Reproductive dynamics of Pacific bluefin tuna (Thunnus orientalis) off the Nansei Islands, southern Japan

To describe seasonal changes in the reproductive traits of Pacific bluefin tuna (Thunnus orientalis), we examined the spawning season, spawning fraction (SF), spawning interval, batch fecundity (BF) and monthly changes in the horizontal distribution of spawning fish (SP) in the Nansei Islands and the waters off southwestern Honshu Island, Japan. SP were observed at sea surface temperatures (SSTs) between 19 and 29 ?, and the frequency of SP tended to increase with SST. SP were observed from early May to mid-July and most frequently in late June. The SP distribution in the Nansei Islands tended to expand northward with time. Mature female fish were widely distributed between the Nansei Islands and the waters off southwestern Honshu Island throughout the study period. The total SF during the spawning season was 0.24, which was equal to the 4.17-day spawning interval. The SF tended to increase with time. The number of days since the start of the fishing season (Days) positively affected the probability of the occurrence of specimens with postovulatory follicles among mature fish. The mean and relative BFs during the spawning season were 9.74 +/- 6.72 (million oocytes) and 64.8 +/-& nbsp;38.4 (oocytes/g), respectively. The BF was positively affected by increases in the fork length (FL) and Days. These results suggest that the reproductive traits of Pacific bluefin tuna in the Nansei Islands are influenced by environmental and maternal conditions.

Automated summary

This study investigated the reproductive traits of Pacific bluefin tuna in the Nansei Islands and waters off southwestern Honshu Island. The results showed that the distribution of spawning fish was influenced by sea surface temperatures, with a peak in late June. Mature female fish were widely distributed in the study area. The spawning fraction increased over time, and batch fecundity was positively affected by fork length and days since the start of the season.