Habitat and behaviour of adult yellowfin tuna (

Yellowfin tuna (Thunnus albacares) is an economically important species for longline and trolling fisheries in the waters off the southwestern coast of Taiwan, yet this species' movement patterns remain poorly understood. This study provides the first information on the movement and behaviour of adult yellowfin tuna using pop-up satellite archival tags in the waters off the southwestern coast of Taiwan. In total, 11 tuna (ranging from 116 to 135 cm in fork length) were tagged and released from 2011 to 2013. Seven fish were successfully tracked to provide information on depth and temperature preferences as well as horizontal movements. The majority of the vertical movements (30.3%) of yellowfin tuna occurred in the 50-m depth range in mixed layers. The mean swimming depth was 74.4 m (±50.7 m) during the daytime and 94 m (±72.5 m) at nighttime, which was a contrast to the findings in other waters. The maximum diving depth was 1000 m, where the water temperature was approximately 4 °C. This value was similar to the measurements made by a CTD near a depth of 1000 m, where the water temperature was approximately 4.2 °C and the O2 level was 3.0 mg l−1. One of the tuna travelled 190 NM (straight distance) in 37 days, with most of its horizontal movements (70.6%) occurring at temperatures that ranged from 26 to 28 °C, suggesting that yellowfin tuna have a preference for this temperature range throughout the period of PAST observation.