Effectiveness of eugenol as an anesthetic for adult spotted sea bass (Lateolabrax maculatus)

Spotted sea bass, Laterlabrax maculatus, is one of the most commercially important marine fish in China. It is very easy to suffer from stress and physical damage due to its aggressive nature and dorsal spines, which results in severe mortalities during handling and transportation. Therefore, most spotted sea bass are sold as fresh cooled products rather than live fish in domestic market, thus limiting its market value and market scale as well as the industry development. Anesthetics could be useful to reduce the physical stress during handling and transportation. However, few studies have been done to investigate the application of anesthetics in spotted sea bass. In the present study, the anesthetic efficacy of eugenol (2-methoxy-4-prop-2-enyl-phenol) in adult spotted sea bass was extensively investigated. Firstly, the acute toxicity of eugenol was measured. The estimated 10-min LC₅₀ and 24-h LC₅₀ were 98.13 mg•L⁻¹ and 19.73 mg•L⁻¹. Secondly, the needed time periods for induction and recovery from anesthesia were measured and compared to clove oil and MS-222. Based on the time criteria of ideal induction (less than 3 min) and recovery (less than 10 min), the lowest effective concentrations for spotted sea bass were 60 mg•L⁻¹ for eugenol, 120 mg•L⁻¹ for clove oil and 140 mg•L⁻¹ for MS-222 at around 20 °C. In addition, the anesthetic efficacies of different concentrations of eugenol at water temperatures of 20 and 30 °C were compared. Results showed that longer induction time to deep anesthesia and shorter recovery time were observed when fish were exposed to 20 and 30 mg•L⁻¹ eugenol at 30 °C compared to that at 20 °C. On the contrary, shorter time to induce deep anesthesia and longer time to recovery were found when fish were exposed to 40 and 50 mg•L⁻¹ eugenol at 30 °C compared to that at 20 °C. Lastly, water quality and survival of spotted sea bass subjected to different concentrations of eugenol during simulated transportation were evaluated. Current study indicated that the addition of eugenol did not show significant improvements compared to the control group. In conclusion, eugenol was an effective anesthetic for inducing anesthesia on spotted sea bass, but the addition of it did not improve the transportation of spotted sea bass.