Variation in dive behavior of Cuvier’s beaked whales with seafloor depth, time-of-day, and lunar illumination

Depth distributions were analyzed from a study of 19 Cuvier’s beaked whales Ziphius cavirostris that were tagged with satellite transmitting instruments off southern California, USA. Over 113 000 depth measurements were made over the equivalent of ∼200 sampling days. The mean foraging depth was 1182 m (SD = 305 m), and the mean of the maximum depth of all foraging dives was 1427 m (SD = 298 m). Mean foraging depths increased with seafloor depths up to a maximum of ∼1300 m at a seafloor depth of 1900 m, but decreased slightly to a mean of ∼1200 m at seafloor depths of 2000–4000 m. Near-bottom habitat appears to be important for foraging; whales spent ∼30% of their foraging time within 200 m of the bottom at seafloor depths of 1000–2000 m. However, little foraging time was spent near the bottom at seafloor depths greater than 2000 m. The percentage of time spent at near-surface depths (< 50 m) was more than twice as high at night (25%) than during the day (12%). Lunar light also appears to affect diving, with 28% of dark nights and only 17% of brightly moonlit nights spent at these near-surface depths. The apparent avoidance of surface waters during daytime and on brightly moonlit nights is consistent with avoidance of visual predators. A considerably greater fraction of time was spent foraging at night (24.8%) than during the day (15.7%), possibly due to energetic constraints imposed by predator avoidance during the day.