Red tilapia can grow and productive in a wide range of salinity water, the direct transfer from fresh water to marine water or vice versa results in fish died because severe dehydration which might impair the function of NaK -ATPase activity and is not enough to compensate for ionic exchange requirements at high salinity. The acclimatization success from fresh to marine water relates with many factor such as methods of acclimatization, temperature, fish age, salinity level and feeding. Feeding and their systems during acclimatization periods of fish are considered un determined factors. Moreover, f eeding of fish is essential to compensate for the energy consumed in osmoregulatory mechanisms and modifying the physiological case under acclimatization conditions. Therefore, it is believed that optimum feeding level has a vital role to raise the quality acclimatization and survival rate of fish.
A 15 day acclimation experiment was conducted to determine the optimum feeding level during an acclimatization period to marine water for improving survival rate and gonadosomatic index (GSI) of adult red hybrid tilapia in high salinities. Red tilapia were randomly distributed into 8 square plastic tanks with dimensions of (55 cm × 38 cm × 29 cm; L×W×H) and water volume 60 liter. Fish with an average initial weight 48.75±2.39g were stoked at 6 fish/ tank (3 male + 3 female). All tanks were provided with continuous aeration. Red tilapia were reared under natural photoperiod conditions. Fresh water was replaced with marine water for raising the salinity rate of 3.8 ppt. every day for 11 day. After this period the salinity of water was equal the salinity of Mediterranean Sea (38 ppt). This trial tested four different levels of feeding rates during fish adaptation from fresh water to marine water. The first treatment (T1 ) was a control group and their fish did not feed the second treatment (T2) fish fed with feeding rate 0.5% of their biomass, while the third treatment (T3) and the fourth treatment (T4 ) fish fed with feeding rates 1% and 2% of their body weight, respectively. Fish fed a commercial diet containing 27% crude protein (floating pellets 3mm) Table (2) Feed was offered by hand in two meals/day (9:00 a.m. and 16:00 p.m.). Water temperature, dissolved oxygen (DO) and pH, were recorded daily before adding the marine water and measured by multi parameter water quality analyzer (MULP-8C). The statistical analyses appeared significant differences among treatments and confirmed that the growth and GSI indicators are affected by different feeding levels. 0.5% (T2 ) and 1% (T1) as feeding rates for adult red hybrid tilapia during the acclimatization period had the best and highest in these indicators in comparison with the control treatment and T4 (table 1 and 2).
It could be conclude that the determination of feeding rate of the acclimated fish is very important to improve the acclimatization quality of adult-red tilapia. Where, the feeding rates 0.5% and 1% are suitable to mitigate the stressful conditions that accompany the acclimatization to high salinities. Also, a higher feeding level than 1% for the acclimated adult red tilapia leads to the adverse results.