Movement patterns of large bigeye tuna (Thunnus obesus) in the open ocean,determined using ultrasonic telemetry

The horizontal and vertical movements of large bigeye tuna (Thunnus obesus Lowe, 1839; 25 to 50 kg) captured in the south Pacific Ocean (French Polyne sia) were determined using pressure-sensitive ultrasonic transmitters. Bige ye tuna swam within the first 100 m below the surface during the night-time and at depths between 400 and 500 m during the daytime. The fish exhibited clear relationships with the sound scattering layer (SSL). They followed i ts vertical movements at dawn and dusk, and were probably foraging on the o rganisms of the SSL. Bigeye tuna did, however, make regular rapid upward ve rtical excursions into the warm surface layer, most probably in order to re gulate body temperature and, perhaps, to compensate for an accumulated oxyg en debt (i.e. to metabolize lactate). The characteristics of these dives di ffer from those reported from previous studies on smaller bigeye tuna (simi lar to 12 kg) near the main Hawaiian Islands. During the daytime, the large fish in French Polynesia made upward excursions approximately only every 2 .5 h, whereas smaller fish in Hawaiian waters made upward excursions approx imately every hour. Our data are the first observations on the role of body size in the vertical behavior of bigeye tuna.