maintaining that the farms respect “animal welfare standards among the highest in the world” and that the images spread by activists would offer a distorted representation of reality.
Is Scottish salmon really “premium”?
Despite the ongoing controversy over intensive farming, salmon consumption continues to grow in Italy too. According to 2025 data, around 90% of the salmon consumed in our country comes from Norway, while Scottish salmon occupies a more exclusive niche, linked above all to restaurants and premium lines of supermarkets.
In 2024, Scotland exported around 101,000 tonnes of salmon to 48 countries, confirming the enormous economic weight of the sector. Yet, behind the image of “quality” salmon, there remain the same critical issues that concern much of industrial aquaculture: high mortality, overcrowding, diseases and animal welfare problems.
Even in Italy the debate is open. APIs he has repeatedly underlined how Europe is growing thanks to fish farming, while our country remains far behind giants such as Norway, which produces around 1.5 million tonnes of salmon per year.
But perhaps the point is not just to produce more. More and more people are starting to wonder how intensive fish farming really is different from cattle or pig farming. Because even behind salmon considered “premium” the same questions about animal suffering, environmental impact and real sustainability remain open.
Source: Animal Equality
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