Evaluation of menthol depressant activity in rainbow trout (Oncorhynchus mykiss) under different pH conditions

Abstract

The action of menthol on physiological and anesthesiological variables in rainbow trout was evaluated. Clinically healthy juvenile trout were used, of both sexes, formed in group A (n=20) of 201.4 ± 60.1 g of weight subjected to 100 ppm/L of menthol in water at pH 6.4 and in group B (n=20) of 187.3 ± 48.6 g of weight exposed to identical concentration of menthol with the water adjusted to pH 8.0 with sodium bicarbonate. The fish were placed in containers with water at 16.7 °C and electrical conductivity of 0.10 mS cm^-1. The work methodology was: 1) acclimatization of fish in a container without drug and recording of the basal respiratory rate (RR) after adopting a normal swimming behavior; 2) transfer to a second container with menthol and recording of times of loss of lateral balance and anesthesia; 3) transfer of anesthetized fish to a third container without drug and record of RR and its behavior during the recovery period. In group A, anesthesia was achieved in 127.7 ± 46.3 s with a strong reduction in baseline RR from 132.4 ± 12.6 to 40.4 ± 14.4 mov/min and total recovery occurred in 189.7 ± 64.2 s. In group B, anesthesia and recovery time were observed at 93.4 ± 34.3 and 181.7 ± 68.0 s, respectively, and a reduction in RR from 137 ± 14.3 to 43.6 ± 10.4 mov/min. The results indicate that menthol in water adjusted to pH 8.0 is a good alternative for rainbow trout fish farming practices.