Evaluating the concentration of emamectin benzoate in Atlantic salmon tissues after sea lice treatments

Pharmaceutical treatments are an important part of integrated sea lice control strategies in all major salmon producing countries in the world. Ensuring that in-feed medication is delivered appropriately, and that fish achieve an adequate tissue concentration of the medication is critical to a successful treatment. The muscle tissue concentration of emamectin benzoate was assessed post SLICE® treatment in 867 Atlantic salmon from 147 treatments. Approximately 90% of the muscle samples that included skin had emamectin benzoate levels above 60 ng/g (ppb); however, only 63.8% of muscle samples without skin were at or above this threshold. Further, we found a wide range of tissue concentrations across our samples, with some fish as high as 398 ng/g. The variation in the emamectin benzoate concentration in our samples was associated with the following factors: feeding rate, whether or not the muscle sample contained skin, year of the sample, laboratory analyzing the tissue, prescribed dose, and condition factor, after controlling for temperature, day of the sample post-treatment, and pen effect. Interestingly, the positive relationship between condition factor and the concentration of emamectin benzoate in the tissue sample was dependent on the dose of medication in the feed. In other words, increasing the dose of emamectin benzoate had a greater effect on the concentration of drug in tissues for fish with a high condition factor compared to fish with a low condition factor. Our findings suggest that the feeding rate and fish feeding behavior within a pen may be important factors to consider when applying medicated feed to fish populations to ensure a more even distribution of medication between animals.