Images of severely diseased, dead and dying salmon at an Icelandic fish farm, obtained by the Guardian, have been described by one veterinary expert as an “animal welfare disaster” on a scale never previously seen. The drone footage, shot last week over an open-pen sea cage in the country’s remote Westfjords region, shows salmon suffering from such a severe infestation of sea lice that huge numbers of the fish are having to be prematurely slaughtered. Up to 12 pens are believed to be affected by the parasites, which contained about a million fish last month, although the exact numbers have not been confirmed. A specialised vessel, the Hordafor III, sent from Norway to euthanise the fish, is seen in the footage. The parasite, a crustacean known as salmon lice – Lepeophtheirus salmonis – feeds on the salmon’s skin, causing open sores that stress and weaken the salmon’s immune system. In extreme cases, it can cause mass deaths. Berglind Helga Bergsdóttir, a specialist in fish diseases at MAST, the Icelandic food and veterinary authority, said that such a high infestation of lice had never been seen before in Iceland. “The lesions don’t get much worse than this,” said Bergsdóttir. The wounds were worsened by bacteria which make them deeper and bigger, she said. “What has surprised everyone here is how quickly it happened.” The incident comes at a difficult time for the open-pen salmon-farming industry in Iceland. In August, the escape of 3,500 salmon from a farm owned by Arctic Fish, one of two companies with pens in Tálknafjörður, the site of the current lice outbreak, caused outrage among environmentalists and led to an investigation by Icelandic police. Arctic Fish is one of the country’s largest salmon-farming companies, owned by Norwegian-based Mowi, the world’s largest salmon producer. The escape sparked a protest by Björk. The Icelandic singer is releasing a song with the Catalan star Rosalía to benefit activists fighting salmon farms, which she claimed were having a “devastating effect” on Icelandic wildlife. Mast stressed that the two companies owning sea pens at Tálknafjörður – Artic Fish and Arnarlax – had taken the decision to humanely slaughter the affected salmon, which are not permitted to be used for human consumption. Arnarlax said that not all the pens were necessarily affected. Once slaughtered, the fish would be processed for animal feed, it said. Mast is investigating the cause of the incident, Bergsdóttir said, which escalated after the authority gave permission for the outbreak to be treated with insecticide. Worryingly, the insecticide did not kill all of the lice and the infestation grew, Bergsdóttir said. “In two to three weeks, there were huge increases in lice.” One theory the agency would investigate is whether the lice had mutated, she said. “We know lice are very adaptable and one method they use is mutation, especially with the drugs.” Trygve Poppe, a salmon expert and former professor of fish health at the Norwegian school of veterinary science, said: “It is the worst I have ever seen, both in the extent of the lesions and the number of fish affected. It’s an animal welfare disaster.” Infestations of salmon lice were increasing across the industry, in Norway, Scotland and Iceland, he said. This year, eight open-pen farms were given permission to use insecticides including emamectin, azamethiphos and deltamethrin. Lice cause pain and blood loss in the fish, Poppe said. “You can see from the pink heads, the skin barrier between the fish and the water is completely gone,” he added. “The fish are leaking, causing a loss of balance. Most of the fish are just moribund. We are seeing the terminal stages of their lives.” Veiga Grétarsdóttir, a kayaker and activist who took the drone footage of the disposal of the salmon at Tálknafjörður last week, said she was shocked by the condition of the fish. “I don’t think I saw one healthy fish,” said Grétarsdóttir, who collaborates with the Icelandic Wildlife Fund. “They were moving very slowly, like zombie fish; dead but moving.” A spokesperson for Arctic Fish said: “Due to increased sea-lice pressure, Arctic Fish has decided to take out fish at Tálknafjörður. Increased sea-lice pressure in the area has unfortunately made it necessary. “The situation escalated very fast and the company is doing all it can to take out the fish as soon as possible.” He said that “the process of having the correct measures against lice approved and applied is far too bureaucratic”, which led to escalating numbers of lice. “Arctic Fish acknowledges the responsibility of the welfare of our fish but we also see an urgent need for an adjusted framework where the bureaucracy isn’t compromising the welfare [of the fish].” A spokesperson for Arnarlax said it had experienced higher pressure from lice than usual and had taken the decision to extract some salmon to ensure the “good biological status” of the remaining fish. He said the company, which now used a non-chemical method of delousing, had not lost 12 pens, but did not elaborate on the numbers destroyed. He confirmed that, after culling, the fish would be turned into animal feed.