A binaural acoustic recording tag reveals details of deep foraging in beaked whales

Abstract

A new acoustic and orientation recording tag (DTAG) for marine mammals contains two hydrophones, 25 mm apart, sampled at 192 kHz. Data are stored with loss-less compression in 6.6 GB of memory giving a 9.5 h recording capacity. Stereo DTAGs have been attached with suction cups to Cuvier’s and Blainville’s beaked whales in Liguria and the Canary Islands. Both species make regular clicks centered at 40 kHz during foraging dives. The DTAGs recorded clicks from the tagged whale and other whales nearby as well as echoes from targets in the water. The angle-of-arrival (AoA) of these sounds was determined both by cross-correlation and time-delay fitting of signals from the two hydrophones on the tag. The AoAs of clicks from tagged whales reveal that they turn their heads from side-to-side while foraging while the AoAs of echoes are consistent with a beam-width of about 20. By scanning a narrow acoustic beam, the whales may be able to search a large water volume while reducing clutter from multiple echoes. Frequent clicks from untagged whales with distinct AoAs indicate that several whales forage together during deep dives and such group cohesion may be a contributing factor to strandings of these species related to use of naval sonar.