CHANGES IN OVARIAN DEVELOPMENT AND PLASMA-LEVELS OF SEX STEROID-HORMONES IN THE WILD FEMALE JAPANESE SARDINE (SARDINOPS-MELANOSTICTUS) DURING THE SPAWNING PERIOD

In order to clarify the spawning pattern of Japaneses sardine (Sardino ps melanostictus) during the spawning period, changes in ovarian histo logy, frequency distributions of oocyte diameter and plasma levels of oestradiol-17beta (E2) and 17alpha,20beta-dihydroxy-4-pregnen-3-one (1 7alpha,20beta-diOH-P) were examined in female fish captured in the reg ion off Kyushu and Shikoku in Japan. With the development of the ovary , the gonadosomatic index (GSI) increased gradually and the size of oo cytes became larger. When the GSI exceeded 13, the translucent hydrate d eggs began to dominate and ovulation subsequently occurred. After ov ulation, GSI decreased to less than 3. Post-ovulatory follicles were f ound only in the lower GSI ranges and atretic oocytes were always obse rved in the ovary. Plasma E2 levels increased along with ovarian devel opment but decreased after completion of yolk accumulation. On the oth er hand, 17alpha,20beta-diOH-P showed the highest level just before an d after ovulation. Ovarian histology and hormonal profiles suggest tha t the sardine spawns repeatedly during the spawning period. Based on t he concentration of 17alpha,20beta-diOH-P, the female Japanese sardine was estimated to ovulate before the middle of the night.