Spawning Characteristics and Steroid Hormone Profiles in the Wild Female Japanese Sardine Sardinops melanostictus

In the present study, the changes in ovarian development and serum steroid hormone levels of the wild female Japanese sardine, captured off Kyushu and Shikoku at the main spawning ground during the spawning period, were examined to understand the mode of ovarian development, spawning characteristics and endocrine control of reproduction of the female sardine. Based on the occurrences of hydrated mature oocytes and newly formed postovulatory follicles in the ovaries, the spawning of the Japanese sardine in this spawning ground was estimated to start at 20:00--23:00. Serum estradiol-17β was maintained at a relatively high level during the vitellogenic stage (1.15-1.69 ng/ml), but the level rapidly decreased (0.46 ng/ml) at final oocyte maturation, and thereafter increased to a high level (1.79 ng/ml) just after spawning. The putative maturation-inducing hormone (MIH) of the Japanese sardine, 17α,20β-dihydroxy-4-pregnen-3-one, showed a low, constant level (0.28-1.95 ng/ml) during vitellogenesis, whereas the concentration was remarkably high (71.63 ng/ml) at final oocyte maturation, and showed a marked decline (3.11 ng/ml) just after spawning. Thus, a clear shift in the predominance of estradiol-17β, which induces oocyte growth, to MIH during final oocyte maturation was confirmed in the Japanese sardine. The ovarian histology and these hormonal profiles during the maturation cycle supported the possibility of multiple spawning of this species within the spawning period.