Salmon farms are transferring sea lice to wild salmon in a much larger area of British Columbia's coastal waters than first thought, according to new research by B.C. scientists.

Fisheries scientists have long linked salmon farms in the Broughton Archipelago, between the north end of Vancouver Island and the mainland, with the sea lice that wild juvenile salmon pick up as they migrate past farms on their way to the open ocean.

The latest study, published in the current issue of the peer-reviewed Canadian Journal of Fisheries and Aquatic Sciences, shows juvenile salmon picking up sea lice throughout the Georgia Strait to the south and in Finlayson Channel on the Central Coast, northwest of Bella Coola.

Prof. John Reynolds of Simon Fraser University says the research underscores the value of moving salmon farms with open net pens out of migration routes of wild salmon and ultimately into land-based closed containment systems.

Several studies have shown sea lice can kill small juvenile salmon. Parasite outbreaks from salmon farms have been linked to the collapse of wild sea trout and Atlantic salmon populations in Norway, Scotland and Ireland. The lice attach themselves to the exterior of the salmon and feed on surface tissues like skin, blood and muscle and can cause stress, viral or bacterial infection and ultimately death.

Reynolds and two colleagues, Prof. Michael Price of the University of Victoria and Alexandra Morton, the director of the Salmon Coast Field Station, sampled juvenile pink and chum salmon over two years and found much greater infection rates for juvenile salmon that pass by fish farms, compared with those not exposed to the farms.

"Depending on the region, you could have as much as a seven- or 30-fold increase in the percentage of the fish that have sea lice on them after they passed by the fish farms," Reynolds said.

The researchers found fewer than five per cent of the juveniles that had not been exposed to fish farms had sea lice. Areas with the most fish farms, such as the Georgia Strait, had the highest infection rates of wild fish.

"Given the current intensity of farmed salmon produced in British Columbia, and the proposed expansion of the industry, there is concern that lice outbreaks and negative impacts on wild salmon populations could occur elsewhere," the researchers write in their paper.