The Effects of Dietary Orange Peel Fragments Enriched with Zinc and Vitamins C and E on the Antioxidant and Immune Responses of Nile Tilapia under Stress Conditions

This study aimed to evaluate the effect of dietary orange peel fragments (OPFs) enriched with vitamins C (C) and E (E), as well as zinc (Zn) on the growth performance, hematological profile, immunological parameters, antioxidant capacity, and fillet lipid peroxidation of Nile tilapia subjected to heat/dissolved oxygen-induced stress (HDOIS), transport-induced stress (TIS), and <i>Aeromonas hydrophila</i> infection (BC). A group of 500 male Nile tilapia (2.7 ± 0.03 g) was randomly distributed in twenty-five 250 L aquaria (20 fish/aquarium) and fed diets containing OPFs (6 g kg^-1), OPFs/C (6 g kg^-1/1.8 g kg^-1), OPFs/E (6 g kg^-1/0.4 g kg^-1), OPFs/Zn (6 g kg^-1/0.21 g kg^-1), or OPFs/C/E/Zn (6 g kg^-1/1.8 g kg^-1/0.4 g kg^-1/0.21 g kg^-1) for 100 days. The diets were formulated to contain 30% crude protein and 17 MJ kg^-1 gross energy. After the feeding period, three groups of fish were independently subjected to a different type of stress: HDOIS (34 °C) for two days; TIS for four hours, or BC for 15 days. The hematological profile, antioxidant capacity, and fillet lipid peroxidation were determined before and after all the stress treatments, along with immunological parameters, which were investigated only for the fish subjected to bacterial infection. In summary, the results showed that growth was not affected by the OPFs, nor by the OPFs enriched with C, E, and Zn; bacterial infection determined anemia for the fish fed any of the experimental diets; the OPFs did not prevent lipid peroxidation under TIS and BC; on the other hand, when enriched with C/E/Zn, lipid peroxidation decreased under HDOIS and TIS. In conclusion, the OPFs enriched with C/E/Zn showed a synergistic effect that promoted an increase in antioxidant enzyme activity, a decrease in lipid peroxidation, and the maintenance of the hematological profile under HDOIS and TIS, but they were not able to maintain the health status under BC.