Residue Depletion of Tritium-Labeled Ivermectin in the Muscle Tissues of Aquacultured Atlantic Salmon, Tilapia, and Catfish following Oral Treatment

Abstract The residue depletion and metabolism profiles of tritium-labeled ivermectin in the muscle tissues of aquacultured Atlantic salmon Salmo salar, hybrid tilapia (Nile tilapia Oreochromis niloticus × Mozambique tilapia O. mossambicus; hereafter, “tilapia”) and channel catfish Ictalurus punctatus following oral treatment are reported. Fish were administered 3H-ivermectin at the dose level of 0.1 mg/kg of body weight (9–10 μ curies [Ci]) in a gel capsule via a stomach tube. At each postdose withdrawal time (1, 7, 14, 21, and 28 d), six fish of each species were sedated, euthanized, bled, and scaled, and fillets with adhering skin (except for catfish) were collected. The muscle fillets were homogenized in dry ice and an aliquot of each was extracted in acetonitrile followed by a high-performance liquid chromatographic analysis to determine the presence of parent ivermectin and its potential metabolites. The analysis of the muscle extracts revealed that the parent 3H-ivermectin is the major residue present for the three fish species studied. The results showed that the highest radioactive residue concentrations (ivermectin equivalents) of 21 and 6 ng/g were obtained on postdose day 1 for channel catfish and tilapia, respectively. For Atlantic salmon, the highest concentration of 14 ng/g was detected on day 7. By postdose day 21, the ivermectin radioactive residue depleted to less than 10 ng/g for all three species. No significant radioactive residue concentrations of metabolites were detected on any of the sampling days. These results suggest that parent ivermectin could serve as a marker residue to monitor its unauthorized use in Atlantic salmon, tilapia, and channel catfish.