Ecological Impacts of Emamectin Benzoate, a Sea Lice Therapeutant, on Benthic Communities in the Vicinity of Norwegian Aquaculture Facilities

Sea lice (<i>Lepeophtheirus salmonis</i> and <i>Caligus elongatus</i>) are a major challenge for aquaculture in Norway and beyond. The aquaculture industry uses medicinal (e.g., emamectin benzoate (EMB)) and non-medicinal (e.g., thermolicers) methods to control sea lice. Non-medicinal methods can harm fish, while medicinal treatments can accumulate on the seafloor and impact the environment. Nevertheless, research on EMB's effects on non-target fauna in Norway remains limited. This study is the first to integrate field and modeling analysis of EMB and benthic ecology in Norway, aiming to (i) map EMB distribution around farms, (ii) assess impacts on benthic communities, and (iii) compare EMB's effects on crustaceans to other stressors. Results show EMB had the strongest effect on benthic abundance and species richness, compared to other stressors (e.g., organic matter). EMB concentrations at 0.2 ng/g, near the SEPA EQS of 0.272 ng/g, reduced species diversity and disrupted community structure, with crustaceans being most sensitive. EMB was found to be widespread, persistent, and to accumulate in sediments long after use. Dispersion modeling suggest harmful concentrations could extend 1 km from farms. These findings highlight the need for improved monitoring, regulation, and sustainable lice management to protect biodiversity in aquaculture intensive areas.