Search

Reproductive characteristics are one of the key factors for understanding population dynamics in fishes. This study describes the spawning habitat of Pacific bluefin tuna (Thunnus orientalis) in the Kuroshio and the Kuroshio–Oyashio transition area. A total of 551 adult fish (fork length > 120 cm) were collected during 1994–2007 (except in 2001 and 2003) and 2016. Based on a histological analysis of the gonads, we concluded that actively spawning females were mainly present in late May to early July. Relative batch fecundity was significantly lower than that reported in the Sea of Japan but was similar to that observed in the Nansei area. Whereas most spawning adults in the Sea of Japan are 3–6 years old and those in the Nansei area are over 10 years old, the majority of the specimens in this study ranged from 160 to 180 cm in length, which corresponds to 6–8 years of age. Our results indicate that Pacific bluefin tuna spawn in the study area and provide important data for understanding the spatiotemporal differences in spawning strategy with growth in this species.

Recently, based on the histological studies it is suggested that Pacific bluefin tuna (PBF) spawns in the Kuroshio–Oyashio transition, off the Pacific coast of northeastern Japan, in addition to the previously identified two spawning areas of the northwestern Pacific around the Nansei Islands and the Sea of Japan. Distributional surveys for PBF larvae have not been conducted in the Kuroshio–Oyashio transition, and thus, we conducted PBF larval surveys at twenty stations in this area in early August 2018 to obtain evidence for the spawning of PBF. Twelve PBF larvae (3.9–7.2 mm in body length) were collected at six stations where sea surface temperature ranged from 27.2 to 28.2°C. Otolith microstructure analysis indicated that age of these larvae ranged from 4 to 11 days after hatching and the larvae hatched in late July. Growth of the collected larvae was comparable to those in the other two spawning grounds. Therefore, PBF spawns, hatches, and at least survives to the postflexion stage, 11 days after hatching in the Kuroshio–Oyashio transition. This fact potentially has a large impact on recruitment processes of PBF if they survive to recruitment in this third spawning ground. [ABSTRACT FROM AUTHOR]

The Pacific bluefin tuna Thunnus orientalis is a highly migratory species, some tuna exhibiting a trans-Pacific migration between the Western Pacific Ocean (WPO) to the Eastern Pacific Ocean (EPO) at the juvenile stage. To detect a signature for the eastward trans-Pacific migration, we analyzed otolith stable isotope ratios of carbon (δ13Coto) and oxygen (δ18Ooto) along the growth trajectory for 20 migrants (age 3–5 years) from the EPO and 10 residents (age 2–3 years) from the WPO. The δ18Ooto values for migrants recorded during the eastward trans-Pacific migration were higher than those for residents during the same period. The higher δ18Ooto values for migrants reflect the low water temperatures encountered during their migration. The δ13Coto values were higher in the migrants than in the residents, suggesting that the higher values of carbon isotope ratios of both dissolved inorganic carbon and the diet in the EPO are recorded in the δ13Coto profiles. Our study demonstrates that otolith isotope analyses can successfully detect trans-Pacific migration of the Pacific bluefin tuna. [ABSTRACT FROM AUTHOR]