Five freshwater lochs in the Highlands and Islands have been damaged by pollution from fish farms, according to a long-term scientific study by University College London.
The study concludes that faecal waste and uneaten food dropped into the lochs from over 50 salmon and trout cages since the 1980s and 1990s is “highly likely” to have altered the water quality, risking algal blooms and posing dangers to wildlife.
Researchers insisted they had to keep the identities of the lochs secret in order to protect “the confidentiality of commercial information”. One of the funders of the study was the Norwegian salmon farming multinational, Mowi, previously known as Marine Harvest.
But now the names and locations of the polluted lochs can be revealed, thanks to a freedom of information battle won by the campaign group, WildFish. They are Loch Arkaig, north of Fort William; Loch Earn, west of Crieff; Loch Langavat, on the Isle of Harris; Loch of Buttersone, near Dunkeld; and Loch Shin, near Lairg in Sutherland.
WildFish accused Mowi of trying to hide the names of the lochs, and claimed that the industry could not be trusted. Open net salmon farming could not be practiced without “unacceptable environmental impacts”, the group said.
Mowi, however, dismissed the study as “inconclusive”, arguing that salmon farming did not harm lochs. The company counter-attacked WildFish, accusing it of failing to fund wild salmon conservation.
The study by researchers from University College London (UCL) and the Scottish Government’s wildlife agency, NatureScot, was published in the Journal of Environmental Management in August 2024. It examined samples of sediments taken from the bottom of seven lochs between 1996 and 2015.
The sediments were analysed to discover the composition of algae over 100 to 200 years to see if the introduction of fish farms made any difference. Wastes from fish cages can pollute lochs causing algae to bloom, depriving underwater plants and animals of the light and oxygen they need to survive.
In three lochs – Arkaig, Earn and Langavat – researchers found “marked”, “striking” and “major” changes in the algae after the start of fish farming in the 1980s. In another two lochs – Butterstone and Shin – they also detected changes after fish farms arrived, though they were less marked.
The researchers concluded that at the five lochs the introduction of salmon or trout farms was “highly likely to have contributed to alterations in water quality”.
At the two other lochs they studied – Loch Huamavat on the Isle of Lewis and Loch Shiel, west of Fort William – changes were “subtle” and could not be linked to the arrival of fish farms. The researchers suggested this could be because of the differing scale of fish farming operations, the siting of fish cages, and the way the water flowed.
‘Plain wrong’ to keep lochs secret
When the UCL study was published, the lochs were only identified as A, B, C, D, E, F and G. “The authors do not have permission to share data”, it said, because of “the need to preserve anonymity of the study sites”.
The campaign group, WildFish, described this as “plain wrong” on the grounds that, if lochs had been polluted by fish farms, the public had a right to know. It asked the researchers to name the lochs, but they refused, citing “the confidentiality of commercial information”.
WildFish then filed a freedom of information request to UCL, which was also refused. So the group appealed to the UK Information Commissioner, who issued a decision in March 2025 ordering UCL to name the seven fish farms.
WildFish’s solicitor, Guy Linley-Adams, blamed the secrecy on the salmon farming company that helped fund the research, Mowi. “If you sponsor academic research and that research shows that your activities may have caused long-term harm to the environment, don’t you have a moral obligation to come clean?” he said.
“Mowi didn’t seem to think so. Time and again, we see that you just cannot trust the Scottish salmon farming industry.”
Linley-Adams argued that open-net salmon farms should be shut down. Externalising costs and impacting the environment was “part of the industry’s basic business model”, he claimed.
“Open net salmon farming cannot be practiced without unacceptable environmental impacts outside the cages, on the seabed, in the water column and on wild marine species”.
The Scottish Greens called on the salmon farming industry to be more transparent. “The changes to water quality coinciding with the arrival of fish farms is not surprising,” said Green MSP, Ariane Burgess.
“Waste and chemicals are released into our waters by farms, which harm wildlife, damage the seabed and marine environment and even put its own industry at risk.”
She added: “I have been urgently calling for a pause on both new salmon farms and the expansion of existing salmon farms across Scotland, to buy time for the government to genuinely assess the long-term environmental impact.”
According to Mowi, the UCL study related to research from over 20 years ago that “does not take account of more recent environmental data relating to the same lochs”. The company “willingly participated” to help understand the impact of fish farms on freshwater lochs.
“Freshwater lochs where Mowi has salmon farms were examined as part of this study, as well as lochs with farms operated by other companies,” the company said. “The report is inconclusive in determining whether or not fish farming in freshwater lochs leads to ecological changes.”
After the names of the seven lochs were released, Mowi published findings from its own 37-year monitoring programme of water quality in Loch Arkaig, which the UCL study said had been altered by fish farming.
The company concluded that salmon cages in the loch had not done any harm as they “did not alter the nutrient status of the water body”. It announced in January 2025 that it was going to cease farming salmon in the loch in April.
Study of lochs ‘inconclusive’
Mowi’s chief operating officer in Scotland, Ben Hadfield, attacked WildFish. “It is disappointing that WildFish has chosen to highlight a report that is inconclusive in determining whether or not fish farming in freshwater lochs leads to ecological changes,” he told The Ferret.
“However, this is what we have come to expect from an organisation that chooses to campaign for the closure of open net salmon farms, rather than investing in conservation initiatives to boost the wild salmon population in Scotland.”
He added: “Instead, at Mowi, we are investing in continuous scientific research to properly assess and understand the impact of our operations on the nutrient status of freshwater lochs.
“This is not a regulatory requirement, we do it because it gives us a better understanding of the environment in which we operate and because we are committed to farming sustainably.
“We are also proactive in supporting conservation initiatives which aim to address declining wild fish numbers in Scotland by restoring habitats, protecting against predators and supporting river restocking.”
NatureScot, which also helped fund the UCL study, said it was happy to be involved in “this open access, peer-reviewed paper which adds to the knowledge about the impacts of the fish farming industry”.
It pointed out that the fish farms were all given the go-ahead before 1997, and that most of the samples had been taken over a decade ago.
But a spokesperson added: “It offers useful information about the scale of the impacts and some of the factors that might be helpful to both the industry and regulators in considering siting and management of current and future developments.”
Researchers at University College London did not respond to requests to comment.
The Ferret is an independent, investigative journalism cooperative dedicated to holding power to account. Since 2015, we have reported on corruption, environmental issues, human rights, and political influence in Scotland. Our work is supported by members who believe in journalism that challenges, exposes, and informs. If you value in-depth, fact-based reporting, you can support our work by becoming a member. Join us today: https://theferret.scot/subscribe
Cover image of Loch Arkaig thanks to iStock/Cablach.
Who would believe a word that the Fish Farmers say, unfortunately helped by the Scottish government. Time for fish farms to be on-shore.
It’s not just commercial fish farming that needs looked at…
Studies should be undertaken to examine whether stocking levels set by angling syndicates can cause environmental impacts similar to fish farms.