Menthol as anaesthetic for lambari Astyanax altiparanae (Garutti & Britski 2000): attenuation of stress responses

The anaesthetic potential of menthol was evaluated in lambari Astyanax altiparanae by exposing fingerlings to concentrations 50, 100, 150, 200, 250 and 300 mg L^?1 and measuring the induction and recovery times to deep anaesthesia, the mortality rates during and 96 h after procedure and after 6 min of continuous exposure. The effect of menthol on stress responses were evaluated by comparing glucose and cortisol levels of juveniles subjected to anaesthesia (50 mg L^?1), stress (air exposure) or pre‐anaesthesia associated to stress. All concentrations induced deep anaesthesia within 0.5 to 1 min, with recovery between 1.83 and 4.16 min, without mortality during the induction or up to 96 h after exposure. Induction time decreased and recovery time increased linearly as the menthol concentration increased. Continuous exposure to 50, 100 and 150 mg L^?1 concentrations resulted in mortality rates of 0%, 20% and 80% respectively. Anaesthesia or air exposure increase blood glucose but prior anaesthesia with menthol suppressed the elevation of cortisol caused by stress. Menthol has an anaesthetic effect and attenuates the stress response in lambari and 50 mg L^?1 is the most effective concentration for inducing deep anaesthesia in 1.0 min, safe for up to 6 min exposure.