Integrative assessment of behavioral and cardiorespiratory modulation induced by basil (Ocimum basilicum L.) essential oil in tambaqui (Colossoma macropomum)

The use of natural anesthetics, such as essential oils, is increasingly relevant in aquaculture to improve the management of economically important species such as tambaqui ( Colossoma macropomum ). This study investigated the anesthetic potential of basil ( Ocimum basilicum L.) essential oil (OBEO), primarily composed of estragole (77.71%) and eucalyptol (6.43%), through behavioral, respiratory, and electrocardiographic (ECG) analyses. Tambaqui juveniles ( n = 90) were exposed to immersion baths containing 76, 85.5, 95, or 104.5 mg L⁻ 1 OBEO. OBEO produced a concentration-dependent behavioral anesthetic state characterized by loss of postural reflex and immobility, with shorter induction latencies and longer recovery times at higher concentrations. Because responses to noxious stimuli were not evaluated, these endpoints demonstrate immobilization appropriate for routine handling rather than a confirmed surgical plane of anesthesia. Sinus bradycardia increased with concentration, reaching a 40.83% reduction in heart rate at 104.5 mg L⁻ 1 relative to the control. RR, PQ, ST, and QRS duration increased at higher concentrations, whereas QRS complex amplitude remained unchanged. Opercular frequency and power also decreased in a concentration-dependent manner. During the 5-min washout in anesthetic-free water, behavioral and most mean cardiorespiratory variables moved toward control values, although recovery was slower and some ECG variables remained different at the highest concentration. Under these short-term experimental conditions, no arrhythmias or persistent gross physiological abnormalities were observed. OBEO therefore shows potential as a reversible immobilizing/anesthetic agent for routine handling of juvenile tambaqui; broader conclusions regarding food-fish safety and practical use require pharmacokinetic, residue, toxicological, and withdrawal-period studies.