Commercial fish farms must change the way they operate to ensure the survival of wild fish stocks, says an author of a study that found parasites from the farms kill as much as 95 per cent of wild young salmon that pass by them.

"We’re going to have big problems if we’re losing 95 per cent of fish," Martin Krkosek, a PhD candidate at the University of Alberta Centre for Mathematical Biology, who led the study, told CBC. "We’ve got to protect the wild fish from these fish farms."

The study, published Monday in the Proceedings of the National Academy of Sciences in the United States, found that sea lice from fish farms are killing large numbers of wild juvenile salmon that migrate past the facilities.

Farms typically contain their fish in net pens that are open to the water, making it possible for the parasites to be transmitted to the environment around them, Krkosek said.

Exposed to lice sooner

In the absence of the fish farms, the juvenile salmon would normally have between three and five months to grow before they would be exposed to sea lice, which are typically carried by adult salmon, Krkosek said. The juvenile salmon would enter the ocean without lice months before the adult salmon return.

But the farms artificially concentrate large numbers of sea lice near salmon migration paths, forcing the juvenile salmon to pass through clouds of the parasites, the study found.

The mortality rates rise from a low of nine per cent early on in the spring season to the peak of 95 per cent as more juvenile salmon have increasing exposure to rising numbers of sea lice, according the study.

Biologists and mathematicians in coastal British Columbia conducted the study, which was funded by the federal government and organizations that include the David Suzuki Foundation, the Canadian Sablefish Association, and the British Columbia Wilderness Tourism Association.

The study's co-authors are with the University of Victoria and the Raincoast Research Society, both in B.C., and the University of Hawaii.

Changes needed

The findings suggest that the fish farming industry needs to make dramatic changes to the way in which it conducts its operations if wild fish stocks are to survive, initially by getting the sea lice infestations under control, Krkosek said.

"In the short term they can start by increasing the amount of drugs, adding them more frequently, using more heavy doses," to kill the sea lice on the farmed fish, he suggested. "But there's a problem with that, because parasites can build resistance to the drugs, so you have to use more to kill them.

"That's a problem, too, because the drugs can be harmful to humans, so you can't eat the fish for a month after it's farmed."

The only workable solution to ensure the survival of wild fish stocks is to keep the fish farms and their byproducts physically separated from them, Krkosek said.

"It doesn't make any sense to put them in the corridor where they're near migrating salmon," he said. "We need to move fish farms to places where they're not near migration paths and move to a closed containment system, where there's an actual physical barrier between the farms and the open water."

Such a system would see the water used by farms treated before it is pumped in and out of containment tanks to protect both the farmed and wild fish, he said.

Despite the urgency of the problem, Krkosek said he expects the study to come under fire from some quarters.

"There are a few people who are going to be severely critical of the paper," he said. Asked who he was referring to, he responded: "People who are funded by the fish farming industry or who work for the industry."