Behavioral and electrophysiological evaluation of the efficacy of Origanum vulgare essential oil as anesthesia in Amazonian zootechnical fish.

The search for safe and effective anaesthetic agents for aquaculture is essential to improve fish welfare and management practices. This study evaluated the anaesthetic efficacy of Origanum vulgare essential oil (OVEO) in juvenile tambaqui (Colossoma macropomum), an important Amazonian species. A total of 108 fish were exposed to immersion baths with increasing OVEO concentrations (19.04-57.12 mg L⁻1). Behavioral endpoints, electrocardiographic activity, opercular movements, and plasma glucose were assessed to determine induction, maintenance, and recovery profiles. At 19.04-38.08 mg L⁻1, OVEO induced rapid loss of equilibrium with full recovery, establishing a safe anaesthetic window. Higher concentrations (47.60-57.12 mg.L⁻1) produced deep anesthesia but also concentration-dependent bradycardia, reduced opercular frequency, increased glycaemia, and prolonged recovery times. Electrophysiological analysis revealed a progressive reduction in heart rate (up to 48.5%) and prolongation of RR and PQ intervals, without QRS morphology alteration, indicating reversible sinus bradycardia rather than conduction block. Opercular electromyography also demonstrated concentration-dependent respiratory depression with complete reversibility at lower concentrations. These results provide the first detailed electrophysiological evidence of OVEO-induced anesthesia in C. macropomum, supporting its use as a natural, effective, and sustainable anaesthetic for Amazonian aquaculture, contributing to improved animal welfare and management strategies.