الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
Stress is critical factor in fish health that important to studied. The things that cause stress in fish’s live called (stressors) include:
Salinity, ammonia, over crowding, handling and poor or unfavorable environment.
Stress can be described as the physiological response to stressor. In other describe stress are internal physiological state that is caused by external condition.
Stress is categorized as acute or chronic and severe or mild. The degree to which stress affects any particular fish is demined largely by the severity of stress its duration and the health of dish.
Stress affect on fish by 2 different ways which are disturbance in homostatic equilibrium and behavioral and physiological changes.
In this study, 3 experiments were designed on oreochromis niloticus Fingerling with average body weight 25.00±5.00 gm/fish were used to study the effect of different levels of salinity, ammonia and stocking density on fish health and growth and show some different in molecular changes in fish.
At the first experiment studied the effect of different concentration of salinity on fish which are most important environmental parameter that affect on survival, growth and reproduction. Fish divided into 4 equal groups with 2 replication. The first group kept as a control 5 ppt of salinity, the 2nd group exposed to 10 ppt of salinity, 3rd group 15 ppt and 4th group 20 ppt of salinity. The time of the experiment was 8 weeks.