A study of dichlorvos (Nuvan; 2,2 dichloroethenyl dimethyl phosphate), a therapeutic agent for the treatment of salmonids infected with sea lice (Lepeophtheirus salmonis )

Sea lice (Lepeophtheirus salmonis ), ectoparasites of salmonids, are creating concern within the aquaculture industry of Canada. The prophylactic agent of choice to treat for these parasites is dichlorvos. Larval, juvenile, and adult lobsters (Homarus americanus ), species of zooplankton, species of phytoplankton, mussels (Mytilus edulis ) and periwinkles (Littorina littorea ) were exposed to dichlorvos at various concentrations in the laboratory and mortalities were recorded. Dichlorvos was also released from an ocean field site and at a fish farm while stage IV larval or juvenile lobsters were suspended in the water column in areas adjacent to the release site to determine the effects of dichlorvos on their survival. Results indicate that dichlorvos is not toxic to mussels or periwinkles at 1.0 ppm, for 1 h exposure, but is toxic to larval lobsters, adult lobsters, zooplankton and phytoplankton. In field trials larval and juvenile lobsters housed adjacent to sea-cage operations were not killed following treatment with dichlorvos.