Gonad and blood samples were taken from the captive female Japanese sardine Sardinops melanostictus between 1988 and 1989, and changes in serum levels of gonadal steroids were correlated with the annual gonadal cycle. Under captive conditions, female fish did not mature and spawn spontaneously, although oocytes developed up to the end of vitellogenic growth. Based on evidence from ovarian histology, the annual gonadal cycle of the Japanese sardine was divisible into four periods, i.e., immature (June to October), vitellogenesis (November to December), spawning (January to March), and post-spawning (April to May). The pattern of seasonal change in the gonadosomatic index (GSI) showed an inverse correlation to change in water temperature and reflected the degree of ovarian maturity. The serum estradiol-17 β level increased from its lowest concentration (0.12 ng ml-1) in September to a peak (1.14 ng ml-1) in March. Serum 17 α,20 β-dihydroxy-4-pregnen-3-one (17 α,20 β-P) was detectable at low levels (<0.3 ng ml-1) between October and February, but was below the assay detection limit (0.06 ng ml-1) at all other times. Testosterone was not detectable (<0.06 ng ml-1) in the serum of any fish throughout the year. The effects of several steroids on the maturation of follicle-enclosed oocytes of sardine were examined in vitro, and 17 α,20 β-P was found to be the most potent inducer of maturation. This suggests that post-vitellogenic oocytes of the Japanese sardine in captivity have an ability to respond to an appropriate hormonal effector and subsequently to resume meiotic maturation.
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