Implications of eucalyptol anaesthesia on juvenile tambaqui Colossoma macropomum: Examining behavioural, muscular, cardiac, and haematological responses

Abstract

This study aimed to evaluate the anaesthetic efficacy and safety of eucalyptol (ECL) in promoting full body immobilisation (anaesthesia), muscle relaxation, and cardiac activity in juvenile tambaqui, C. macropomum. Fish with an average weight of 30.8 ± 4.3 g and a 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 for behavioural assessment. Thereafter, concentrations of 600, 700, and 800 μL.L−1 were arbitrarily selected for further testing, including electrophysiological recordings (EMG and ECG) over 300 s. Additionally, blood samples were collected before and after anaesthesia at 700 μL.L−1 ECL to analyse haematological responses. 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 full body immobilisation (full anaesthesia). The electromyogram revealed significant decreases in muscle contraction power during anaesthetic induction at the three tested concentrations, indicating pronounced muscle relaxation. The electrocardiogram demonstrated a significant reduction in heart rate in fish induced with 700 μL.L−1 (41 ± 4 BPM) and 800 μL.L−1 ECL (35 ± 5 BPM), and the traces from exposure to ECL (600, 700, and 800 μL.L−1 ECL) showed slight, albeit reversible, arrhythmia, which preceded a bradycardic state. All animals gradually returned to sinus rhythm within the appropriate time threshold, suggesting a safe condition for recovery at all concentrations. Haematological results showed that only glucose levels (146.625 ± 30.95 mg.dL−1) were significantly altered compared to the control (76 ± 19.94 mg.dL−1) and ethanol (97.5 ± 16.15 mg.dL−1) groups, with homeostasis resuming after manipulation. 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 and cardiac depression without posing a life-threatening scenario. ECL can act safely and reversibly as an anaesthetic agent for use in non-invasive procedures in tambaqui and potentially in other teleost species.