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.
Toxic fish farm pesticide polluted ten lochs across Scotland
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,...