Field trials to evaluate the efficacy of emamectin benzoate in the controlof sea lice, Lepeophtheirus salmonis (Kroyer) and Caligus elongatus Nordmann, infestations in Atlantic salmon Salmo salar L.

Three field trials were conducted to evaluate the efficacy of emamectin ben zoate as a treatment for sea lice, Lepeophtheirus salmonis (Kroyer) and Cal igus elongatus (Nordmann), infestations on Atlantic salmon Salmo salar (L.) . Trials were carried out at sea temperatures of 13.0-15.5 degrees C and 7. 2-8.5 degrees C. Salmon naturally infested with sea Lice, with mean weights of 438, 513 and 2662 g, respectively, were held in experimental pens on co mmercial sites. At day - 1 or -2, 20 or 30 fish were sampled from each pen to determine pre-treatment numbers of lice. Emamectin benzoate was administ ered in-feed at a dose of 50 mu g kg(-1) biomass day(-1) for 7 consecutive days. Sea lice were counted again on days 7, 14 and 21, and comparisons mad e with untreated control fish. Treatment with emamectin benzoate was effective against chalimus and motile stages of sea lice. In all three trials, treated groups were surrounded by pens of heavily infested fish and L. salmonis numbers increased over time on control fish by 87-284%, whereas over the same period, L. salmonis were reduced on treated fish by 68-98%. In the low temperature trial, reductions were slower but numbers were still 90% lower than on control fish at day 2 1. At the end of the third trial, both control pens were treated with hydro gen peroxide owing to heavy lice burdens. However, L. salmonis numbers rapi dly increased again and at day 55, fish treated only with emamectin benzoat e still had 80% fewer lice than control fish. In the two summer trials, lar ge numbers of C. elongatus were rapidly reduced by treatment with 82-84% ef ficacy by day 21. Despite the potential for continuous re-infestation, oral treatment with emamectin benzoate presented an effective means of controll ing all parasitic stages of L. salmonis and C. elongatus on fanned salmon, and in one trial, numbers remained lower on treated fish for at least 55 da ys. (C) 2000 Elsevier Science B.V. All rights reserved.