Studies on diving behavior of odontocetes using the suction-cup attached TDR tag.

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We investigated diving behavior of sperm whales and Indo-pacific bottlenose dolphins using the suction-cup attached TDR tag. The tag was deployed to 2 and 4 sperm whale off Kumano coast and off Ogasawara Islands, respectively, and 4 bottlenose dolphins off Amakusa Islands, Kyushu.All tagged sperm whales repeatedly dived for 40-45 min duration down to 400-1300m. Most of their dives were U-shaped-and thought to be for foraging. At the bottom of the dive, whales were very active in terms both depth and speed with occasional bursts of speed. These suggest that sperm whales take an active searching and pursuing strategy while foraging. Dive depth was shallower in the night. This may reflect the vertical migration of the prey or they feed different types of prey according to depth. The latter was also suggested by the fact that swimming speed at the bottom and number of bursts both increased with depth. We think that whales feed on larger and scarcer prey in the deeper water.Previous studies suggested that the suction-cup attached tag was not feasible for bottlenose dolphins because of their intense reactions for the tag. However, we showed it is not always unfeasible for bottlenose dolphins. Indo-Pacific bottlenose dolphins usually traveled or rested in the most of daytime and made a deep regular dive at night. They tended to come back to the same depth in the deep dive at night, suggesting they foraged at the bottom of the sea. This species spent most of their time in the water shallower than 20 m. Number of surfacings for respiration was clearly correlated with the time they stayed near surface. We can estimate the school size using this relationship.