November 6, 2020


Salmon along Canada's British Columbia coast more likely to encounter pathogens near salmon farms



A new study has found wild salmon along the British Columbia, Canadian coastline to have an almost triple possibility to encounter harmful pathogens near active salmon farms.


A study from the University of Toronto tested 58 sites along the BC coastline for the past three years and found 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 the BC waters but stocked farms have the potential to act as reservoirs of hosts for pathogens.


Lead researcher Dylan Shea, a PhD candidate at the University of Toronto's Ecology and Evolutionary-biology Department, said the occurrence of DNA varied by times and locations, adding that the study relates only to the presence of DNA, not the risk of infection to fish.

"There are many unknown 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 micro-parasites we studied," said Shea.


Two of the most commonly detected agents include the erythrocytic necrosis virus, which attacks red blood cells, affecting fish's metabolism, and candidatus syngamydia salmonis, which causes 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 on October 21.


Veterinarians on salmon farms verified young salmon are free from a variety of pathogens, including some identified in the recent study before they are transferred to open-net pens and undergo regular screenings thereafter.


However, Shawn Hall, a spokesperson for the BC Salmon Farmers Association noted that "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 fallow salmon farms. Based on this major exclusion of other test sites, it is clear that study results lack 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 need 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 that the findings call into question recent Department Fisheries and Oceans Canada (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," the foundation's 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 First Nations before deciding later this year on whether to renew aquaculture licenses in the Discovery Islands.


A DFO spokesperson said the University of Toronto's study is among the 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, exposure dose and duration are all factors that play a role, and are analysed when assessing risk."


 - Oak Bay News