1Nagasaki Prefectural Institute of Fisheries, Nagasaki 851-2213, 2Faculty of Agriculture, Kyushu University, Fukuoka 812-8581, Japan
Eects of initial oocyte diameter on ovulation time, quantity and quality of eggs in yellowtail, Seriola quinqueradiata, in inducing ovulation with HCG were examined. After measuring the oocyte diameter through ovarian biopsy, all sh (n=33) were divided into three groups according to their oocyte size: 650-700, 700-750, and 750-800 μm, and injected with HCG (500 IU/kg BW). Ovulation was induced in All the sh of the three groups, and the relationship between elapsed time until ovulation (T, hr) and oocyte diameter (D, μm) was represented as follows: T=-0.082D+105.99 (R2=0.51). 530 thousand eggs per sh could be obtained from sh with oocyte diameter over 700 μm, while only half the number of eggs could be collected from sh with oocyte diameter of 650-700 μm. High fertilization rates of over 90% could be obtained from all three groups regardless of the initial oocyte diameter when the articial insemination had been performed just after ovulation. However, the hatching rates rose with the increase of initial oocyte diameter. Thus, measuring the initial oocyte diameter before the HCG injection enables estimation of the ovulation time, number and quality of ovulated eggs in yellowtail.