Reproductive biology of female Pacific bluefin tuna, Thunnus orientalis, in the Sea of Japan

A histological analysis of ovaries was conducted to examine reproductive characteristics of female Pacific bluefin tuna, Thunnus orientalis (Temminck and Schlegel, 1844), in the known spawning ground in the Sea of Japan. A total of 1040 females caught with purse seines were sampled from late May to early August in 2011 and 2012. The spawning season began in mid-June and continued until early August. The sex ratio for the catch in 2012 was nearly 1:1. The estimated spawning time was 1700-2200 h based on the occurrence of 0 h postovulatory follicles. The estimated mean spawning frequency and mean spawning interval were 0.91 per day and 1.10 days, respectively, indicating that mature females spawn nearly every day in the Sea of Japan. The fork lengths at 50% and 95% mature were 114.4 cm and 133.6 cm, respectively. Batch fecundity was positively correlated with fork length, gilled and gutted body weight, and gonad weight. The mean batch fecundity was 6.4 million oocytes, and the mean relative batch fecundity per gilled and gutted body weight was 122 oocytes. This study provides some comprehensive information on reproductive biology of female Pacific bluefin tuna in the Sea of Japan.