Diving behaviour of Cuvier's (Ziphius cavirostris) and Blainville's (Mesoplodon densirostris) beaked whales in Hawai'i

Beaked whales (family Ziphiidae) are thought to be among the longest and deepest diving mammals, and some species appear to be prone to mass-strand in response to high-intensity sonar. We studied diving behaviour of Cuvier's (Ziphius cavirostris G. Cuvier, 1823) and Blainville's (Mesoplodon densirostris (Blainville, 1817)) beaked whales in Hawaiian waters using suction-cup-attached time-depth recorders. Six whales, two Cuvier's and four Blainville's, were tagged and 41 h of dive data were collected. While Cuvier's beaked whales were found in significantly deeper water depths (median depth = 2079 m) than Blainville's beaked whales (median depth = 922 m), several aspects of diving were similar between the two species: (i) both regularly dove for 48-68 min to depths greater than 800 m (maximum 1408 m for Blainville's and 1450 m for Cuvier's); (ii) ascent rates for long/deep dives were substantially slower than descent rates, while for shorter dives there were no consistent differences; and (iii) both spent prolonged periods of time (66-155 min) in the upper 50 m of the water column. Based on time intervals between dives for the Cuvier's beaked whales, such long dives were likely aerobic, but both species appeared to prepare for long dives by spending extended periods of time near the surface.