A comprehensive peer-reviewed study debunks the false claim by anti-aquaculture activists that ‘no salmon farms mean no sea lice’.
By Fabian Dawson
SeaWestNews
A comprehensive eight-year scientific study has dismantled the central argument made by anti-fish farming activists that removing salmon farms from British Columbia’s coastal waters results in lower sea lice levels on wild fish.
Published in the Journal of Fish Diseases, the peer-reviewed paper found that sea lice levels on wild salmon in 2024 were among the highest recorded during the last eight-year period in the Discovery Islands, despite the closure of salmon farms.
“A similar pattern with a low sea lice infestation in 2023 and higher levels of sea lice infestation in 2024 was also observed in the Broughton Archipelago and other areas with and without salmon farms,” said Lance Stewardson, Director of Mainstream Biological Consulting Inc. and one of the study’s authors.
“These findings demonstrate that the evidence does not support the narrative of ‘no salmon farms mean no sea lice,” said Stewardson, who holds an ongoing annual contract with B.C. aquaculture producers Cermaq Canada, Grieg Seafood BC, and MOWI Canada West to conduct sea lice assessments on wild juvenile salmon.
The other authors of the study are Dr. Simon Jones, a research scientist with Fisheries and Oceans Canada, and Crawford Revie, a professor at the University of Strathclyde Department of Computer and Information Sciences in Scotland.
“Our findings disprove the claim that salmon farms are the sole driver of sea lice on wild Pacific salmon in the near-shore environment and underscore the need for continued monitoring,” said Stewardson.
Sea lice are naturally occurring parasites found on many species of marine fish, they pose no risk to humans. Studies have consistently shown that, in BC regardless of the presence or absence of salmon farms, there is wide variability in sea lice prevalence in coastal locations.
Farm-raised salmon are free of sea lice when they are entered into the ocean, but during the migration season adult wild salmon may pass sea lice to farm-raised salmon.
The latest study comes at a pivotal moment, as Ottawa faces growing legal and political pressure over its plan to phase out ocean salmon farming in BC — a move largely driven by activist campaigns that have long used sea lice as a cornerstone argument against the industry.
The proposed ban threatens to wipe out 4,560 jobs across BC, including about 1,000 held by Indigenous workers and could expose Canadian taxpayers to as much as $9 billion in compensation claims from salmon farmers, suppliers, and impacted First Nations.
According to the study, the removal of Atlantic salmon aquaculture facilities from the Discovery Islands began in 2021. Since 2022, there have been no active farms in the region. But despite their absence, sea lice levels on wild juvenile pink and chum salmon in 2024 were among the highest documented since 2017.
Data for the study was collected in collaboration with local First Nation stewardship staff. The research team analyzed sea lice prevalence (percentage of infested fish) and mean abundance (number of lice per fish) in wild juvenile salmon across 16 sites in the Discovery Islands region.
The study concluded that while there was a reduction in sea lice after farms were removed between 2020 and 2022, lice levels rose again in 2024, even though no farmed Atlantic salmon remained in the area. This strongly suggests that natural environmental factors and other marine species are playing a dominant role in lice outbreaks.
These findings are consistent with 2024 data from the Broughton Archipelago and support an earlier Canadian Science Advisory Secretariat (CSAS) 2022 Science Response, which “concluded no statistical correlation between sea lice counts on wild and farm-raised salmon populations.”
As detailed in the study, environmental drivers such as water temperature, salinity, natural host reservoirs (like herring and stickleback), and ocean currents likely play a much more significant role in the sea lice life cycle than the presence or absence of fish farms.
This data validates what salmon farmers, and their Indigenous partners have said for years that policy decisions impacting their industry must be based on evidence, not activism.
Activists, however, have already begun questioning the study, pointing to potential conflicts of interest given that Stewardson’s consultancy work with aquaculture companies. But the authors note the data was independently reviewed, transparently collected under government protocols, and supported by First Nations Fisheries Guardians on-site.
They emphasized that the real threat to wild salmon stems from a failure to understand the complex environmental factors influencing sea lice outbreaks. The authors also cautioned that attributing sea lice infestations solely to salmon farms oversimplifies the issue and risks misguiding conservation efforts.
(Main file image shows a salmon smolt)