Reproductive condition, spawning season, batch fecundity and spawning fraction of skipjack tuna Katsuwonus pelamis caught around Amami-Oshima, Kagoshima, Japan

To illustrate the spawning characteristics of skipjack tuna Katsuwonus pelamis in the Northwestern Pacific Ocean, we examined seasonal changes in the histological condition of gonads, spawning fraction, and batch fecundity (BF) of skipjack tuna caught around Amami-Oshima in the northern Nansei Islands, Japan, from May 2011 to July 2013. Females classified as mature appeared from June to October, and mature males were present from April to November. Spawning began in June when the sea surface temperature (SST) exceeded 24 A degrees C and peaked in August. Although the SST was still above 24 A degrees C, spawning ended in September when the SST started decreasing continuously. The minimum size (fork length, FL) at first maturity was 40.5 cm for females and 37.6 cm for males. The BF (mean +/- A SD) and relative batch fecundity (RBF) were estimated at 93,700 +/- A 21,800 oocytes and 56.8 +/- A 14.3 oocytes/g, respectively. The total spawning fraction (spawning intervals) during the spawning season was estimated at 0.53 (1.88 days). These results indicate that the spawning activity of skipjack tuna around Amami-Oshima changes seasonally depending on SST.