PARAMETERS ASSOCIATED WITH THE STRIPED GREY MULLET Mugil cephalus L. FRIES MORTALITIES IN IRAN.  

Nine years old cultured breeders of Mugil cephalus L. specimens were subjected to eight artificial propagation treatments from December till February in 2003.Some female breeders received two injections in an interval of 24 hours and some others received a gradual daily injection of 500 IU HCG per kilogram of body weight for five days and then subjected to two injection protocol. Male breeders were given 5-10 mg of 17-α MT in addition to HCG.The result of these artificial propagations was the production of 117 - 2000000 larvae in six trials for each breeder. The temperature and salinity of water during artificial propagation process were 20-23^° C and 32 ppt respectively. There have been two trials for larvae culture. The final density of larvae was 20 individuals per liter. The water temperature and salinity in larvae culture tanks were 22-24°C and 32-33 ppt respectively.

The larvae were fed from the second day post-hatching with Nannochloropsis oculata with the density of 500000 cells/l, rotifers ( Brachionus plicatilis) with density of 20 individuals/ ML. and also Artemia Nauplii with the density of 3-200 individuals/l. The particle size of dry feed used was 100,300 and 500 micron. The daily water exchange in larval culture tanks at first, the second and third weeks of post-hatching were 10, 20-30 and 40-50 percent respectively.

At the first round of larval culture trials which carried out by means of ten circular tanks with water depth 80 cm in each; whole larvae died 12 days post-hatching. However at the second experiment, which carried out by three black color circular fiberglass tanks with 3000 liters volume and working water depth of 100 cm, the survival rate estimated 0.9 percent 50 days post-hatching.

There are two vertical migrations during first two weeks of the grey mullet larvae culture. The first sinking takes place after the second day post-hatching and probably related to the rapid absorption of the yolk sac and the resulting change in specific gravity. The second sinking happens 8^th -11^th days post-hatching and probably related to fully absorption of oil globule and the resulting increase in specific gravity. These migrations have been coincided with high larval mortalities (critical points).

It seems that the characteristics of larval culture tank (such as shape, volume and depth), the nutritional circumstances of larvae and the aeration procedure are some important and effective factors to pass on these critical periods.