Diving ecology of sperm whales

点击翻译按钮获取中文摘要

We deployed suction-cup attached data loggers to 13 sperm whales (6 off Kumano Coast and 7 off Ogasawara Islands) in this 3-year study period. Data from a total of 18 whales have been obtained since 2000. The sperm whales off Ogasawara Islands showed a clear diel pattern in their diving depths. They dived around 800-1300m in the daytime but the depths shallowed to around 400m just after the sunset. Vertical water temperature profile obtained by the data logger showed a thermocline between North Pacific Subtropical Mode Water and deeper water mass around 400m, and whales tended to dive to the depth of the thermocline at night. This border of water mass is thought to concentrate organisms conducting vertical migration including sperm whales' prey. On the other hand, thermocline was not observed off Kumano Coast and we consider that vertical concentration of organisms does not occur in this area and the dive depths do not show diel changes. There were significant differences in the number and direction of the burst, which indicates chase event for prey, between off Kumano Coast and Ogasawara Islands, suggesting sperm whales take different prey in these two areas. Descend rate was correlated with dive depth. This indicates whales adjust dive angle according to the dive depth. However initial descent rate (descent rate in the first 120 sec.) was not correlated with terminal dive depth, but with final bottom depth of the former dives and descent rate in the 120-240 sec was correlated with terminal depth. This suggests sperm whales start a dive with a descent rate so that they can reach bottom depth of the former dive, but adjust their target depth according to the information from echolocation signals, which whales start to emit a few minutes after the submergence.