Reproductive traits and seasonal variations in the spawning activity of female albacore, Thunnus alalunga, in the subtropical western North Pacific Ocean

To clarify the reproductive activity of albacore, Thunnus alalunga, in the subtropical western North Pacific Ocean (WNPO) (13–25°N, 133°E–171°W), reproductive traits, such as the sex ratio, spawning season, batch fecundity, size at maturity and spawning fraction, were investigated. The sex ratio (5 cm intervals) was skewed towards females in each fork length class less than 95 cm and was skewed towards males in each fork length class larger than 95 cm. Spawning capable fish appeared throughout the year, except in January, and primarily occurred between April and September. The size at 50% maturity in females was estimated to be a fork length of 87.5 cm (approximately 5 years). The estimated total spawning fraction and spawning interval during the spawning season were 0.35 and 2.83 days, and the spawning fractions from May to August were larger than those in the other sampling months. The estimated batch fecundity and relative batch fecundity (mean ± SD) were 1.16 ± 0.67 million oocytes and 59.1 ± 31.5 oocytes/g, respectively (fork length range: 88.6–104.8 cm). There was a positive correlation between the fork length and batch fecundity (r2 = 0.27). These results suggest that albacore in the WNPO spawn year round, while peak spawning occurs between May and August (i.e., between boreal spring and summer).