New study suggests fish farms raise risk of exposure to infectious disease for wild B.C. salmon

“This suggests that the management of salmon farms and the conservation of wild salmon are absolutely not distinct issues,” said the lead author on the new paper, University of Toronto PhD candidate Dylan Shea.

“Wild salmon conservation should be taken into account when making decisions related to salmon aquaculture.”

The authors of the study, published Wednesday in the journal Proceedings of the Royal Society B, also include scientists from Fisheries and Oceans Canada.

The research comes at a critical time for federal management of wild salmon and B.C. fish farms.

Just last month, the government concluded that open-net fish farms in B.C.’s Discovery Islands pose a “minimal risk” to wild salmon, meeting a deadline set by the 2012 Cohen Commission. The commission’s report had called for the prohibition of fish farms in the area by Sept. 30, 2020, unless there is proof they pose only a “minimum risk of serious harm to the health of migrating Fraser River salmon.”

Officials are now consulting with local First Nations before making a final decision on aquaculture licences in the Discovery Islands — located between Vancouver Island and B.C.’s mainland — by the end of the year.

‘Notoriously difficult’ question to answer

Shea’s research looked at 58 active and inactive farm sites, and compared the probability of detecting pathogen DNA.

“This has been something that’s been notoriously difficult to assess because it’s not as simple as visualizing sea lice on the outside [of a fish],” Shea said.

The presence of genetic material in the water doesn’t necessarily mean there are viable pathogens as well, Shea acknowledges, but it does suggest that wild salmon are at higher risk of infectious disease near active fish farms.

The researchers screened for 29 different microorganisms that can infect salmon, and detected the presence of 22 of those.

That includes bacteria like Moritella viscosa, which causes skin lesions that lead to chronic health issues in salmon, and Tenacibaculum maritimum, which causes mouth rot. Other pathogens included in the study may cause less severe infections.

“We don’t really know what the impact of some of those less severe pathogens are when the infection is in a migrating salmon that has hundreds or thousands of kilometres of open water to cover, and then the infection has time to increase in severity,” Shea said.

Study co-author Martin Krkosek, an associate professor of biology at the University of Toronto, said the results are particularly interesting in light of the COVID-19 pandemic. As he points out, the outbreak of the novel coronavirus is believed to have begun with transmission from wild animals to humans.

“There’s a broad diversity of pathogens that are being passed between domesticated and wild animals, and that’s one of the main mechanisms from which you get these new diseases,” Krkosek said.

“Dynamics that underlie the emergence of COVID and other human infectious diseases are still at play when we’re thinking about things like fisheries management. We’ve got a diversity of pathogens here whose transmission dynamics are being augmented by the introduction of domesticated fish populations in British Columbia.”