Behavior of skipjack tuna (Katsuwonus pelamis) associated with a drifting FAD monitored with ultrasonic transmitters in the equatorial central Pacific Ocean

• The ultrasonic transmitter data for 15 skipjack tuna in the Pacific were analyzed.
• The daytime swimming depth of the fish was shallower when associated with a FAD.
• A diurnal pattern was partly observed in the vertical behavior of skipjack tuna.
• The monitored fish made occasional excursions from the FAD.
• The daytime swimming speed was higher than that observed during nighttime.

The behavior of skipjack tuna Katsuwonus pelamis released at a drifting fish aggregating device (FAD) in the equatorial central Pacific Ocean was investigated using ultrasonic transmitters. We conducted two types of field observations: one to track a single fish using a continuous transmitter, and the other to simultaneously monitor multiple individuals with coded transmitters. In the first examination, the behavior of one individual (fork length (FL) of 65.2 cm) was monitored when the fish was both near and away from the FAD. In the second examination, the behaviors of 14 individuals (FL between 36.0 and 64.0 cm) were monitored only when these were associated with the FAD. The behaviors of the fish were monitored 82.0 and 154.5 h using continuous and coded transmitters, respectively. The fish monitored with a continuous transmitter made several excursions from the FAD (maximum distance of approximately 9700 m) during the monitoring period. The swimming depth during daytime was deeper than that observed during nighttime, and this difference was markedly more obvious when the fish was away from the FAD. Additionally, the daytime swimming depth was deeper when the fish was away from the FAD. The swimming speed of the fish monitored with a continuous transmitter was usually less than 3 KT and was higher during daytime compared with nighttime. The fish monitored with coded transmitters occasionally left the FAD, particularly during nighttime, partly demonstrated diurnal vertical movements, and were mainly distributed in the mixed layer or upper part of the thermocline. In addition, the results showed a synchronicity in the vertical movements of two or more fish. This study indicated a clear difference between the on-FAD and the off-FAD vertical movements, occasional excursions from the FAD, and diurnal movements of the fish, especially when away from the FAD.