This study investigated the effects of combined dietary fish meal and oil replacement by soybean extract and increasing amount of plant oil mixture on growth performance, whole body and tissue amino acid and fatty acid compositions in Nile tilapia (Oreochromis niloticus). Diets in which 50% of the fish meal crude protein was replaced by soy bean extract along with 60, 80 and 100% of fish oil replacement by mixture of soy, canola and linseed oils (v/v, 1:1:1) were used in the study. Four iso-nitrogenous and iso-energetic experimental diets were prepared and fed to juvenile Nile tilapia (25 fish per tank with average wet weight of 16.24 ± 0.66 g) twice in equal portions during morning (09:00-10:00 h) and afternoon (17:00-18:00) at a fixed rate of 4% BW.d-1 for 84 days. At the end of 84 day grow out period growth parameters and whole body and muscle tissue amino acid and fatty acid compositions were assessed. Fish in each dietary treatment more than doubled its initial average weight and no statistical differences (P>0.05) were found among dietary treatments in terms of measured growth parameters. It was also demonstrated that diets supplemented with soy bean extract and increasing amount of plant oil mixture proportionally increased the crude protein contents and significantly decreased (P<0.05) the crude ash levels in fish whole body samples compared to that of fish fed fish meal and oil control diet. It appeared that except for one or two amino acids, fish fed diets supplemented with soy bean extract and increasing amount of plant oil mixture tended to increase the muscle tissue essential and non-essential amino acid levels and that L-lysine and DL-methionine used in these diets were effectively used for growth in juvenile Nile tilapia. Except for several fatty acid classes, fish whole body and tissue fatty acid compositions generally mimicked the feed fatty acids following the growth trial. It was also understood that DHA was probably deposited whereas EPA was used for energy production in fish fed diets supplemented with soybean extract and increasing amount of plant oil mixture. Furthermore, fish fed diets supplemented with soy bean extracts and plant oil mixtures had significantly higher (P<0.05) intermediate metabolites like 18:3n-6 and 20:3n-6 of the n-6 LC HUFA biosynthesis in whole body and tissue samples compared to that of fish fed the control diet implying Nile tilapia in this conditions might have activated the Δ5 and Δ6 elongation and desaturation enzymes. It was demonstrated that the partial replacement of dietary fish meal and 100% replacement of dietary Journal of Aquaculture Engineering and Fisheries Research, 4(1), 106-119 (2018) Journal abbreviation: J Aquacult Eng Fish Res 107 Introduction The total production of aquatic organisms from wild and farming has almost reached 168 million tons in 2016 and nearly 60% of this production level comes from the farming activities only (FAO, 2016). Feed cost comprises of approximately 60% of total farm expenditures and sustainability of fish farming relies heavily on the availability of low cost good quality aqua feeds. Fishmeal and oil are finite and therefore expensive ingredients that are widely used in aqua feeds. Fishmeal is also considered an indispensable feed ingredient for farmed fish and crustacean species for several reasons such as its high protein content, balanced amino acid profile and palatability. However, the uncertainty in annual production levels and per unit price urges aquaculture scientist to find alternatives to fishmeal and oil that are cheaper and sustainable to produce in order to continuation of sustainability of aquaculture production (Tacon and Metian, 2008). Higher annual production rates and much lower per unit prices compared to that of fishmeal are the advantages of plant proteins to be considered as viable alternatives for fishmeal protein in aqua feeds (Teoh et al., 2011). However, imbalanced amino acid compositions, several anti-nutritional factors and therefore the lower nutrient digestibility are the main disadvantages that may limit the effective use of plant protein sources in aqua feeds (Teoh et al., 2011). Soya products have been used effectively in commercial diet formulations of many farmed fish species including Nile tilapia. But previous studies demonstrated that successful replacement of dietary fishmeal by soya products in Nile tilapia was only possible when dietary amino acid profile is balanced by AA supplementation (El-Saidy and Gaber 2002, Furuya et al., 2004, Figueiredo-Silva et al., 2015). Dietary lipids provide fish not only with the energy they require for daily maintenance but also the essential fatty acids for healthy growth and development through the formation of many biologically active cellular compounds such as prostaglandins and eicosanoids, sterols, phospholipids and the bio activation of lipid soluble vitamins. Unlike fish oil, plant oils lack n-3 LC-PUFAs and contain high amounts of other fatty acid classes such as SFAs (Saturated Fatty Acids), MUFAs (Mono Unsaturated Fatty Acids) and n-3 / n-6 PUFAs (Short Chain Polly Unsaturated Fatty Acids) (Teoh and Ng, 2016). Perilla and linseed oils are examples of plant oils that are rich sources of n-3 PUFAs. However, soy, corn and sunflower oils are classified as plant oils that contain high amounts of n-6 PUFAs. Canola and palm oils are, on the other hand, categorized as MUFA and SFA rich plant oil resources respectively (Teoh and Ng, 2016). Previous studies conducted in many farmed warm freshwater fish species including tilapia demonstrated that partial and total replacement of dietary fish oil by vegetable oils did not compromised growth rates as long as dietary essential fatty acid requirements of fish has been provided (Steffens et al., 1995; Alava, 1998; Li et al., 2016). In addition, blend of different plant oil use in diets as a lipid source has been demonstrated to be more effective than a single plant oil source in meeting the fatty acid requirement of Nile tilapia thereby resulting in better growth and feed efficiency rates (Teoh et al., 2011). However, major concern for dietary plant oil use in aqua feeds appear to be the lowering impact of these oils on the whole body and fillet n-3 LC-PUFA content of fish (Turchini et al., 2011). Studies investigating the effects of combined replacement of dietary fishmeal and oil on growth and whole body and tissue nutrient content and fatty acid composition of Nile tilapia are limited. Therefore, this study investigated the effects of dietary combined replacement of fishmeal and oil by soybean extract (50% of fishmeal crude protein) and blend of plant oil sources (60, 80 and 100% replacement of fish oil by soy, canola and linseed oil blend, v/v 1:1:1) on growth, nutrient utilization, whole body and tissue amino and fatty acid compositions of juvenile Nile tilapia using experimental diets with high crude lipid content. Materials and Methods Fish and the maintenance Juvenile Nile tilapia used in this experiment was obtained from the freshwater fish culture unit of Faculty of Fisheries, Çukurova University, Adana, Turkey. Following their arrival, fish were acclimatized to feeding and laboratory conditions for approximately 2 weeks using commercial fish pellets (Çamli Feed Company: 45% CP and 15% CL). After the acclimatization period, a total of 300 Juvenile Nile tilapia (16.2 ± 0.7 g AIBW) was randomly assigned to 12 (25 fish per tank) rectangular fiber glass culture tanks (120 cm L × 50 cm H × 32 cm W and 190 l rearing volume) designed as a semi-recirculating rearing unit. Before the experimental feeding began, fish were starved for 2 days. During the experimentation, fish were fed twice a day in the morning (09:00-10:00 h) and the afternoon (17:00-18:00 h) at a fixed feeding rate of 4% BW.d-1. Uneaten feed and feces were siphoned out daily throughout the experimental period and the water taken out was replenished with the same temperature freshwater added fish oil by soy bean extract and plant oil mixture did not compromise growth rates or drastically change whole body and tissue amino acid and fatty acid compositions in juvenile Nile tilapia.