Tolerance of Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), to emamectin benzoate, a new orally administered treatment for sea lice

Atlantic salmon (weight range 289-484 g) and rainbow trout (weight range 16 6-387 g), field in seawater, were fed medicated diets containing graded lev els of emamectin benzoate for 7 days. Nominal dose rates were 0, 100, 250 a nd 500 mu g kg(-1) body weight day(-1) (equivalent to 0, 2, 5 and 10 times recommended dose rate for treatment of sea lice). Fish were observed for a further 7 days to determine the effects of treatment. Calculated actual dos e rates, compensated for feed concentrations determined by chemical analysi s, feed intake and weight gain during the course of the study, were 0, 70, 173 and 356 mu g kg(-1) body weight day(-1) for Atlantic salmon, and 0, 88, 218 and 413 mu g kg(-1) body weight day(-1) for rainbow trout. No mortalit y was observed, which could be related to treatment and unequivocal signs o f toxicity were recorded only at the highest dose used in these studies. Si gns of emamectin benzoate toxicity in both species included lethargy, dark coloration and inappetance. Atlantic salmon also showed loss of coordinatio n. Fish in the high-dose groups exhibiting signs of toxicity showed no evid ence of recovery during the 7-day post-treatment period. No pathognomonic s igns of emamectin benzoate toxicity were identified during gross necropsy o r histopathological examination. (C) 2000 Elsevier Science B.V. All rights Reserved.