الفهرس 
Introduction
Experimental DesignOnly 14 pages are availabe for public view
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Abstract
The present study was carried out to investigate the effect of dissolved oxygen (DO) levels and fish density (SD) or fish size (FS) on performance, physiological variables, and innate immunity of Nile tilapia, Oreochromis niloticus (L.) throughout two experiments. The first experiment was based on a bifactorial design with three levels of DO [low (LDO): 1.0 - 1.5; medium (MDO): 2.5 – 3.0; and normal (NDO): 6.0 – 6.5 mg/L] and two stocking densities (0.2 and 0.4 g/L; SD1 and SD2, respectively). Fish (0.7± 0.03 g) were fed up to satiation twice a day on a 35 % crude protein diet for 12 weeks. The second experiment was also carried out under the same DO levels and fish density was 2.1 g/L i.e. 50 or 15 fish/aquarium for small (3.7 g) and large (12.9 g) fish, respectively. After the feeding trial in both experiments, fish were intraperitoneally infected by pathogenic bacteria, Aeromonas hydrophila, and fish mortality was observed for 10-days post challenge. The obtained results showed a reduction in DO concentration from 6.5 to 3.0 or 1.5 mg/L caused a reduction in fish growth and feed utilization. This reduction in fish performance increased significantly by increasing fish density. However, the highest growth was observed at NDO – SD1 group. Further, crude protein and total lipids contents decreased significantly at LDO conditions, but total lipids content decreased also at high SD groups. Additionally, glucose, activities of aspartate amninotransferase (AST) and alanine aminotransferase (ALT), creatinine, and uric acid in fish sera increased significantly, while total protein and total lipid decreased significantly at LDO condition and/or at high density. Furthermore, fish resistance to pathogenic bacteria, A. hydrophila infection, NBT, and lysozyme values were significantly affected by DO, SD, and their interaction. The other experiment showed that the larger fish consumed more feed but exhibited worse growth than the smaller ones. Regarding fish-body composition, moisture content was affected only by FS, while crude protein, lipid content and total ash were significantly affected by DO level, FS, and their interaction. Glucose, AST and ALT, creatinine, and uric acid in sera increased significantly with decreasing DO level at larger fish. Meanwhile, total protein and total lipid in fish sera decreased significantly by decreasing DO level at smaller fish. The fish resistance to pathogenic bacteria A. hydrophila infection, NBT values and lysozyme activity decreased as DO level decreased at small fish. These results indicate that fish reared at NDO exhibited better performance and health than other treatments.