Use of post-bomb radiocarbon dating to validate estimated ages of Pacific bluefin tuna, Thunnus orientalis, of the North Pacific Ocean

•The validity of the estimated ages of the Pacific bluefin tuna using post-bomb radiocarbon dating.•The Δ14C of the whole otoliths of 0-1-year of age fishes coincided with the reference values of the western Pacific.•The declining of Δ14C of the otolith cores of adults coincided with the reference values, and values were 10‰ lower.•The remained portions of the otolith formed after 2-years of age would dilute the Δ14C values of the otolith cores of adults.•The estimated ages based on the number of opaque zones in otolith sections were supported by post-bomb radiocarbon dating.

Estimation of the age of Pacific bluefin tuna (PBF), Thunnus orientalis, is one of the most important processes involved in assessment of its stock biomass. Recently, the ages of PBF have been estimated by analyzing otolith thin sections. However, the accuracy of this estimated age has not yet been confirmed. In this study, the estimated ages of PBF were validated by a novel use of post-bomb radiocarbon dating. The analysis was based on a metric of the abundance of 14C relative to 12C (Δ14C) in the core portion of the otoliths of PBF. The Δ14C of the otolith core portion was measured by accelerator mass spectrometry and compared with reference values of corals in the western Pacific. The decrease of the Δ14C of the whole otoliths of PBF that were 0-2-years of age from 1982 to 2010 coincided with the decrease of Δ14C reference values. This result suggests that post-bomb radiocarbon dating can be used to estimate ages of adult PBF. The Δ14C of the otolith core portions of fish estimated to be 2-27-years of age gradually declined as birth year approached the present day. This trend was consistent with the trend of the reference values and 10‰ lower. The portions of the otolith formed after 2-years of age remained on the surface of the adult otolith core; these portions would dilute the Δ14C values of the otolith cores of adults and provide an explanation for the depletion. These results support the use of otolith thin sections to estimate PBF ages.