Environmental effects of the anti-sea lice (Copepoda: Caligidae) therapeutant emamectin benzoate under commercial use conditions in the marine environment

Sea lice infestation can pose a particular problem to intensive cage aquaculture of salmonids. It is most often treated by therapeutants added to the water or incorporated into fish feed. This study investigated the environmental effects of one such in-feed treatment, emamectin benzoate, under commercial use conditions. Atlantic salmon were fed the commercial preparation of emamectin benzoate at a production fish farm on the west coast of Scotland. Sediment dwelling fauna, large mobile fauna and sentinel organisms were monitored for potential impacts over a 1 year post-treatment period (23 September 1997 to 23 September 1998). Additionally, sediment and water samples, transplanted blue mussels and captured large fauna were analysed for accumulated emamectin benzoate and its desmethylamino metabolite to investigate the long term environmental fate of emamectin benzoate.Results from analysis of sediments suggested that limited deposition of emamectin benzoate took place during and up to 1 week post-treatment, and may have continued up to 4 months of post-treatment. This was consistent with the release of the active ingredient from fish excretion and defecation after treatment. Quantifiable amounts of emamectin benzoate were found 10 m from the cages 12 months after treatment. However, clear reduction in overall sediment concentrations indicated that the deposited emamectin was degrading over time. Emamectin benzoate was not detected in water samples during the field trial. Quantifiable concentrations of emamectin benzoate were found in blue mussels deployed up to 100 m from the treatment cages at 1 week post treatment, but found only at 10 m from the cages at 1 month post-treatment. This suggests that while mussels were actively accumulating emamectin benzoate immediately after treatment by 1 month this had largely been depurated.Macrobenthic faunal analysis indicated that there was no evidence that the occurrence of emamectin benzoate, or its desmethylamino metabolite, in sediments around fish farm cages after treatment had any toxic impacts on organisms in either water column or sediments.