Commercial trials using emamectin benzoate to control sea lice Lepeophtheirus salmonis infestations in Atlantic salmon Salmo salar

Two trials were conducted at commercial salmon farms to evaluate the effica cy of emamectin benzoate (Slice(R), 0.2% aquaculture pre-mix, Schering-Plou gh Animal Health) as a treatment for sea lice Lepeophtheirus salmonis (Kroy er) and Caligus elongatus Nordmann infestations in Atlantic salmon Salmo sa lar L. Trials were carried out in 15 m(2) commercial sea pens, at temperatu res of 5.5 to 7.5 degrees C and 10.8 to 13.8 degrees C. Each pen was stocke d with 14 000 to 17 500 fish with mean weights of 0.44 to 0.74 and 1.33 to 1.83 kg. Fish were naturally infested with sea lice at the start of each tr ial. At Day -1, samples of 10 or 15 fish were taken from each pen to determ ine pre-treatment numbers of lice. Emamectin benzoate was administered in f eed, to 4 replicate pens, at a dose of 50 mu g kg(-1) biomass d(-1) for 7 c onsecutive days (Days 0 to 6). Sea lice were counted again, between Days 7 and 77, and comparisons made with untreated control fish. Despite adverse w eather conditions, wide variations in fish weights and exposure to new infe stations, treatment was effective against chalimus and motile stages of L. salmonis. In the autumn trial, efficacy at Day 27 was 89%, and lice numbers remained lower on treated fish than on control fish 64 d from the start of treatment. In the winter trial, reductions in lice numbers at low temperat ures were slower but good efficacy was achieved by Day 35. Although control fish had to be treated with hydrogen peroxide at Day 21, fish treated only with emamectin benzoate on Days 0 to 6 still had 89% fewer lice than contr ol fish at Day 35. There were very few C. elongatus present, but at the end of both trials numbers were lower on treated fish. No adverse effects were associated with treatment of fish with emamectin benzoate.