Sea Lice from Salmon Farms May Threaten Wild Salmon Populations

Young pink salmon infected with sea lice.

[Image courtesy of Alexandra Morton]

A new article in Science suggests that sea lice outbreaks at salmon farms on Canada's West Coast may cause the local extinction of wild salmon within 10 years.

Because of their crowded conditions, salmon farms are prone to infestations of parasitic crustaceans called Lepeophtheirus salmonis, or salmon lice. These small organisms attach themselves and feed on tissues from salmon and other fish.

While sea lice naturally occur on wild adult salmon, juvenile fish are usually protected against them because infected adult salmon typically live offshore when the juveniles enter the sea.

But as salmon farms increase in the rivers and coastlines of Western Canada, juvenile salmon are increasingly coming in contact with sea lice as they swim near salmon farms during their seaward migration.

Wild pink salmon routed in the Broughton archipelago (red), and salmon farms (black)

View a larger image of this map.

[Image courtesy of Alexandra Morton]

Martin Krkošek, a graduate student at the University of Alberta, Edmonton, and colleagues compiled data on the numbers of pink salmon in rivers in the central coast of British Columbia, comparing populations that were and were not exposed to salmon farms, from 1970 to the present.

Using a mathematical model of population growth rates, the researchers found that sea lice from farm-raised salmon are dramatically impacting local wild pink salmon.

The team concluded that sea lice have reduced the wild salmon population to the point that wild salmon, a vital economic resource, could be locally extinct in just four salmon generations—or possibly less, considering the salmon population decline is already underway—if the outbreaks continue.

"These results suggest that salmon farms can cause parasite outbreaks that erode the capacity of a coastal ecosystem to support wild salmon populations," the author wrote.

[Image courtesy of Alexandra Morton]

In a 13 December teleconference for reporters, Krkošek, along with several co-authors, called the sea lice infestation in juvenile salmon "severe," adding that it only takes one or two lice to kill a young fish.

To reduce wild juvenile exposure to sea lice from farm-raised adult salmon, the authors urged salmon farms to build barriers that would prevent the spread of the parasite. "The infestation occurs because farmed salmon are raised in open net pens where the parasites are freely transmitted between wild and farmed salmon," Krkošek said in the teleconference.

The report has met some resistance from industries involved in salmon farming who claim the statistics of juvenile salmon mortality are exaggerated and its implications are still unclear.

Kathy Wren and Benjamin Somers

14 December 2007