Essential oil of citronella modulates electrophysiological responses in tambaqui Colossoma macropomum: A new anaesthetic for use in fish

The aim of this study was to evaluate the efficacy of essential oil of citronella, Cymbopogon nardus (EOC) as an anaesthetic for fish, using juvenile tambaqui Colossoma macropomum as a model. The times to induction and recovery after short-term anaesthesia were evaluated by concentration-response trials. Moreover, electrophysiological responses in muscle (EMG), electrocardiographic data (ECG), opercular beat intensity (OBI) and rate (OBR) were also evaluated. Juveniles were placed in aquaria containing six concentrations of EOC (200, 400, 600, 800, 1000 and 1600 mu L L-1) and the times of anaesthetic induction and recovery were registered. Sham control fish and fish exposed to ethanol-added water were used as controls. With the exception of the lowest concentration used (200 mu L L-1), deep anaesthesia was attained at all other subsequent concentrations, being the use of 600 mu L L-1 sufficient to promote rapid induction (< 3 min) and uneventful recovery (< 5 min) from deep anaesthesia; while the concentration of 200 mu L L-1 was sufficient to induce sedation. EOC elicited a conspicuous depression on muscle contraction power with loss of muscle tonus. Although transient cardiorespiratory depression occurred during deep anaesthesia, gradual increases in the amplitudes of the differences in potential and frequencies in EMG, OBI and ECG, corroborate the reversibility and smooth transition to normal swimming behaviour during recovery from anaesthesia with EOC.