Temporal aspects of macroscopic changes in rainbow trout (Salmo gairdneri) oocytes before ovulation and of ova fertility during the post-ovulation period; effect of treatment with 17α-hydroxy-20β-dihydroprogesterone

Ovulation induced by a single injection of 17α-hydroxy-20β-dihydroprogesterone (17α–20β P) in rainbow trout at the subperipheral germinal vesicle stage was compared with spontaneous ovulation. Three criteria were considered: the time course of the maturation-ovulation sequence, the embryo and fry viability, and the rate of aging of ova retained in the body cavity after ovulation. Following treatment with 17α–20β P at 13°C, the various stages of oocyte maturation took place between day 1 and day 4 and ovulation occurred on days 3, 4, and 5 with a peak on day 4. The same timing was observed in the case of spontaneous ovulation.Treatment with 17α–20β P did not affect embryo or fry viability until the end of yolk sac resorption.Lastly, in both natural and steroid-induced ovulation, ova fertility had decreased significantly by the 9th day of retention in the female body cavity, at 13°C. The overall rate of ova aging was similar in both groups. These data demonstrate that ovulation induced by 17α–20β P at the subperipheral germinal vesicle stage is close to the natural, spontaneous, process