Abstract
The present study investigates the relationship between oocyte development and serum steroid hormone levels in captive Pacific herring, Clupea pallasii, during the first reproductive cycle. The process of oocyte development in Pacific herring belongs to the group-synchronous type. Maturity of the ovary was divided into six periods based on histological observation (i.e. immature (April to September), onset of vitellogenesis (August to October), progress of vitellogenesis (October to December), completion of vitellogenesis (December to March), maturation and spawning (March to April) and spent (late April)). The pattern of seasonal change in the gonadosomatic index (GSI) well reflected the ovarian maturity. Serum vitellogenin levels showed good correlation with change in GSI, which increased from September to a peak (4.2±0.3 mg/mL) in March. Serum estradiol-17β (E2) levels elevated from September and reached a peak (15.8±4.2 ng/mL) in December, and remained comparatively high until March, suggesting that the active vitellogenin synthesis during vitellogenesis is controlled by the high E2 level. 17, 20β-Dihydroxy-4-pregnen-3-one showed a single sharp peak (2.4±0.28 ng/mL) in early April of the second year, suggesting it was a maturation-inducing steroid in this species.
