Rosmarinus officinalis essential oil components promote anesthesia in Colossoma macropomum (Cuvier, 1818) through behavioral and cardiorespiratory modulation

Anesthetic 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 anesthetic, yet its cardiophysiological effects in Amazonian species remain underexplored. This study assessed the behavioral, electrocardiographic, and respiratory responses of Colossoma macropomum (tambaqui) exposed to ROEO, aiming to define safe and effective concentrations for use as a plant-derived anesthetic in aquaculture. Juvenile tambaqui were exposed to immersion baths containing 100–200 µL L⁻¹ of ROEO for 5 min. Behavioral 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 anesthetic 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 200 µL 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 anesthetic effect in C. macropomum , maintaining sinus rhythm and physiological recovery. These results highlight ROEO as a biodegradable, low-toxicity, and effective alternative to synthetic anesthetics, reinforcing its applicability for sustainable and welfare-oriented aquaculture practices.