Reproductive cycle, spawning frequency and batch fecundity of the female whitefin jack Kaiwarinus equula in the East China Sea

Sexual maturity, annual reproductive cycle, spawning frequency, and batch fecundity of the female whitefin jack, Kaiwarinus equula, were examined in specimens collected from the East China Sea between April 1996 and October 1999. The fork length (FL) at 50% sexual maturity was 172.4 mm. Females with mature and spawning stage ovaries were collected between May and October, when the mean gonadosomatic index was also high. This indicates that the spawning season extends from May to October. In the spawning stage ovary, postovulatory follicles continued to degenerate and disappear until the most advanced oocytes attained the migratory nucleus and mature stage, suggesting that this species is a multiple spawner. Throughout the spawning season, atretic rates were higher in smaller females (≤200 mm FL) than in larger ones, suggesting that smaller females have a shorter spawning season. Time course sampling showed that final oocyte maturation occurred during the daytime and subsequent ovulation occurred in the evening. Spawning frequency, determined from the percent of females with migratory nucleus and mature stage oocytes, was 27.7%; this means that the average interval between spawnings was 3.6 days. Some females showed histological evidence of spawning on two consecutive days. Batch fecundity (BF) in 17 females with migratory nucleus or mature oocytes was related to FL (mm): BF = (1.50 × 10^-7)FL^4.89(178 ≤ FL ≤ 246 mm).