The parasites live in the slime that covers the fish
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.
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.”
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.
This story and images were taken from the following BBC news article.
