The spread of parasitic sea lice from salmon farms to wild salmon is a far bigger problem than had previously been imagined, a new study claims.
The parasites live in the slime that covers the fish
Researchers looking at a salmon farm in Canada found that infection levels in wild juvenile salmon near the farm were 73 times higher than normal.
Sea lice are crustacean parasites that can also affect fish. Both wild and farmed salmon are at risk.
Details appear in the science journal Proceedings of the Royal Society B.
During the course of their research, the scientists studied 5,500 young wild pink and chum salmon over 60km (37 miles) of their migration route in British Columbia, Canada.
The researchers were able to isolate the effect of a single farm on infection levels in a wild population because the facility was anchored in a long, thin fjord and the wild fish had no choice but to pass by the farm on their seaward migration.
The University of Alberta team sampled fish every 1-4km along the route, documenting the effect on the salmon as they moved towards the farm.
Zone of infection
Juvenile salmon carried almost no sea lice prior to the farm but became heavily infected as they approached it.
"Our research shows that the impact of a single farm is far-reaching," said lead author Marty Krkosek.
"Sea lice production from the farm we studied was four orders of magnitude - 30,000 times - higher than natural. These lice then spread out around the farm.
"Infection of wild juvenile salmon was 73 times higher than ambient levels near the farm and exceeded ambient levels for 30km of the wild migration route."
Previous studies on sea lice transfer have been disputed
Sea lice can lower the fitness of salmon - and in some cases be lethal - as they create open lesions on the surface of the fish that compromises its ability to maintain its salt-water balance.
When infection rates are high enough, the parasites feed on the fish at rates greater than the fish can feed itself, literally eating the fish alive. Young salmon are much more vulnerable due to their small size.
Andrew Dobson, an animal epidemiologist from Princeton University, US, said researchers were reporting similar effects in Scotland, Norway and Ireland.
"Sea lice are the kiss of death for salmon - these are fish that are already declining in the wild in Britain. This is not helping at all is it?" said Mike Donaghy, freshwater policy officer for WWF Scotland.
He added that in Scotland, fish farms were often located close to the entrances to salmon rivers, exacerbating the problem.
The farmed salmon industry in Scotland alone is worth £500m ($940m) per year.
Brian Simpson, chief executive of Scottish Quality Salmon in Perth, commented: "The real issue is how we control sea lice - this is an area where the farmers and the wild fish interests come together. Both sectors want no sea lice."
Mr Simpson said that salmon farmers on the west coast of Scotland were working together successfully with other interest groups on voluntary "area management agreements" to monitor sea lice and minimise the risk of infection for both farmed salmon and wild fish.
Mr Donaghy agreed that area management agreements had shown that sea lice infestations could be slashed in salmon farms, reducing the numbers available to infect wild populations.
"The industry has ways of dealing with this. We want to encourage them to do it more - taking a scientific approach - and think more about where they locate these particular fish farms," he explained.
Previous studies on the transfer of sea lice to wild salmon from farms have been dismissed by some in the industry. But experts say the precision of the data sampling and mathematical modelling in the latest study mean it will be hard to ignore.
Some Scottish salmon farmers argue that the decline in wild salmon and sea trout along Scotland's coastline began long before the advent of farming.