Menthol as anesthetic for juvenile Lophiosilurus alexandri: Induction and recovery time, ventilatory frequency, hematology and blood biochemistry

Anesthesia is an indispensable procedure during the handling of fish. This work evaluated the effects of menthol as anesthetic for juvenile Lophiosilurus alexandri. Menthol concentrations of 0 (control), 25, 50, 150, 300 and 450 mg L−1 were tested using 14 animals for each concentration, with each fish being a replicate. Blood was sampled from seven of the juveniles in each group shortly after anesthetic induction and from the other seven at 24 h post-anesthesia and procedures to collect biometric measures, i.e., handling and air exposure. Induction time showed a linear response plateau effect (LRP) with an inflection point of 142 mg L−1, while a dose-response relationship was observed among concentrations for recovery time. Ventilatory frequency (VF) during anesthetic induction had a LRP effect with an inflection point of 127 mg L−1, while during recovery it had a quadratic effect with a maximum point of 305.45 mg L−1. Fish anesthetized with 50 mg L−1 of menthol had the lowest hematocrit values (P