الفهرس 
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Abstract
This study was conducted at National Institute of Oceanography and Fisheries and at Genetics laboratory of Animal Husbandry and Wealth Development Department, Faculty of Veterinary Medicine, Alexandria University. The aim of this work was to detect changes in body weights of Nile tilapia and the expression level of Na+/k+-ATPase α-1a and α-1b, insulin-like growth factor 1 (IGF-1), growth hormone (GH) and Glutathione s-transferase (GST) in Nile tilapia exposed to different salinity concentrations (6 ppt, 16 ppt and 20 ppt). Fish were allotted into 3 groups with salinity 6 ppt, 16 ppt and 20 ppt. After 2 months, tissues samples (Gills, liver and brain) were collected from three fish in each group, then RNA was isolated and cDNA was synthesized. Quantitative real time PCR were done to detect the expression level of target genes in different salinity concentration. The result showed that, net weight gain of fish increased numerically at 16 ppt group but there was no significant differences of weight gain among the studied groups.Expression level of Na+/K+- ATPase α1-a revealed significant decrease at 16 ppt. Furthermore, the expression of Na+/K+- ATPase α1-a showed decrease in 20 ppt group than 16 ppt. The expression level of Na+/K+ ATPase α1-b after two months of experiment was increased significantly in 16 ppt group and increased in 20 ppt relative to the control group. The IGF-1 showed significant increase in 16 ppt group relative to 6 ppt group. It also increased in 20 ppt group. The GH expression analysis revealed that mRNA transcript level was relatively increased in 20 ppt group and in 16 ppt group relative to the control group. The expression level of GST in liver tissue revealed more increase in 16 ppt group than 20 ppt group and the control group.In conclusion, the brackish water can used for cultivation of Nile tilapia without effect on growth rate because expression of IGF-1 increase at 16 ppt and net weight gain increased at 16 ppt. So aquaculturists can replace freshwater with brackish water without any adverse effect on growth of fish and overcome shortage of fresh.