WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2023 47 Throughout the experiment, temperature, pH, and dissolved oxygen concentration were monitored daily and maintained within optimal levels for Nile tilapia, at 24-27 C, pH 7.25-8.25, and 3-5mg/L DO. The photoperiod was fixed at 12 hours light and 12 hours of darkness. At the end of the experiment, tilapia were counted and weighed, and all growth performance parameters – final body weight (FBW), weight gain (WG), percent weight gain (WG percent), feed intake (FI), feed conversion ratio (FCR), specific growth rate (SGR), and protein efficiency ratio (PER) – were measured. Additionally, six tilapia from each group were collected for whole-body proximate chemical analysis. Moreover, blood samples were collected from six tilapia from each group for determination of serum metabolites – triglycerides (TG), cholesterol, aspartate aminotransferase (AST), alanine aminotransferase (ALT), immunoglobulins (IgG, and IgM) and anti-oxidant enzymes [serum superoxide dismutase (SOD) and reduced glutathione (GSH)]. Results There was no significant difference in tilapia growth performance parameters with the reduction in dietary protein levels from 32 to 28 percent. When dietary protein levels were further reduced to 26 percent, growth indices and protein efficiency ratio were reduced and FCR increased. Indispensable amino acid (IAA) supplementation of low-protein diets (26 and 28 percent) at 120 percent of the NRC requirements resulted in a significant improvement in all growth performance indices (Fig. 1) compared to 26 percent protein at 100 percent of the NRC IAA levels. Reducing dietary protein with IAA supplementation of low-protein diets (26 and 28 percent) at 120 percent of the NRC requirements did not affect whole-body moisture and crude protein, while fat and ash content increased. Triglycerides, cholesterol, AST and ALT concentrations increased with dietary protein levels reduced to 26 percent. With IAA supplementation at 120 percent of the NRC requirements, these serum metabolites decreased. Serum immunoglobulins (IgG and IgM) and GSH were reduced with reducing dietary protein to 26 percent. With IAA supplementation at 120 percent of the NRC requirements, IgG and IgM increased. Conclusions This study demonstrated that supplementing low-protein diets (26 and 28 percent) with IAAs at 120 percent of the NRC requirements promoted better growth performance in tilapia than in those fed low protein diets at 100 percent of NRC levels. From an economic point of view, it is better to use the low dietary protein level (26 percent) supplemented with IAAs at 120 percent of the NRC requirement to allow reduction in the inclusion of expensive protein ingredients such as fishmeal and soybean meal. Notes Sara Youssef and Shimaa M.R. Salem, Department of Nutrition and Nutritional Deficiency Diseases, Faculty of Veterinary Medicine, Mansoura University, 35516 Egypt Rania E. Mahmoud, Faculty of Veterinary Medicine, Mansoura University, 35516 Mansoura, Egypt Tarek I. Mohamed, Department of Animal Nutrition, Faculty of Veterinary Medicine , Mansoura University, Mansoura 35516, Egypt References Ahmad, N., P.J.A. Siddiqui, M.K. Khan, A. Ali, M. Tahir, N.U. Akbar, M. Amin, M. Attaullah and I.D. Boneri. 2020. Effects of partial substitution of fishmeal by soybean meal in Nile tilapia Oreochromis niloticus diet. The Journal of Animal and Plant Sciences 30:364-370. Bahnasawy, M.H. 2009. 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This study demonstrated that supplementing low-protein diets (26 and 28 percent) with IAAs at 120 percent of the NRC requirements promoted better growth performance in tilapia than in those fed low protein diets at 100 percent of NRC levels. From an economic point of view, it is better to use the low dietary protein level (26 percent) supplemented with IAAs at 120 percent of the NRC requirement to allow reduction in the inclusion of expensive protein ingredients such as fishmeal and soybean meal.