Effect of sea lice on the survival of wild Atlantic salmon smolt in areas with and without open net-pen salmonid aquaculture in Newfoundland, Canada

Amplification and transmission of sea lice (e.g. Caligus spp. and Lepeophtheirus spp.) from Atlantic salmon Salmo salar aquaculture operations has been hypothesized as a contributing factor to the decline of wild salmonid populations. Here, a randomized controlled trial, consisting of paired releases of 4024 antiparasitic-treated (emamectin benzoate; EMB) and 4023 sham-treated wild Atlantic salmon smolt over 2 yr (2023 and 2024), and collected from 2 rivers in Newfoundland, Canada, was used. Migration routes included coastal marine environments with and without Atlantic salmon and steelhead trout Oncorhynchus mykiss aquaculture. The objectives were to determine if EMB treatment was associated with differences in smolt survival in 2023 and 2024 between coastal marine environments and years, and to infer the likely contribution of sea lice-related marine mortality to the decline of some wild Atlantic salmon populations in the region. Overall, the effect of EMB treatment on adult returns was dependent on the river system, with more sham-treated smolt compared to treated smolt returning as adults to Conne River (area with aquaculture), and conversely, more EMB-treated smolt compared to sham-treated smolt returning as adults to Campbellton River (area without aquaculture). While the pattern was consistent between years for Campbellton River, this was not the case for Conne River. Results suggest that sea lice can be a likely source of direct or indirect marine mortality for wild Atlantic salmon smolt even in areas without salmonid aquaculture, affirming the importance of establishing non-aquaculture reference baselines and accounting for interannual variability when interpreting impacts of salmonid aquaculture.