Cuvier’s beaked whales (Ziphius cavirostris) routinely perform deep dives of over 800m depth to forage on cephalopods, but no study has empirically demonstrated whether they forage at the seafloor. We analysed data from digital acoustic recording tags (DTAG) placed on five Cuvier’s beaked whales off Cape Hatteras, and on two Cuvier’s beaked whales off Southern California Bight. Since this species has been documented to strand coincidentally with mid-frequency active sonar (MFAS), animal movement and acoustics were measured with and without MFAS exposure. Returning echoes of echolocation clicks were used to estimate seafloor distance during foraging dives. Echolocation clicks were only recorded during 16 deep (mean depth: 1365 ± 263m) foraging dives and seafloor echoes were identified for 14 dives. Whales foraged between 432-1924m with 55% of all prey capture attempts in the 850-1150m range. Seafloor detection was dependent on the distance between tag placement and the whales sound producing organ. Most foraging activity (70%) was recorded within 200m of the seafloor, suggesting a preference for benthopelagic slow moving prey. MFAS exposure led to echolocation cessation/delay, limiting the whale’s echolocation and hence foraging activity. We discuss a potential opportunistic foraging strategy related to navigation of bathymetrically complex, heterogenous, and lightless environments.
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