Acoustic tracking of bigeye thresher shark Alopias superciliosus in the eastern Pacific Ocean

Abstract

Acoustic telemetry was used to identify the short-term horizontal and vertical movement patterns of the bigeye thresher shark Alopias superciliosus in the eastern tropical Pacific Ocean dur- ing the summer of 1996. Two immature female sharks, 175 and 124 cm PCL (precaudal length), were tracked for 96 and 70 h, respectively, demonstrating very distinct crepuscular vertical migrations sim- ilar to those reported for the megamouth shark. The bigeye threshers stayed at 200 to 500 m depth during the day and at 80 to 130 m at night. The deepest dive extends the known depth distribution of the species to 723 m. No 'fly-glide' behavior (rapid ascents followed by slower acute-angled descents) was observed for the 2 sharks. However, the opposite behavioral pattern of slow ascents and rela- tively rapid descents during the night was observed. Since bigeye threshers have large eyes extend- ing upwards onto the dorsal surface of the cranium, it may be more efficient for them to hunt prey which are highlighted against the sea surface from below. Estimated mean swimming speed over the ground ranged from 1.32 to 2.02 km h -1 , similar to swordfish and megamouth sharks, and slower than that reported for tunas, billfishes, and other pelagic sharks.