This work was conducted to evaluate the effect of anise essential oil as a feed additive on growth performance and immunity of Nile tilapia (Oreochromis niloticus) exposed to two different challenges; reduced levels of dissolved oxygen (LDO) and/or infection with Aeromonas hydrophila. Juveniles were randomly distributed to six replicated experimental groups; groups 1, 2, and 3 were reared at normal levels of dissolved oxygen (NDO) (5 – 6 mg/l) and received 0, 1, and 2 ml/kg diet of anise essential oil, respectively. In addition, groups 4, 5, and 6 which were reared on LDO levels (2 – 3 mg/l) and received the same concentrations (conc.) of anise oil (0, 1, and 2 ml/kg diet, respectively). All groups were kept at a density of 30 fish/m3 and fed on a commercial basal diet (30% crude protein) for 60 days consecutively at rate of 3% body weight. Results cleared an enhancement of growth parameters of fish treated with anise oil, where final body weight, weight gain and average daily gain were increased and feed conversion ratio was decreased significantly than control groups even under hypoxic conditions. Moreover, groups that received anise oil showed stronger immunity against A. hydrophila infection synchronized with developed antioxidative capacity, where results indicated a significant increase in WBCs in all treated groups and globulin in a dose-dependent manner under hypoxic conditions. Additionally, using oil at a concentration of 2 ml/kg diet considerably raised superoxide dismutase (SOD) level under normal and hypoxic conditions and lowered the level of nitric oxide (NO) after challenge with bacteria at a concentration of 1 ml/kg diet at NDO and at 2 ml/kg diet at LDO.
Key words: Nile Tilapia, Oreochromis Niloticus, Anise essential oil, low DO.
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