Parasitic sea lice found in salmon farms are driving nearby populations of wild salmon toward local extinction, according to a scientific paper published Thursday.

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A young pink salmon infected with sea lice. ((Courtesy of Alexandra Morton/Science))

Researchers from the University of Alberta, Dalhousie University and the Salmon Coast Field Station in Echo Bay, B.C., said that if outbreaks of sea lice continue at their present rate, the population of wild pink salmon will drop 99 per cent in four salmon generations, or about eight years.

Martin Krkosek, with the centre for mathematical biology at the University of Alberta, said the rate of decline is so steep "it's arresting."

"Something has to happen immediately to turn this situation around," he said in a podcast accompanying the study, whichwas posted online Thursday in advance of Friday publication in the journal Science.

Krkosek said salmon farms are prone to infestations of a parasitic crustacean called Lepeophtheirus salmonis, or salmon lice. Salmon lice attach themselves to the exterior of pink salmon and feed on surface tissues like skin, blood and muscle and can cause stress, viral or bacterial infection and ultimately death, the authors said.

In wild pink salmon populations, the fish aren't normally exposed to salmon lice until they enter the ocean, two to three months into their lives, Krkosek said. But since salmon farms are often found inland, the young pink salmon are exposed at an earlier age, when they are less able to handle the parasites.

Using a mathematical model of population growth rates, Krkosek and his colleagues showed that salmon lice from farms are driving the wild pink salmon to local extinction.

"If we wait another 10 years to get more data, based upon what we're seeing right now, the fish are going to be long gone," he said.

The study is the latest to paint a bleak picture of wild pink salmon populations in the Broughton Archipelago, the group of islands and inlets off the northeast coast of Vancouver Island.

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Wild pink salmon routes in the Broughton archipelago (red), and salmon farms (black). ((Courtesy of Alexandra Morton))

The group looked at populations of wild pink salmon from 1970 to the present in 71 rivers: 64 where wild salmon populations were not exposed to salmon farms and seven where the fish had to pass at least one salmon farm.

In 2005, Krkosek and colleagues published a study in the London-based journal Proceedings of the Royal Society B, arguing sea lice populations were exploding off B.C.'s coasts.

Last year,marine ecologist John Volpe of the University of Victoria saidin a study published in the Proceedings of the National Academy of Sciences that sea lice from fish farms kill up to 95 per cent of juvenile salmon that pass by.

The issue, said Krkosek, is the free exchange of parasites and bacteria between open-net farms and the area outside the farms. He suggested moving farms from migration routes and moving to more closed containment of fish farms, replacing net pens with actual physical barriers.