Salmon farms breed lice blight

Fishing for wild salmon becomes harder as parasites take their toll. Picture / Glenn Jeffrey

Salmon farms spread deadly parasites to wild salmon, according to one of the first studies to quantify the precise scale of the problem.

Scientists have found that a single fish farm can infect wild salmon up to 32km away with sea lice, and the number of parasites can be thousands of times greater than natural background levels.

The study was carried out off the coast of British Columbia in Canada, but scientists believe the findings are relevant to Europe, where salmon are also intensively farmed.

Marty Krkosek, of the University of Alberta, who led the research team, says the study involved sampling about 5500 wild juvenile salmon that were on a migration route up to 60km from the fish farm.

"Our research shows that the impact of a single farm is far-reaching. Sea-lice production from the farm that we studied was four orders of magnitude - 30,000 times - higher than natural," says Dr Krkosek.

"These lice then spread out round 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 migration route."

Salmon spawn in rivers and their young migrate out to sea, where they spend much of their lives before returning to the same river to breed.

It is on the journey from river to sea that they are most vulnerable to parasites.

Normally, sea lice infect only adult salmon, and rates are kept relatively low because they are widely dispersed out at sea.

In salmon farms, population is kept unnaturally high, making infection far more problematic.

Sea lice sap the strength of infected salmon. Adults usually survive because they are bigger, but juveniles are at higher risk of being killed, or are weakened and succumb to predators.

"The youngest fish can be only days old when they encounter a farm. Some of these fish are so young that their egg sacks haven't been fully absorbed when they are infected," Dr Krkosek says.

The scientists, whose study was published last week in the journal Proceedings of the Royal Society B, believe putting salmon farms in fiords or sea lochs near river estuaries can expose young salmon to parasites that they would not normally encounter.

Each sea louse can produce between 300 and 800 eggs during its lifetime, so infected juvenile salmon can quickly spread the parasites to other young fish.

Dr Andrew Dobson, a biologist from Princeton University in New Jersey, says parasites from fish farms can have a significant, negative impact on wild fish stocks in the area.

"We're seeing similar effects in Scotland, Norway and Ireland," he says.

Dr John Volpe, of the University of Victoria in British Columbia, says the evidence shows that salmon farms can be a constant source of parasites that attack other species of fish important to the wider marine ecosystem.

"We now realise that lice also attack other fish such as herring, which are the sparkplugs of entire ecosystems - everything depends on them, from salmon to whales to seabirds."

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