Behavior of bigeye (Thunnus obesus) and skipjack (Katsuwonus pelamis) tunas within aggregations associated with floating objects in the equatorial eastern Pacific

The horizontal and vertical movements of bigeye (Thunnus obesus Lowe, 1839) and skipjack (Katsuwonus pelamis Linnaeus, 1758) tunas within large multi-species aggregations associated with moored buoys or a drifting vessel were investigated, using ultrasonic telemetry and archival tags, along with sonar imaging, in the equatorial eastern Pacific Ocean (at 2 degreesS-95degreesW and 2degreesN-95degreesW). Four sets of observations, each consisting of the concurrent monitoring of pairs of skipjack and/or bigeye with implanted acoustic or archival tags, were conducted in May 2002 and 2003. Ultrasonic telemetry data were not collected until 24 h or more after the fish were tagged and released, to avoid any abnormal behavior as a consequence of tagging. The pairs of acoustically tagged bigeye and skipjack, and also the entire aggregations, were primarily upcurrent of the moored buoy and downcurrent of the drifting vessel during the day. At night the aggregations were observed to be more diffuse, and the fish were feeding on organisms of the deep scattering layer. The aggregations returned to positions upcurrent of the buoy or downcurrent of the drifting vessel at dawn, commonly breezing at the surface within cohesive monospecific schools. The bigeye and skipjack had concurrent changes in depth records, occupying significantly greater mean depths at night than during the day, in most cases. When associated with a moored buoy, bigeye depth distributions were deeper during the day and night than those of skipjack, but bigeye depth distributions were shallower during the day and night than those of skipjack when associated with the drifting vessel. Simultaneous depth records of a large and a small bigeye with archival tags associated with a moored buoy also indicated diel changes in depth. The mean depth at night was significantly less than during the day for the larger bigeye, but the mean depth during the day was significantly less than during the night for the smaller bigeye. The mean depths during the day and night were significantly greater for the larger bigeye than the smaller.