Thermal physiology and movements of skipjack tuna (Katsuwonus pelamis) from tag releases off the northern coast of Japan: Possible insights into spawning and wintering strategies.

Understanding movements for spawning of tunas is essential for gaining insight into population dynamics and can provide information on the reproductive characteristics of tuna stocks. Skipjack tuna spawn in tropical and subtropical areas and southward movement of presumed adult fish from the northern extents of their range may be related to spawning. We investigated southward movement patterns of skipjack tuna from the northern habitat limit of the northwestern Pacific Ocean using plastic dart tags and archival tag data. Observations uncovered a new alternative movement group that resides around the northern habitat at least for 9 months (residence group), confirming spawning potential movement toward tropical and subtropical areas (spawning potential group). The spawning potential group, that spent the majority of time in water temperature environments above 24°C in spawning grounds, was considered to be undertaking movement for spawning. Extraordinarily high body temperatures reaching 31°C were found only in the spawning potential group, indicative of spawning activity. The frequency and timing of this high body temperature were also consistent with the current reproductive traits of the skipjack tuna, strengthening this group as potential spawners. Conversely, gonadal immaturity of skipjack tuna in the residence group lasting above 8 months after release in environments unsuitable for spawning (<24°C) suggests motivation for movement was not driven by spawning, but by feeding and avoiding unfavorable environments. Remaining in the northern habitat with short-distance daily movement in a rich prey environment indicates engaging in feeding. However, seasonal cooling in this area pushed the temperature of the skipjack tuna thermal limit southward, leading to this group moving slowly southward to avoid exposure to the lower limit. The southward movement of this group was motivated by thermal limit avoidance. Physiological changes demonstrate the importance of describing the two movement patterns and underlying motivations for patterns from a physiological thermal tolerance and reproductive ecology perspective.