A laboratory study to evaluate the use of emamectin benzoate in the control of sea lice in sea-ranched Atlantic salmon (Salmo salar L.)

The treatment of Atlantic salmon smolts with antiparasitic drugs may be a method to increase their survival after release to the sea. In the present study, age 1+ years tank-reared Atlantic salmon smolts that were the offspring of wild parents were treated against sea lice infection using (SLICE™) emamectin benzoate. This treatment was administered orally for seven days to each of three independent experimental groups during May and early June. The concentration of emamectin benzoate in muscle of the experimental groups was measured 14, 21, 35, 49 and 63 days post-initiation of treatment. To test the protection against sea lice, two separate challenges with Lepeophtheirus salmonis were performed such that the number of days from start of treatment to challenge ranged from 49 to 110 days for the three groups. Observed concentrations of emamectin benzoate in muscle were comparable to previous studies, and were close to zero 63 days post-initiation of treatment. However, a very high degree of individual variation in muscle concentration was observed. In contrast to other studies, no significant protection against sea lice was observed after 49 days post-initiation of treatment. Due to high variation between fish, and the observed short period of protection, it is concluded that the treatment of hatchery-reared smolts for sea ranching with emamectin benzoate prior to release will give a lower degree, and shorter period of protection against sea lice infection than indicated in previous studies.