INTRODUCTION: Thermal physiology is a pivotal biotic factor for the ecophysiology of commercially valuable tuna, influencing not only horizontal but also vertical behaviors. We aimed to examine how the thermal physiology of skipjack tuna ( METHODS: We analyzed archival tag data collected during 2012-2015, with individual time series (Fork length: 38-49 cm, N = 38) of swimming depth, water temperature, and peritoneal body temperature during northward migration from subtropical areas to temperate regions around Japan. We quantified surfacing and diving behavior as an index of vertical behavior and estimated the whole-body heat transfer coefficient ( RESULTS: In the southern mixed layer areas, SKJ were widely distributed at a depth layer <
200 m, whereas they were restricted to the surface in the strong thermocline areas in the north. The dive duration was significantly shortened with a strong thermal gradient during northward migration. We observed minor to no differences in DISCUSSION: Our findings of changes in