Foraging tactics of Baikal seals differ between day and night

Some pinniped species appear to forage during both daylight and darkness. To determine any differences in the foraging tactics between day and night, we attached data loggers with a newly developed time-scheduled release system to 2 free-ranging female Baikal seals Phoca sibirica. The system released the loggers from the seals 24 h after deployment, and allowed us to retrieve the loggers via VHF radio signals. We obtained the first time-series diving data on Baikal seals. The seals dived almost continuously, to an average of 68.9 m, with dives deeper than this (>150 m) being concentrated around dusk and dawn. They showed distinctly different diving patterns between day and night in terms of swimming speed profile and the sequential pattern in maximum dive depth. In the daytime, dives were characterized by higher swimming speeds (mean 1.2 m s(-1)) and upward-directed acceleration events. At night, dives were shallower around midnight and characterized by lower speeds (mean 0.9 m s(-1)) and undirectional deceleration events. We suggest that these differences reflect predation on pelagic fishes by seals using visual cues during the day and predation on swarming crustaceans by seals using tactile cues at night.