COS 233-6 - Parasitic sea lice on farmed and wild salmon in Pacific Canada

Abstract: Salmon farming is one of the most valuable forms of aquaculture worldwide, but its economic and ecological sustainability is continuously threatened by a group of parasites called sea lice. The densely packed fish in open-net salmon farms act as year-round reservoirs for sea lice, which provide a source of transmission to wild juvenile salmon that does not otherwise exist naturally. Of all the salmon-farming regions in the world, Pacific Canada is unique for having substantial populations of both farmed and wild salmon.



Here, we present two scientific updates on salmon aquaculture in the Broughton Archipelago, Pacific Canada’s epicenter for salmon farming and research into its ecological effects. Recent landmark agreements have shifted regulatory power to the Mamalilikulla, ‘Na̱mg̱is, and Kwikwasut’inuxw Haxwa’mis First Nations, which has resulted in the decommissioning of most salmon farms in the region. These agreements also mandated the sharing of historical data collected by the salmon-farming companies with the Nations.

In collaboration with these three First Nations, we analysed (1) whether sea lice have evolved resistance to emamectin benzoate (EMB; the main treatment for controlling outbreaks), and (2) whether sea lice on salmon farms are associated with a reduction in population productivity for wild salmon in the region. We established that EMB sensitivity has dramatically decreased since 2010, concurrent with a marked decrease in the field efficacy of EMB treatments, suggesting that sea lice have recently evolved resistance to the treatment. We also found a clear relationship between sea-louse abundance on wild and farmed salmon, as well as a strong negative association between sea-louse infestation and population productivity in wild pink salmon.

Our results suggest that sea lice in the Pacific Ocean have recently evolved substantial resistance to EMB, and that sea-louse outbreaks on Pacific farms will therefore be more difficult to control in the coming years. This is concerning given the predicted mortality due to sea lice for wild salmon in the region (e.g., 15.2% in 2020 and 6.1% in 2021). These analyses were only possible due to Indigenous-led agreements that created a legal obligation for salmon-farming companies to provide data to First Nations—a landmark and much-needed change in a country with a long history of industrial exploitation of resources in Indigenous territories.