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Diel diving behavior of sperm whales off Japan

Diel changes in the diving behavior of cetaceans have been documented only in some shallow divers, such as smaller toothed whales and some baleen whales. In this study, we attached data loggers to sperm whales Physeter macrocephalus, one of the deepest and longest divers among all marine mammals, to investigate the possible diel patterns in the species. We conducted the exper- iments in 2 areas off Japan (the Kumano Coast and the Ogasawara Islands). Obvious diel patterns of diving behavior were found off the Ogasawara Islands, where the whales dived deeper and swam faster during the day than at night (day-time depth: mean 853 ± 130 (SD) m, n = 35; night-time depth: mean 469 ± 122 m, n = 31; day-time velocity: mean 2.0 ± 0.3 m s -1 , n = 20; night-time velocity: mean 1.5 ± 0.3 m s -1 , n = 19). On the other hand, off the Kumano Coast, whales showed no diel rhythm in diving depths or swimming speed (day-time depth: mean 646 ± 212 m, n = 83; night-time depth: mean 561 ± 229 m, n = 44 and day-time velocity: mean 1.9 ± 0.5 m s -1 , n = 57; night-time velocity: mean 1.6 ± 0.5 m s -1 , n = 33). We suggest that the environmental differences between these 2 areas contributed to the difference in diel behavior of the whales, which was mediated by the diel behavior of their prey.