Yoshinori Aoki ^1* Takashi Kitagawa ² Hidetada Kiyofuji ¹

• ¹ Japan Fisheries Research and Education Agency (FRA), Yokohama, Japan
• ² Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba, Japan

This study investigated the vertical behavior and thermal physiology of skipjack (SKJ) during their northward migration from subtropical to temperate areas in the western Pacific Ocean using a physio-logging technique. We quantified the surfacing rate on the surface and dive duration and estimated the whole-body heat transfer coefficient (𝜆) based on body temperature records as an index of their thermal physiology. SKJ exhibited non-surface-oriented behavior with extended dive duration in the southern mixed layer areas, whereas they demonstrated surface-restricted behavior with occasional short dives in northern areas with strong thermocline. Notably, SKJ in northern areas exhibited greater thermoregulation (with differences in 𝜆 of 2–3-fold between the warming and cooling phases associated with diving) than those in southern areas (minor differences in 𝜆 between the phases). This provides new field evidence of short-term thermoregulation during dives in SKJ, which exhibit distinct patterns across the thermal habitats. This adjustment in 𝜆 during the phases in the northern areas allows SKJ to minimize heat release from the body by reducing 𝜆 during the cooling phase in descent and to absorb heat from the warm water near the warm surface and quickly recover body temperature by increasing 𝜆 during warming phase in ascent. Non-surface-oriented behavior with a long dive duration in warm subtropical areas would enable SKJ to forage efficiently in poor prey environments without requiring thermoregulation. The surfacing behavior, with occasional short dives in temperate areas, can be interpreted as a consequence of thermoregulation, avoiding exposure to cold temperatures beyond the thermocline while foraging in rich prey environments. The spatial difference in thermal habitat suitability would explain why SKJ exploitation by surface fisheries does not thrive in subtropical areas from the viewpoint of encounter rate by fisheries, which rely on the overlapping depth and time at the surface. Our findings of SKJ thermal physiology and its implications for vertical behavior and fishery vulnerability underscore the importance of considering thermal physiology in predicting species responses to ocean environmental changes, thus providing more plausible future stock estimates.