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 postanesthesia 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 < 0.05), while those exposed to 450 mg L-1 had the lowest erythrocyte values (P < 0.001). Other changes in hemato-biochemical parameters are discussed. Concentrations between 50 and 300 mg L-1 of menthol were able to induce deep anesthesia in juvenile L. alexandri with induction and recovery times considered ideal for fish. The use of 50 mg L-1 of menthol was able to reduce VF during induction and recovery and also prevent increased plasma glucose, with little influence on hematological and biochemical parameters after handling.

Automated summary

This study found that menthol can be used as an anesthetic for juvenile Lophiosilurus alexandri, with concentrations between 50 and 300 mg L-1 producing ideal induction and recovery times, reducing ventilatory frequency, and preventing an increase in plasma glucose levels.