Geraniol and citronellol as alternative and safe phytoconstituents to induce immobilization and facilitate handling of fish

Abstract The efficacy of a product as an anaesthetic can be determined through combining behavioural assessment and the evaluation of selected electrophysiological markers, such as the electromyogram (EMG) and electrocardiogram (ECG) for muscle contraction power and cardiac function characterizations, respectively. This study aimed to evaluate the anaesthetic efficacy of geraniol (GRL) and citronellol (CTL) in tambaqui juveniles, Colossoma macropomum, through concentration-response trials, and their effects on behaviour and cardiorespiratory responses. Fish (24.78 ± 2.50 g) were assayed into two experimental groups: I – Behavioural Assessment: In which six concentrations were tested (10; 30; 50; 70; 90; and 110 μL L−1) for each compound. For both isolates the concentration of 10 μL L−1 did not induce any signs of anaesthesia. As for the other concentrations, all fish were anaesthetized and fully recovered according to behaviour evaluation. Concentrations of 70 μL L−1 GRL and 90 μL L−1 CTL were considered the most effective doses as they promoted anaesthesia and allowed for recovery within appropriate time intervals; II – Electrophysiological Characterization: Concentrations of 70 μL L−1 GRL and 90 μL L−1 CTL were used for the recordings of EMG, ECG, opercular beat intensity and rate (OBI and OBR), and heart rate (HR). For both experiments, nine fish per concentration (n = 9) per analysis were used and each animal was considered a replicate. Our results demonstrated that geraniol and citronelol induced full body immobilization, which resulted at least in part, from the myorelaxant properties of these compounds. GRL and CTL at 70 μL L−1 and 90 μL L−1, respectively were sufficient to render fish fully and rapidly immobilized. Both products transiently reduced ventilation during anaesthesia, nevertheless allowing for complete recovery after exposure. Although either isolate significantly decreased heart rates, they did not compromise resumption of normal cardiac function. In general, no mortalities were observed and all animals recovered after exposure. Both products could be considered safe alternatives for fish handling and other aquaculture-related activities that might require fish immobilization.