Olfactory nerve response of masu salmon (Oncorhynchus masou Brevoort) and rainbow trout (O. mykiss Walbaum) to clove oil and MS-222

Two anaesthetics, clove oil and methane sulphonate (MS‐222), were examined for their effects on the olfactory nerve response of masu salmon (Oncorhynchus masou Brevoort) and rainbow trout (O. mykiss Walbaum). Exposing both species to clove oil for 3 min at concentrations of 50 and 100 mg L−1, or for 10 min at 50 mg L−1, did not significantly reduce their olfactory response. Directly applying clove oil anaesthesia to the olfactory epithelium significantly reduced olfactory response though after 20 min, olfactory response recovered to 70% and 52% of pre‐treatment levels in masu salmon and rainbow trout respectively. Compared with the post‐anaesthetic recovery of responses after clove oil (50 mg L−1), buffered MS‐222 (100 mg L−1) with NaHCO3 (100 mg L−1), and unbuffered MS‐222 (100 mg L−1) treatment for 3 min, the response after MS‐222 treatment declined gradually and significantly, but not after clove oil and MS‐222+NaHCO3 treatments. Clove oil appears to be an effective and relatively safe anaesthetic for salmonids with little long‐term impact on their olfactory response, which plays a crucial role in their life history.