الفهرس 
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Abstract
The failure of formation of the functional SB in the Gilthead sea bream larvae has a very important role in incidence of fish deformities and mass mortalities in marine culture under Egyptian conditions. This study was carried out at El-Ballah marine fish hatchery which located on the Suez Canal at Ismailia governorate, which was suffering from low percentages of the ISI in Gilthead sea bream larvae and low survival rates of the produced fish. This study tried to know the possible causes of that problem and its effects on survivability and survival rate At first a preliminary study was made, at which some measurements and information thought to have a role in the failure of the ISI were recorded also we made some examinations that detected the percentages of the functional SBs and its effects on the survivability and the survival rate The measurements and information:1. the water salinity recorded 41 ppt, 2. TGP recorded 105%: 107% Dissolved CO2 recorded 40mg/l4.300 fish of the existing brood stock Were used, distributed on two tanks, 150 fish per tank which were at the 3rd and 4th production season after captivity; its weight was ranged from 0.5: 1kg and they were brought from a hatchery followed to General authority for fish resources development located at 21 kilometer Alexandria governorate on the coastal area of Mediterranean sea. This brood stock is used as a source for the fertilized eggs for the hatching process and obtaining the larvae needed for the study. examinations1- Samples of Gilthead sea bream larvae were taken at 15 dph (age at which the ISI is completed) from 20 tanks, 100 larvae/tank, for examination of the ISI by use of the light microscope. The main average of the ISI was 9.45%2- Another samples of Gilthead sea bream fry at 70 dph, from the previous 20 tanks, 100 fry/tank for application of the floating test at hypersaline solution (50ppt) at which we anaesthetized the fry by use of clove oil, the floated fry means it has functional SB and the sunken one or that was suspended in the water column means they hasn’t SB or it has nonfunctional SB the main average of the float fries was 28% and 72% for the sunken ones3- At age of 90 dph, the survivability was recorded and the survival rates were accounted for the same 20 offspring rearing tanks. The main average of the survival rate was 4.4% from the hatched larvae By use of another egg batches, three experiments has been conducted, at which we improved the factors that we thought to have an effects on the ISI, where only one factor was improved in each experiment and the other factors remained as it was in the preliminary study. Also the same examinations that were conducted in the preliminary study were made but the only difference was that the 20 offspring rearing tanks were divided into 10 tanks under the experiment with improvement of one factor which were ‟ the treated tanks” and the other 10 tanks without any improvement of any factor and remained as it was in the preliminary study which were ‟ the control tanksA.The 1st experiment It was carried out for studying the effect of the water salinity on the ISI, so the water salinity of the treated tanks was decreased only from 41 ppt to 35 ppt from receiving of the fertilized eggs till 90 dph. On the other side the water salinity of the control tanks was not changedthe examinations were made and its results were as following1.The main average for the results of examination of ISI at 15 dph was 12.8% and 8.5% in the treated and control tanks respectively2. The main average for the results of the floating test at 70 dph was 29% floated fry and 71% sunken fry in the treated tanks and 30% floated fry and 70% sunken fry in the control tanks3. The main average of survivability and the survival rate at 90 dph of the treated and the control tanks was 5% and 5.3% respectivelyB. The 2nd experiment It was carried out for studying the effect of the water super saturation on the ISI, so a degassation system was built up for the treated tanks which led to lowering the water TGP from 105%:107% to 95%: 98% and decreased the dissolved CO2 from 40 mg/l to 18 mg/l in the treated tanks on the other side the control tanks was receiving supersaturated water of 105%: 107% TGP and of 40 mg/l dissolved CO2. Then the same examinations were made and their results were as following1. The main average for the results of examination of ISI at 15 dph was 30.8% and 8.3% in the treated and control tanks respectively2. The main average for the results of the floating test at 70 dph was 54% floated fry and 46% sunken fry in the treated tanks and 29% floated fry and 71% sunken in the control tanks3. The main average of survivability and the survival rate at 90 dph of the treated and the control tanks was 9.9% and 5.0% respectivelyC.The 3rd experiment It was carried out for Studying the hereditary effect of the brood stock on the ISI in the offspring and the probability that the deformity of noninflated SB is due to a maternal factors, so only one old brood stock tank (150 fish) was excluded and it was replaced by (150 wild one) newly caught from the Mediterranean Sea at Damietta governorate, its weight ranging from 450: 650 gm, which stayed in the hatchery for one year till they became ready for production of eggs. The eggs produced by the new wild brood stock were received in a separated offspring rearing tanks, which reared till 90 dph. These tanks were the treated tanks, while the control tanks were these offspring rearing tanks that received fertilized eggs from the other old brood stock tank (its source from another hatchery, its weight ranging from 0.5: 1kg and were in the 3rd and 4th spawning season). Then the examinations and tests were made on the offspring and the results were as following:1.The main average for the results of examination of ISI at 15 dph was 82.7% and 11% in the treated and control tanks respectivelyThe main average for the results of the floating test at 70 dph was 86.6% floated fry and 13.4% sunken fry in the treated tanks and 28% floated fry and 72% sunken fry in the control tanks.The main average of survivability and the survival rate at 90 dph of the treated and the control tanks was 22.1% and 5.2% respectively.