[It is] a very brutal form of treatment which clearly causes distress and suffering… Killing fish by overheating, whether accidental or not, is simply inhumanePhilip Lymbery, Compassion in World Farming The Prince of Wales during his visit to a Loch Leven fish farm facility in OctoberCredit:PA WIRE Documents released under the Freedom of Information Act reveal that in the single worst incident 95,400 salmon were killed at a Marine Harvest farm in Loch Greshornish, on the Isle of Skye, in the summer.The fish were being treated for sea lice with a new device called a Thermolicer.The “sudden temperature change” in the water caused by the Thermolicer killed 95 per cent of the lice, but it also caused “significant mortalities” amongst the salmon, according to the documents.Officials estimated that the losses cost Marine Harvest over £2.7 million.Another 20,000 salmon were killed at the Loch Greshornish fish farm by other attempts to rid them of sea lice using chemicals.In an email, Loch Greshornish’s site manager Bill Wright reported to Marine Scotland’s senior fish health inspector that there had been more than 115,000 salmon deaths at the farm between July 3 and September 9 this year, “all due to treatment losses”. Another incident saw more than 60,000 salmon killed by hydrogen peroxide being used to treat them for amoebic gill disease at Marine Harvest’s fish farm in Soay Sound, off the Isle of Harris, in September.The deaths led to thousands of salmon carcasses being transported by lorry from the affected farms to incinerator plants near Wigan for destruction.The campaign group Global Alliance Against Industrial Aquaculture (GAAIA), which obtained the government memos revealing the deaths, called for those responsible to be prosecuted.Don Staniford, director of GAAIA, said: “Serious questions must now be asked how thousands of farmed salmon became overheated, suffocated or even boiled alive by the Thermolicer.”Marine Harvests has contracts to supply fresh and smoke salmon to several supermarket giants, including Sainsbury’s and Tesco.The revelations come just a month after the Prince of Wales visited one of Marine Harvest’s farms, at Loch Leven, which houses more than 550,000 salmon.Following the visit, palace officials praised Loch Leven farm for its sustainable practices which they said “set the benchmark for the Scottish fishing industry”. The carcasses of salmon being transported for destruction following a similar incident Serious questions must now be asked how thousands of farmed salmon became overheated, suffocated or even boiled alive.Don Staniford, Global Alliance Against Industrial Aquaculture Thousands of salmon were poached alive when a lice treatment process at Scottish fish farms went disastrously wrong, it has emerged.More than 175,000 salmon died when the water they were swimming in was overheated, prompting calls for those responsible to be prosecuted for cruelty.The catastrophic mistake by staff at farms run by the Norwegian multinational, Marine Harvest, cost millions of pounds and led to over 600 tonnes of dead salmon having to be incinerated.The deaths were largely responsible for a 16 per cent drop in the company’s Scottish salmon production and there are now fears it may lead to price rises for consumers in the run up to Christmas. The plan was backed by Mr Ewing, who promised to set up an “industry leadership group”.But animal welfare campaigners have said the recent deaths showed the plan to be fundamentally misguided. They claim the industry is already unable to deal with lice infestations because of the conditions in which the salmon are kept.Mr Stanniford said: “Sea lice infestations and infectious diseases such as Amoebic Gill Disease are quite literally choking the Scottish salmon farming industry to death.”The Thermolicer is supposed to works by bathing fish briefly in lukewarm water, capitalising on the parasite’s low tolerance for sudden changes in temperature.But Compassion in World Farming described it as “a very brutal form of treatment which clearly causes distress and suffering to the fish”.The campaign group’s chief executive, Philip Lymbery, said: “Killing fish by overheating, whether accidental or not, is simply inhumane. All current forms of treating sea lice entail problems.”Marine Harvest defended its practices, saying that the dead salmon had already been weakened by gill disease.Steve Bracken, the firm’s business development manager, said: “We regret any loss of fish and are always mindful of the welfare of the fish and aim to continuously improve our methods to address changing environmental circumstances.“We have been dealing with a number of challenges in relation to fish health. We have also faced challenges with amoebic gill disease which is increasing in this part of the world as a result of climate change.” Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Details of the incidents were revealed in a series of documents submitted by fisheries officials to the Scottish government’s rural economy minister Fergus Ewing.In recent months Marine Harvest fish farms in the Hebrides and Wester Ross have suffered a spate of gill disease outbreaks.There are concerns that lice are becoming increasingly resistant to chemical treatment – partly as a result of climate change – forcing fish farms to resort to potentially risky measures to get rid of the pests.According to the company’s latest quarterly report to investors, its production of salmon in Scotland has dropped by 16 per cent since last year as a result of the deaths.The report admits that costs increased due to “incident based mortality” that was “mainly related to gill disease and sea-lice treatment losses”.The deaths are a blow to the vital Scottish fish farming industry, which this year launched a plan to double its business from the current £1.8 billion to £3.6 billion by 2030.