Plasma concentrations of emamectin benzoate after Slice™ treatments of Atlantic salmon ( Salmo salar): Differences between fish, cages, sites and seasons

Abstract

Infestations with the marine copepods Lepeophtheirus salmonis and Caligus elongatus are unavoidable in Norwegian net-pen based salmonid production and a major cause of economic loss and reduced animal welfare. Treatments are mainly performed with pyrethroids and the avermectin emamectin benzoate (Slice™). In this study, plasma concentrations of emamectin benzoate (EB) in salmon receiving standard, oral Slice™ treatments were studied in two fish farms in mid-Norway with postsmolts put to sea in the autumn of 2005. Samples were collected in the autumn of 2005 and repeated sampling was performed from the same sites in the summer of 2006. The tentative concentration of EB in the medicated feed was 10 mg/kg, resulting in a daily dosage of 50 µg/kg body weight at a feeding rate of 5 g medicated feed/kg body weight for seven days. Blood samples were collected the day after ended treatment from 50 anaesthetised fish (metacaine, 80 mg/l) in each of three separate cages, by caudal venipuncture using heparinised vacutainer tubes. Twenty-five randomly selected samples from each cage were analysed for EB by an HPLC method. The overall median concentration was 116 ng/ml, the plasma emamectin concentrations in fish varied from 6 ng/ml (autumn 2005) to 440 ng/ml (summer 2006). There were significant differences in the EB plasma concentrations among fish from the three cages at site 1, both in 2005 and in 2006. At site 2, no significant difference among the three cages could be demonstrated. There was a significant difference between pooled results between the seasons on site 2. No difference was found between the two sites in the autumn, however in the summer there was a highly significant difference between sites 1 and 2. At both sites in autumn 2005 and on site 1 in summer 2006, there were clinical outbreaks of diseases (IPN, HMSI). The multivariate ANOVA model demonstrated a highly significant influence of disease outbreaks on emamectin concentrations in plasma. The model also demonstrated tendencies that site and seasons influenced the EB concentration in blood plasma. There was no correlation between the weight of the fish and the EB concentration. The influence of feeding techniques (hand feeding versus automatic feeding) could not be tested.