Depletion of emamectin residues following oral administration to rainbow trout, Oncorhynchus mykiss

The depletion of emamectin B1a in the edible tissues of rainbow trout (Oncorhynchus mykiss) was studied at two temperatures following treatment with emamectin benzoate (Slice®) in feed. Fish approaching market size (400–1500 g) were held in tanks supplied with temperature-controlled seawater at 6 ± 1 °C (cold water) or 15 ± 1 °C (warm water). In each study the medicated group was offered feed containing emamectin benzoate at a nominal dose rate of 50 μg kg− 1 fish day− 1 for 7 days and the control group was offered unmedicated feed. Actual dose rates, calculated from growth rate and feed consumption data, and measured emamectin benzoate concentrations in feed, were 88.6% nominal in the cold water study (96.6% adjusted for feed assay recovery) and 96.8% nominal in the warm water study (105.1% adjusted for feed assay recovery).Concentrations of emamectin B1a were determined in fillet samples collected at intervals from 6 h to 77 days post-treatment in the cold water study and 6 h to 49 days post-treatment in the warm water study. In the cold water study, mean emamectin B1a residues ranged from 81.8 ± 44.5 ng g− 1 at 1 day post-treatment (102.3 ± 55.7 ng g− 1 adjusted for recovery) to 13.7 ± 10.5 ng g− 1 at 77 days post-treatment (17.2 ± 13.1 ng g− 1). In the warm water study, mean residue concentrations ranged from 64.5 ± 50.3 ng g− 1 at 6 h post-treatment (80.7 ± 62.9 ng g− 1 adjusted for recovery) to 1.6 ± 1.6 ng g− 1 at 49 days post-treatment (2.0 ± 2.0 ng g− 1). In the cold water study, residues in skin and muscle were also determined separately. On average, emamectin B1a concentrations in skin were approximately 1.8 times higher than in muscle.Measured residue levels ranged widely and no detectable residues were found in at least a few individual fish at all time points. This high variability was considered to be due to differences in medicated feed consumption within the experimental population. Depletion of emamectin was faster at 15 °C than at 6 °C. In both studies the depletion curve showed a small secondary peak at around 90 degree-days. This observation is consistent with recirculation of the compound from a body store.