Mortality factors in artificially reproduced grey mullet (Mugil cephalus L.) Fries

Cultured nine year old breeders of Mugil cephalus L. were subjected to eight artificial propagation treatments from December to February, 2003. Some female breeders received two injections in an interval of 24 hours, and others received a gradual daily injection of 500 IU HCG per kilogram of body weight for five days and then subjected to the two injection protocol. Male breeders were given 5-10mg of 17-a MT in addition to HCG. The result of the process was the production of 117-2000,000 larvae in six series for each breeder. Water temperature and salinity during artificial propagation process were 20-23C and 32ppt, respectively. For larvae culture two trials were conducted. The final density of larvae was 20 individuals per liter. Water temperature and salinity in larvae culture tanks were 22-24C and 32-33ppt, respectively. The larvae were fed from their second day post-hatching with algae Nannochloropsis oculata with a density of 500000 cells/l, rotifer Brachionus plicatilis with a density of 20 individuals/ml and also Artemia nauplii with a density of 3-200 individuals/l. The particle size of dry feed used was 100, 300 and 500 microns. The daily water exchange in larval culture tanks at the first, second and third weeks of post- hatching were 10, 20-30 and 40-50 percent respectively. In the first round of larval culture trials which were carried out by means of circular tanks under water 80 cm deep; all larvae died 12 days post-hatching. In the second experiment, which was carried out by three black color fiberglass tanks with water 100 cm deep, the survival rate was 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 is probably related to the rapid absorption of the yolk sac and the resulting change in specific gravity. The second sinking happened 8-11days post-hatching and is probably related to the full absorption of oil globule and the resulting increase in specific gravity. These migrations were found coinciding 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 important and effective factors for the mullet to pass successfully from these critical periods.

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