Diel variation in beaked whale diving behavior

We investigate diel variation in beaked whale diving behavior using data from time-depth recorders deployed on six Blainville's (Mesoplodon densirostris) (255 h) and two Cuvier's (Ziphius cavirostris) (34 h) beaked whales. Deep foraging dives (> 800 m) occurred at similar rates during the day and night for Blainville's beaked whales, and there were no significant diel differences in ascent rates, descent rates, or mean or maximum depths or durations for deep dives. Dive to mid-water depths (100-600 m) occurred significantly more often during the day (mean = 1.59 h(-1)) than at night (mean = 0.26 h(-1)). Series of progressively shallower bounce dives were only documented to follow the deep, long dives made during the day; at night whales spent more time in shallow (< 100 m) depths. Significantly slower ascent rates than descent rates were found following deep foraging dives both during the day and night. Similar patterns were found for the Cuvier's beaked whales. Our results suggest that so-called bounce dives do not serve a physiological function, although the slow ascents may. This diel variation in behavior suggests that beaked whales may spend less time in surface waters during the day to avoid near-surface, visually oriented predators such as large sharks or killer whales (Orcinus orca).