Differences in the reproductive traits of Pacific bluefin tuna Thunnus orientalis among three fishing grounds in the Sea of Japan

Abstract

Differences in the reproductive traits of Pacific bluefin tunaThunnus orientalis(PBF) in the Sea of Japan (SOJ) were investigated to evaluate the reproductive potential in 3 fishing grounds (i.e. northern, central and southern areas). The size distribution of PBF differed among the fishing grounds, with smaller fish (100-130 cm in fork length, FL) being caught in the central and northern areas of the SOJ. Spawning-capable females were found at sea surface temperatures (SSTs) between 17 and 25°C, with most SSTs >21°C. Seasonal changes in SST differed among the fishing grounds, and the period when SST exceeded 21°C differed in each fishing ground. Spawning-capable females were found at different times among the fishing grounds, and were observed throughout the fishing season in the southern area of the SOJ. The mean ± SD relative batch fecundity was estimated to be 69.8 ± 45.3 and 92.7 ± 43.1 oocytes g-1in the central and southern areas of the SOJ, respectively. The total spawning fraction was 0.2, 0.38 and 0.73 in the northern, central and southern areas of the SOJ, respectively. SST and FL were positively correlated with the batch fecundity. In addition, FL, SST and location affected the probability of occurrence of spawning-capable fish among mature fish. Therefore, the difference in the observed reproductive potential of PBF among fishing grounds in the SOJ can be linked to the differences in seasonal increases in SST and the size distribution of adult fish.