The efficacy of metomidate, clove oil, Aqui-S registered and Benzoak as anaesthetics in Atlantic salmon (Salmo salar L.) smolts, and their potential stress-reducing capacity

Atlantic salmon (Salmo salar L.) smolts (45 g) in freshwater (5.4 degree C) were exposed to water containing different concentrations of metomidate from 0 to 10 mg l super(-1), clove oil (eugenol), Aqui-S registered (iso-eugenol) and Benzoak (benzocaine) from 0 to 100 mg l super(-1). All anaesthetic concentrations were mixed and expressed as mg l super(-1) in relation to the active substance of the various anaesthetics. Metomidate, clove oil, Aqui-S registered and Benzoak were effective as anaesthetics for Atlantic salmon smolts at concentrations 2 mg l super(-1) (metomidate) and 30 mg l super(-1) (clove oil, Aqui-S, Benzoak). The fish entered behavioural stage 4 (failure to respond to external stimuli) within 6.2-2.2 min for metomidate concentrations from 2 to 10 mg l super(-1), within 8.1-2.2 min (clove oil and Aqui-S registered ) and 17.4-3.0 min (Benzoak) for concentrations from 30 to 100 mg l super(-1). A concentration of 100 mg l super(-1) of Benzoak, clove oil and Aqui-S registered caused the salmon smolts to enter stage 5 (arrested opercular activity) after 6.4 and 12 min of exposure, respectively. Metomidate solutions 2 mg l super(-1), clove oil and Aqui-S registered solutions 20 mg l super(-1) prevented plasma cortisol elevation above resting level. Benzoak (regardless of dosage) did not prevent plasma cortisol elevations above resting level. While the anaesthetics increased mean plasma lactate concentration significantly in nearly all experimental groups, no such changes in plasma glucose were evident in any of the experimental groups. The eugenol-based anaesthetics (clove oil and Aqui-S registered ) show promise to become effective anaesthetics, with good efficacy at low dosages and with stress-reducing capabilities. They are also inexpensive and easily obtained, and are organic substances safe for both environment and user.