Wild salmon may be almost three times more likely to encounter harmful pathogens near salmon farms, a new study has found.
Scientists from the University of Toronto spent three years testing at 58 sites along the B.C. coastline, finding the environmental DNA (eDNA) of 22 viruses, bacteria and other microscopic organisms had a 2.72 times higher probability of being detected near active salmon farms, versus inactive sites.
The pathogens are naturally occurring in B.C. waters but stocked farms have the potential to act like reservoirs of hosts for pathogens.
Lead researcher, Dylan Shea, a PhD candidate at the university’s ecology and evolutionary-biology department, said the occurrence of DNA varied by times and location, adding the study relates only to the presence of DNA, not the risk of infection to fish.
“There are many unknowns between what we observed and the disease implications for any given fish species … but the findings themselves convey a pretty important message. I can say with confidence that salmon farms do increase the risk of exposure to the suite of microparasites we studied.”
Two of the most commonly detected agents include the Erythrocytic necrosis virus, which attacks red blood cells, affecting a fish’s metabolism, and Candidatus Syngamydia salmonis, causing swelling of the internal organs and skin lesions, which can potentially lead to secondary infections.
The study was published in the scientific journal Proceedings of the Royal Society Oct. 21.
Veterinarians on salmon farms verify young salmon are free from a variety of pathogens, including some identified in the recent study, before they’re transferred to open-net pens. The fish undergo regular screenings thereafter.
Shawn Hall, a spokesperson for the B.C. Salmon Farmers Association, said there are other cautions to note in the latest research.
“The study did not look at viable pathogens, but rather fragments that don’t necessarily cause any concerns related to disease,” Hall said. “Also, the study did not test any control sites – different areas of the ocean. Samples were only taken near operating or fallowed salmon farms. Based on this major exclusion of other test sites, it is clear that study results lacks the comprehensive research support.
“Good science and protecting wild salmon will always be the foundation of responsible ocean-based salmon farming in BC. New studies always needs to be looked at with a critical eye focused on how the research was conducted.”
The David Suzuki Foundation, which supported the study, said in a statement the findings call into question recent DFO risk assessments that found the transfer of nine pathogens from farms to wild fish did not pose a significant threat to wild populations, some of which are at their lowest numbers on record. The assessments were spurred by the 2012 Cohen Commission inquiry that recommended a prohibition on salmon farms in the Discovery Islands by September, 2020, if the threshold for disease transmission exceeded “minimal risk”.
“Uncertainty should inspire precaution,” David Suzuki Foundation marine conservation specialist Kilian Stehfest said. “The study’s findings further highlight the importance of applying a precautionary approach when making management decisions based on incomplete evidence with high levels of uncertainty.”
DFO is currently consulting with area First Nations before deciding later this year on whether to renew aquaculture licences in the Discovery Islands. A DFO spokesperson said the University of Toronto study is among many that will be reviewed and incorporated into the fisheries minister’s decision, but stressed the implications of the latest research are still unclear.
“Exposure to pathogen(s) alone is not sufficient to cause disease — the nature of the host, environmental conditions, and exposure dose and duration are all factors that play a role, and are analyzed when assessing risk.”