Final oocyte maturation and ovulation of captive chub mackerel Scomber japonicus with fully yolk-accumulated oocytes were induced by a single injection of human chorionic gonadotropin. Reproductive parameters, including spawning frequency and batch fecundity, which are required to estimate spawning biomass in pelagic fish by the daily egg production method, were analyzed. Germinal vesicle migration (GVM) occurred at 18-24 h post-injection, and the hydration and ovulation of oocytes were completed at 30 and 36 h post-injection, respectively. The results of the maturation process suggest that fish with GVM-stage ovaries captured in the daytime from the field are capable of spawning on the night following their capture. The oocytes used in the oocyte sizefrequency distribution method for batch fecundity estimates should be at late GVM and more advanced stages. The results of sequential artificial insemination showed that the quality of ovulated eggs held in the ovarian lumen rapidly deteriorated as time progressed after ovulation. This indicates that the fertilization window for the ovulated eggs of chub mackerel lasts only a few hours, and spawning behavior should be performed within a few hours after ovulation in the wild population.KEY WORDS: batch fecundity, chub mackerel, fertilization rate, final oocyte maturation, human chorionic gonadotropin, ovulation.

Food selectivity of seven-band grouper Epinephelus septemfasciatus larvae fed diŠerent sizes of rotifers YUKARI TANAKA, 1a YOSHITAKA SAKAKURA, 2 HISASHI CHUDA, 3b

No abstract

The water flow in larval rearing tanks has been indicated to cause mass mortality of the seven-band grouper (Epinephelus septemfasciatus) larva. Therefore, we tested a new aerating method in an actual scale of intensive rearing tank (8.0 m in diameter, 1.87 m 5 of water depth, 100 m 3 of volume), in which an aerator was set at the center of the rearing tank surrounding cylindrical drain (1.2 m in diameter) to generate the flow filed, and 7 larval rearing trials were performed. Then, we compared the survival rate with the former aeration methods, in which several aerators are located in the rearing tank. The survival rate at 10 days after hatching in the new aeration method (61.5 ± 5.1 %, n=7) 10 was about 3 times higher than the former methods (21.2 ± 13.7 %, n=6). We also examined the flow environment of rearing tanks by quantifying the flow field and discussed the relationship between the flow field in the rearing tank, behavior of larvae and survival. We confirmed that the vertical circulating flow was observed in rearing tanks, and determined effectively the survival and the behavior of grouper larvae in 15 patchiness.

The histological changes in the ovary of sevenband grouper Epinephelus septemfasciatus were examined during vitellogenesis and final oocyte maturation. Ovaries contained oocytes at the perinucleolus stage at the start of the experiment in January. Oocytes at the primary yolk stage appeared when the gonadosomatic index (GSI) was slightly on the rise in March. After that, vitellogenesis progressed and ovaries contained oocytes at the tertiary yolk stage in May. The GSI increased drastically at the same time. These results suggest that vitellogenesis in sevenband grouper starts or is already in progress in March and is completed by May, In order to clarify the process of final oocyte maturation, luteinizing hormone-releasing hormone (LHRH) analog (des Gly 10 , [D-Ala 6 ] LHRH ethylamide) was implanted in fish with oocytes at the tertiary yolk stage. The oocytes developed to the migratory nucleus stage in 18 h and reached the maturation stage in 42 h after implantation. This indicates that final oocyte maturation is completed within 42 h after stimulation of gonadotropinreleasing hormone.KEY WORDS: estradiol-17β β β β , final oocyte maturation, gonadosomatic index, LHRHa, ovarian development, sevenband grouper.