Oregano (Origanum vulgare) essential oil as an additive in diets for Nile tilapia (Oreochromis niloticus) fingerlings reared in salinized water

The present study was undertaken to examine the effects of oregano essential oil on the growth performance, blood glucose levels, body composition and intestinal morphometry of Nile tilapia reared in salinized water at 10 g/L for 64 days. The experiment was set up as a completely randomized design with five treatments and four replicates, in which a control diet and four diets containing increasing concentrations of oregano essential oil (0.75, 1.5, 2.25 and 3.0 g/kg) were tested. The best results for the growth parameters were obtained with the oregano essential oil inclusion level of 0.75 g/kg, but did not differ (p > .05) from inclusion levels of 0.0, 2.25 and 3.0 g/kg. Conversely, the treatment including the oil level of 1.5 g/kg was the only one to produce significantly (p < .05) lower results. The body composition and blood glucose were not significantly affected. Intestinal morphometry analysis revealed a significant effect on villus length, which increased with the proportion of oregano essential oil added to the diet; accordingly, the inclusion level of 3.0 g/kg provided the best results for this variable. Therefore, the addition of oregano essential oil at 3.0 g/kg in the diet of Nile tilapia cultured in salinized water may act as a stimulant to intestinal villus development and, consequently, as a growth promoter.