The use of natural products for fish anaesthesia, such as plant-derived substances, represents an interesting alternative to facilitate handling and mitigate stress responses and their harmful effects in more intensive fish farming. This study aimed to evaluate the anaesthetic efficacy and safety of eucalyptol (ECL) in promoting full body immobilisation (anaesthesia) and muscle relaxation in juvenile tambaqui, C. macropomum.
Fish with an average weight of 30.8 ± 4.3 g and a total length of 11.9 ± 0.7 cm were used. Initially, specimens underwent anaesthetic baths at ECL concentrations of 300, 400, 500, 600, 700, and 800 µL.L–1 ECL for behavioural assessment. Thereafter, concentrations of 600, 700, and 800 µL.L–1 were arbitrarily selected for further electromyographic (EMG) testing over 300 s. A total of 10 juveniles were used in each test and recording, divided into three groups: a) control (baseline recording), b) fish exposed only to the vehicle (water mixed with ethanol), and c) fish subjected to ECL for induction and subsequent recovery.
The results indicated that higher concentrations of ECL more effectively promoted stage A3, i.e., full body immobilisation (anaesthesia) (Figure 1). The EMG revealed significant decreases in muscle contraction power during anaesthetic induction at the three tested concentrations, indicating pronounced muscle relaxation. All animals gradually returned muscle tonus, righting reflex, and normal swimming behaviour within the appropriate time threshold at all concentrations (Figure 2).
In conclusion, ECL was effective in promoting anaesthesia, leading to complete body immobilisation of C. macropomum at concentrations ranging from 300 to 800 µL.L–1 with appropriate induction and recovery times. Concentrations of 600 to 800 µL.L–1 induced pronounced muscle relaxation without mortalities. ECL can act safely and reversibly as an anaesthetic agent for use in non-invasive procedures in tambaqui and potentially in other teleost species.