46 SEPTEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG commercially available synthetic amino acids at 100 and 120 percent of the levels recommended by the National Research Council (NRC 1993) on growth performance, whole-body composition, serum metabolites and immune system and antioxidant biomarkers. Experimental Design A 15-wk growth trial was carried out at the Fish Nutrition laboratory in the Animal Nutrition and Nutritional Deficiency Diseases Department, Faculty of Veterinary Medicine, Mansoura University, Mansoura, Egypt. Male Nile tilapia fingerlings (n=180) with an average initial weight of 6.6 ± 0.0 g were used in this study. The experiment had six isoenergetic (3000 kcal DE/kg) dietary treatments with two replications per diet. Four groups of diets were formulated to contain different levels of dietary protein (32, 30, 28, and 26 percent) in which the calculated indispensable amino acid (IAA) profile met the requirements for Nile tilapia as recommended by the NRC (1993), except for the low protein diet (26 percent) in which methionine was supplemented to raise its level up to the requirement reported by Nguyen and Davis (2009). Test diets used in both experiments contained 414 kcal gross energy, 28 g protein, and 5 g lipid per 100g diet. In the first experiment, seven diets were made using cottonseed meal (CSM. Two other groups of diets were formulated to contain 26 and 28 percent crude protein (CP) in which IAAs were adjusted to be 20 percent above the NRC (1993) requirements. Culture Environment Tilapia fingerlings were adapted to laboratory conditions for about two weeks during which time they were fed the basal diet. After the adaptation period, tilapia fingerlings were equally distributed into twelve glass aquaria (15 fish per aquarium). Globally, tilapia are considered the second most farmed fish species after carps, accounting for 7.5 percent of total aquaculture production (Prabu et al. 2019). In Egypt, they are one of the most widely cultured freshwater fish, accounting for about 66 percent of total Egyptian aquaculture production in 2019 (GAFRD 2021). Currently, one of the major problems facing aquaculture is the increasing cost of fish feed, which represents approximately 60 percent of total production costs, especially in intensive culture systems (Daniel 2018). Protein alone constitutes about 50 percent of the feed cost, being the most expensive ingredient component in aquafeeds (Bahnasawy 2009). Generally, fishmeal (FM) is considered a main source of dietary protein for most commercially farmed fish species because of its high protein content and balanced amino acid profile. Moreover, FM is an excellent source of essential fatty acids (EFAs), digestible energy, minerals and vitamins (Ahmad et al. 2020). However, with the rapid growth of aquaculture and increasing demand for FM for animal feeds, the cost has increased substantially. Therefore, to minimize reliance on FM, farmers can use less expensive protein sources and/ or reduce dietary protein levels in fish diets. However, in both cases, the deficiency in some essential or limiting amino acids will be the most prominent concern (Danielle and Devic 2017). The strategy of synthetic amino acid supplementation has been widely used in tilapia diet formulation, based on the ideal protein concept to ensure a well-balanced amino acid profile for optimum growth performance (Ren et al. 2019, Boaratti et al. 2021) and other ten isonitrogenous and isoenergetic EAA limiting diets (LDs). The aim of this study focused on evaluating the effect of reducing dietary protein levels in Nile tilapia Oreochromis niloticus with dietary supplementation of Effect of Protein Reduction with Indispensable Amino Acid Supplementation in Nile Tilapia Diets Sara Youssef, Shimaa M.R. Salem, Rania E. Mahmoud and Tarek I. Mohamed FIGURE 1. Growth of juvenile Nile tilapia fed experimental diets with reduced dietary protein levels supplemented with indispensable amino acids (IAAs) at 100 and 120 percent of NRC (1993) levels over a 15-wk growth period. * IAAs were supplemented to meet the requirement (100 percent) according to the NRC (1993). ** IAAs were supplemented to meet 120 percent of the NRC (1993) requirements.