The effect of salinity on juvenile Nile tilapia, Oreochromis niloticus (15 ± 2.4 g) reared in unpleasant levels of high stocking density was studied. The study tries to elucidate the effect of moderate salinity levels (8 psu = 8 g NaCl L−1) on stocking density level. To study the effect of water salinity, the two trials: 0 (T1) and 8 psu (T2) were constructed. In each salinity trial, fish were stocked at three densities: 90 fish m−3 (D1), 150 fish m−3 (D2) and 300 fish m−3 (D3). The experiments were conducted for 42 days at 26±0.5°C. The effect of different stocking density levels on growth performance and white muscle and liver metabolism, at T1 and T2, was recorded. In white muscle and liver, the activity of phosphofructokinase (PFK), cytosolic AST and ALT and mitochondrial enzyme glutamate dehydrogenase (GDH), aspartate- and alanine- aminotransferases (AST and ALT) was significantly increased in D2 and D3 at the trial T1 and D3 at the trial T3 in comparison with D1 of each trial. This increase in enzymes activity indicates an increased rate of energy expenditure. The unpleasant effect of increased stocking density level on growth rate recorded in D2 at T1 was alleviated in D2 at T2. This reflects an increased tolerance to an increased level of stocking density when fish subjected to the salinity level, 8 psu.
Key words: Stress, phosphofructokinase (PFK), glutamate dehydrogenase (GDH), cytosolic- and mitochondrial aspartate- alanine- aminotransferases (AST and ALT)
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