The growth performance, antioxidative capacity, and histological features of intestines, gills, and livers of Nile tilapia reared in different water salinities and fed menthol essential oil

Abstract

Farming Nile tilapia in brackish water is one of the choices in the areas suffering from a lack of fresh water. Hence this study tested the effects of different salinity levels and dietary menthol essential oil (MNT) on the productivity of Nile tilapia. Two test diets were formulated with MNT (0.25%) or without MNT and fed to Nile tilapia stocked in 0, 5, 10, and 15 ppt salinity levels for 60 days. The growth performance, feed efficiency ratio, and survival rate indices were markedly (p < 0.05) affected by different salinity levels, dietary MNT, and their interaction. All groups showed significantly higher survival rate than fish in 15 ppt without MNT feeding (p < 0.05). The histopathological investigation of gills, intestines, and livers was prominently affected by increased salinity, while the addition of MNT clearly improved the morphological abnormalities. Fish in 15 ppt without MNT had the highest ALT and AST levels, while fish in 0 ppt with MNT had the lowest ALT and AST levels (p < 0.05). The blood glucose level was markedly increased in 10 and 15 ppt with MNT (p < 0.05), followed by fish in 5 ppt with MNT and 15 ppt without MNT. The blood cortisol level was markedly increased in 10 and 15 ppt with MNT (p < 0.05), followed by fish in 5 ppt with MNT, then 0, 5, and 10 ppt without MNT. Fish in 0 and 5 ppt salinity levels had higher SOD, CAT, GPx, and lower MDA levels (p < 0.05) than fish in 10 and 15 ppt without MNT feeding. In all groups, the SOD, CAT, and GPx were markedly enhanced by the addition of MNT while MDA levels were reduced (p < 0.05). The regression analysis illustrated that the best performances of Nile tilapia could be occurred by 1.84–8.04 ppt. At the same time, the inclusion of MNT resulted in increased tolerance of Nile tilapia up to 14.85 ppt based on the overall regression analysis results. In conclusion, dietary MNT relived the impacts of brackish water conditions on the productivity and the histological structure of gills, intestines, and livers of Nile tilapia.