Study finds link to salmon farms in infection of wild salmon

By Scott Lingley

Dr. Mark Lewis

March 29, 2005 - Two University of Alberta researchers have co-authored a study that establishes a connection between salmon farms and an increase in sea lice infections among wild salmon populations on the British Columbia coast.

The study, by U of A biology grad student Martin Krkosek, Dr. Mark Lewis, Canada Research Chair in mathematical biology at the U of A, and Dr. John Volpe of the University of Victoria, took advantage of innovative field techniques and state-of-the-art models of disease transfer to provide evidence that salmon farms along migration routes on the B.C. coast have extensive and far-reaching impacts in contributing to sea lice infections among wild pink and chum salmon. Their findings have been published in the current edition of the U.K.-based scientific journal Proceedings of the Royal Society B.

“Initially we were just interested in trying to model the disease progression, but we found out fairly quickly there was no agreement on whether the farms actually contributed to disease in wild salmon when it came to sea lice,” said Lewis, who supervises Krkosek’s research. “So we had to back up a step and say, 'what kind of evidence can we find for or against infection of wild salmon from farms?' ”

Where traditional observation methods would have led to destruction of specimens, Krkosek developed a simple, non-lethal means of catching juvenile salmon for study, then re-releasing them, enabling him to do comprehensive individual observation of a large number of specimens along the 60-kilometre migration route.

“Marty collected a data set where he managed to examine more than 5,000 of these small fish before they got to the farm, when they were near the farm, and after they had passed the farm, so he had a lot of spatial resolution and he replicated this several times on different channels at different times,” Lewis said. “Immediately one could see a big jump in infection levels once the fish passed the farm.

“Where the mathematics comes in is using the biology that we understand about infection and coming up with a model that makes predictions and seeing how well the predictions are correlated with the data . . . with the model you can go in and say we know with a high degree of precision that it is the farm that causes the infection and then, also, using the model we can work out the level at which the farm is producing these lice.”

The study found that sea lice production from a single farm is 30,000 times higher than if the farm weren’t there. And because the mesh separation between the salmon farm and the migration route is permeable, wild salmon are swimming past a concentrated source of infection.

“Sea lice are a natural disease found in salmon and the ironic thing is that the farmed salmon are actually catching the disease from wild salmon. But when there are so many salmon in this confined space, then the lice levels skyrocket because there are so many other fish that can get infected,” Lewis said.

He noted that part of the environmental concern about mounting rates of sea lice infection is that wild salmon are being exposed to the parasites at an early stage of development. A full-grown adult may have the body mass to withstand sea lice infection, which feed on the host fish and leave lesions on the body, but young pink or chum salmon might only be three centimeters long and half a gram in mass and would have a much lower lethal limit for lice infection.

Lewis said more research is needed to determine the potential ecological as well as economic impact of increased sea lice infection on wild salmon migration routes, but the present study should clear away some of the controversy about the source of the problem.

“What we are hoping the research will do is to communicate with the industry, with academics and with government that there’s a big problem with farmed salmon infecting wild salmon as they’re going out to sea,” Lewis said. “This was something that there was a lot of contentious debate about, so we feel that this paper has really managed to resolve that and allows us to move on to how we’re going to deal with the issue. The next part of the problem, though, is quantifying the effect of the lice on the mortality of the fish.”


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