The duration of efficacy following oral treatment with emamectin benzoate against infestations of sea lice, Lepeophtheirus salmonis (Kroyer), in Atlantic salmon Salmo salar L.

The duration of efficacy of emamectin benzoate in the oral treatment of sea lice, Lepeophtheirus salmonis, infesting Atlantic salmon, Salmo salar L., was evaluated in a tank study. One group of salmon was treated at a nominal dose of 50 mu g kg(-1) biomass day(-1) for 7 consecutive days and a second group was untreated. Fish were then redistributed to 16 tanks, each holdin g 17 control and 17 treated fish. On days 34, 41, 48, 55, 62, 69, 76 and 83 , two tanks were challenged with L. salmonis copepodites. Eight to 14 days after each challenge, fish were anaesthetized and numbers of lice recorded. Treatment with emamectin benzoate prevented development of copepodites for up to 62 days from the start of treatment, and chalimus numbers remained l ow for 69 days. Treated fish, challenged from days 34 to 69, had significan tly (P < 0.01) fewer lice than control fish. Treated fish challenged at day s 76 and 83 still had fewer lice than control groups, although differences were not statistically significant for both replicates. When chalimus appea red on treated fish challenged at days 69-83, survival of chalimus to adult stages was lower than on control fish. Louse egg production on treated fis h challenged at days 62-83 was not reduced compared to control groups.