Ocean basin connectivity of Pacific bluefin tuna (Thunnus orientalis), linking natal origin and trans-Pacific movements into population dynamics

Abstract

Uncertainties about stock structure continue to complicate fisheries management, particularly for highly migratory species. A prime example is Pacific Bluefin Tuna (PBF, Thunnus orientalis), which is widely distributed throughout the North Pacific and western South Pacific, although the dynamics within this range are not well understood. The PBF stock assessment is structured under the assumption of a single stock in the Pacific Ocean with two discrete spawning grounds in the western Pacific Ocean (WPO). One located around the Philippines north to the Ryukyu Islands (hereafter: East China Sea) with spawning occuring from April to June and one in the Sea of Japan from July to August. Age-0 fish remain in waters around Japan, but at 1-2 years old, an unknown portion of fish migrate to the eastern Pacific Ocean (EPO) where they remain for several years before returning to the WPO. While the general pattern of these trans-Pacific migrations has been documented, the dynamics and timing are poorly understood and questions remain about how many fish migrate to the EPO and from which spawning ground as well as what environmental/biological factors influence migration. It is also not clear to which spawning ground these EPO fish return. In addition, adult PBF are collected in the western South Pacific (SWPO) around New Zealand; however, the dynamics of the connectivity between the SWPO and the North Pacific remains uncertain. The aim of this project was to use natural chemical tags in PBF otoliths to identify natal origin and quantify the dynamics of trans-Pacific migrations of this species.