FISH farms are responsible for the deaths of up to 95% of young wild salmon migrating out to sea, according to a new report.

Concentrations of sea lice are 30,000 times higher around fish farms in coastal waters than in deep waters. The new research warns that young wild salmon are dying after swimming through plumes of lice from infected fish farms.

“We know that fish farms raise sea lice levels, and we know that sea lice kill fish,” said the report’s author, Martin Krkosek, a mathematical biologist at the University of Alberta. “This is the first study to estimate the total impact.”

The report is the latest controversy to hit salmon farming. Scientists warned two years ago that some farmed salmon contained potentially harmful levels of cancer-causing toxins and that consumers should consider restricting the amount they eat.

In the research, to be published this week by America’s National Academy of Sciences, an international team of biologists, mathematicians and environmentalists studied young salmon on their 37-mile migratory passage past open-net fish farms off the coastline of British Columbia, western Canada.

They sampled fish at regular intervals along the route, documenting the effect on the salmon as they swam down it.

Young salmon carried almost no sea lice before reaching the fish farms, but became heavily infected as they approached them and swam through the plumes.

Post-mortems were conducted on the fish that died. The researchers estimated that 9% died in early spring when the sea lice population was low, while 95% were killed later in the season when sea lice numbers swelled.

The lice survive by eating flesh and tissue, and are not generally dangerous to adult species.

They are, however, a risk to juveniles because their scales and outer tissue are softer and cannot protect vital organs against attack.

“It takes only one or two sea lice to kill a juvenile pink or chum salmon,” added Krkosek. “The juveniles are so vulnerable because they are so small — only one to two inches long.”

Dr Mark Lewis, a mathematician and biologist at the University of Alberta who also worked on the project, said: “Only a small fraction of juvenile salmon survive to return as adults. The fish-farm sea lice are reducing that fraction even more.”

The UK has 390 active fish farms, mainly in the coastal waters of Scotland. The Scottish salmon industry alone is estimated to be worth £700m a year, and supermarket sales have now risen to more than 160,000 tonnes a year, nearly all of which comes from farmed fish.

Environmentalists claim the problem of sea lice in fish farming is as widespread in Scotland as it is in British Columbia. Anglers are also concerned at the impact on salmon stocks. According to environmentalists, there has been a 45% decline in wild salmon over the past two decades.

Paul Knight, from the Salmon and Trout Association, which represents anglers, said: “We are aware that fish farms harbour sea lice. However, this report highlights the devastating effects that such farms really have on the rest of the sealife population. Quite simply, more needs to be done.”

Currents can sweep significant numbers of sea lice up to 12 miles away from the pens. Though pesticides reduce the problem, experts say the most effective solution is land-based containment tanks with sea water pumped into them. Although this would increase costs, trials in parts of British Columbia have successfully prevented lice and diseases spreading into the natural aquatic environment.

Another option — putting pens offshore — would reduce sea lice in salmon migratory routes but could spread lice to other wild fish.

The Scottish Salmon Producers Organisation, a body representing the Scottish fish farm industry, said it was working to promote long-term sustainable salmon farming and wild salmon fisheries.

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