Rosmarinus officinalis essential oil components promote anaesthesia in Colossoma macropomum (Cuvier, 1818) through behavioural and cardiorespiratory modulation.

Anaesthetic management plays a crucial role in maintaining the welfare and physiological stability of fish during routine aquaculture and research procedures. The essential oil of Rosmarinus officinalis (ROEO) has emerged as a promising natural anaesthetic; however, its cardiophysiological effects in Amazonian species remain underexplored. This study assessed the behavioural, electrocardiographic and respiratory responses of Colossoma macropomum (tambaqui) exposed to ROEO, aiming to define safe and effective concentrations as a natural anaesthetic agent in a Neotropical teleost species. Juvenile tambaqui were exposed to immersion baths containing 90.60-181.20 mg L-1 ROEO for 5 min. Behavioural endpoints (latency to loss and recovery of postural reflex), electrocardiographic parameters (heart rate, QRS amplitude, R-R, P-Q and Q-T intervals) and opercular frequency were monitored. The oil composition was determined by gas chromatography. Statistical analyses were performed using one-way ANOVA, followed by Tukey's test (p < 0.05). ROEO produced a clear dose-dependent anaesthetic response, characterized by faster induction and prolonged recovery times at higher concentrations. Cardiac monitoring revealed reversible bradycardia (up to -56%), elongation of R-R and Q-T intervals and a slight reduction in QRS amplitude without arrhythmias. Opercular frequency decreased by up to 38% at 181.20 mg L-1, indicating moderate respiratory depression. All effects were reversible within the recovery period. Eucalyptol (47.5%), camphor (19.3%) and α-pinene (12.2%) were the main phytoconstituents identified. Rosmarinus officinalis essential oil provides a safe, reversible and concentration-dependent anaesthetic effect in C. macropomum, maintaining sinus rhythm and physiological recovery. These results highlight ROEO as a biodegradable, low-toxicity and effective alternative to synthetic anaesthetics, reinforcing its applicability for sustainable and welfare-oriented aquaculture practices.