EFFICACY OF IVERMECTIN FOR CONTROL OF THE SALMON LOUSE LEPEOPHTHEIRUS-SALMONIS ON ATLANTIC SALMON

The efficacy of orally administered ivermectin against the common salmon louse Lepeophtheirus salmonis on Atlantic salmon Salmo salar was investigated under laboratory conditions. Both 3 and 6 doses of ivermectin at a targeted dose of 0.05 mg kg-1 fish administered in the feed every third day arrested the development and reduced the intensity of infection by L. salmonis. This is the first report of an efficacious treatment against the chalimus stages of sea lice. Serious head and dorsal body lesions, typical of L. salmonis feeding activity, which developed on the control fish were absent from the ivermectin-treated fish. Ivermectin fed at these dosage regimes resulted in a darkening of the fish, but appeared not to reduce their feeding activity.