Reproductive condition, batch fecundity, and spawning fraction of large Pacific bluefin tuna Thunnus orientalis landed at Ishigaki Island, Okinawa, Japan

In order to illustrate the reproductive activity of Pacific bluefin tuna (Thunnus orientalis) around the Yaeyama archipelago, an area known to be the species’ major spawning ground, gonadal condition of large individuals obtained by longline fishing was assessed during the commercial fishery season between late April and early July in 2008 and 2009. Almost all of the specimens landed at Ishigaki Island during these two seasons were larger than 200 cm in fork length (FL), and, while the sex ratio was nearly 1:1 in total, a significant bias existed towards males when only individuals with FL values greater than 230 cm were considered. All females were sexually mature and sperm was observed in the testes of all males throughout the fishing season. Average spawning intervals (fraction) and average batch fecundity (mean ± SD) were estimated at 3.29 days (0.30) and 15.4 ± 10.2 million oocytes, respectively. Ovarian and testicular maturation phase based on histological assessment did not show any significant changes over the two-year study period, while commencement of spawning varied from year to year. We determined that spawning activity of Pacific bluefin tuna around the Yaeyama archipelago persisted throughout the fishing season, and likely varied due to the annual fluctuations in environmental conditions at the spawning ground.