Effects of oocyte diameter on ovulation time, quantity and quality of eggs in yellowtail Seriola quinqueradiata in inducing ovulation with HCG

Abstract

Effects 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 fish (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 fish 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 fish could be obtained from fish with oocyte diameter over 700 μm while only half the number of eggs could be collected from fish 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 artificial 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.