The effects of sediment organic carbon and chemical residence time on the acute toxicity of sea lice chemotherapeutants to benthic invertebrates

Chemotherapeutants are commonly used to manage sea lice outbreaks in salmonid aquaculture. Among the classes of chemotherapeutants used are avermectins; these tend to persist in the sediments underneath salmon farms and may directly impact nearby benthic fauna of marine ecosystems. The present study sought to determine how two environmental factors – namely, sediment organic carbon (OC) and chemical residence time – can modify the toxicity of emamectin benzoate (EB; formulation: Slice®) and ivermectin (IVM) in two species of benthic invertebrates: the amphipod Eohaustorius estuarius and the polychaete Neanthes virens. In both species, sediment OC significantly reduced toxicity, an effect that was more pronounced for IVM and combination exposures. Four months of chemical residence time reduced toxicity in E. estuarius but did not affect toxicity in N. virens. This research provided novel insight into the effects of two environmental factors that potentially impact avermectin toxicity in nontarget species underneath salmon farms.