The U.S. is lagging in access to an approved immediate-release sedative, i.e. a compound that can be safely and effectively used to sedate fish and has no withdrawal period. AQUI-S® 20E (10% active ingredient, eugenol) is under investigation as an immediate-release sedative for freshwater finfish. Because of its investigational status, data are needed to characterize the depletion, distribution, and identity of AQUI-S® 20E residues in fillet tissue. Rainbow trout (Oncorhynchus mykiss) were exposed to uniformly ring labeled ¹⁴C-eugenol at a nominal concentration of 10 mg/L for 60 min in 18 °C water. Fish (n = 6) were sampled immediately after the exposure (0 min) then at 30, 60, 120, and 240 min. Eugenol concentrations and characterization of ¹⁴C residues in the fillet tissue were determined by high pressure liquid chromatography and flow-through liquid scintillation counting techniques. Total ¹⁴C-residue burdens in fillet tissue were determined by tissue oxidation and static liquid scintillation counting techniques.

Maximum eugenol and ¹⁴C-eugenol equivalent residue concentrations in the fillet tissue were measured immediately after the exposure (44.5 and 38.8 μg/g, respectively). Eugenol was the primary ¹⁴C-residue (> 90% of all ¹⁴C-residues) in extracts from fillet tissue taken from fish sampled immediately after the exposure (0 min) and from fish sampled at 30 and 60 min after the exposure. The depletion of ¹⁴C-eugenol residues from the fillet tissue was rapid (t_1/2 = 26.25 min) after transferring the exposed fish to fresh flowing water.