Comparative efficacy of clove oil and 2-phenoxyethanol as anesthetics in the aquaculture of European sea bass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata) at different temperatures

The efficacy of clove oil as an anesthetic was evaluated in juvenile European sea bass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata), and was compared to the commonly used 2-phenoxyethanol through a series of experiments simulating aquaculture activities. Firstly, using as a criterion the acquisition of complete anesthesia (stage A5) in <3 min and recovery (stage R5) in <10 min, the optimal doses at 25 °C were determined to be 40 mg/l of clove oil for both species, and 350 mg/l and 300 mg/l of 2-phenoxyethanol for European sea bass and gilthead sea bream, respectively. At 15 °C, the optimal doses for the European sea bass were determined to be around 30 mg mg/l clove oil and 300 mg/l 2-phenoxyethanol, and for gilthead sea bream 55 mg/l clove oil and 450 mg/l 2-phenoxyethanol. Increasing the exposure time of fish to the optimal anesthetic dose for 5, 10 or 15 min after stage A5 anesthesia prolonged recovery time (ANOVA, P <0.001), especially in gilthead sea bream, which also suffered significant mortality (10–83%). As expected, the lower temperature resulted in significantly longer anesthesia induction and recovery times (ANOVA, P <0.001), presumably due to the positive relationship between temperature, and opercular ventilation rates (ANOVA, P <0.001) and metabolism. Finally, repeated exposure to anesthetics at 0 h, 3 h and 24 h increased significantly the induction time to stage A5 anesthesia (ANOVA, P <0.001), suggesting the development of a slight tolerance, especially to the clove oil. The study demonstrated that clove oil can be used as an effective anesthetic in European sea bass and gilthead sea bream aquaculture, at almost 10-fold lower doses than 2-phenoxyethanol. The observed differences in (a) dose response, (b) anesthesia induction and recovery times, (c) ventilation rates and (d) mortality after prolonged exposure among the two species, underscore the need to undertake extensive studies with the specific fish species, anesthetic andexperimental procedure employed, before clove oil or any other anesthetic is proposed for commercial use in an aquaculture species.