Vertical Habitat Use by Japanese Jack Mackerel <scp> <i>Trachurus japonicus</i> </scp> Inferred From a Biologging Study in Tokyo Bay

ABSTRACT The movement ecology of Trachurus japonicus in the adult stage remains poorly understood because observing their underwater behavior over long periods is challenging. This study aimed to examine vertical habitat use by T. japonicus using electronic tags. Ninety fish were tagged and released in November 2022 in Tokyo Bay, Japan. Tracking data from November 2022 to July 2023 were obtained from nine fish recaptured in the bay. The fork length of the recaptured fish at release ranged from 25.5 to 35.0 cm. Mean swimming depths ranged from 24 to 109 m. Maximum swimming depth was 348 m. T. japonicus exhibited seasonal changes in swimming depth; they moved to deeper waters during winter and returned to shallow waters during spring. Moreover, diel vertical migration was observed, involving movement from shallow depths at night to greater depths during the day. Mean ambient water temperatures ranged from 15.1° C to 17.2° C . Minimum and maximum values were 9.9° C and 26.0° C , respectively. Notably, water temperatures during winter were the highest in the middle layer at depths of approximately 40–80 m. Our findings suggest that the fish moved to the mouth of Tokyo Bay, where a thermohaline front developed during winter. However, the tracking data also indicated that not all individuals moved to the mouth of the bay. There was a significant positive linear relationship between mean swimming depth and body size of the fish. Therefore, winter vertical habitat‐use patterns of T. japonicus may be related to individual body size.