Using Playback Experiments to Estimate the Hearing Range and Sensitivity in Humpback Whales

Despite increasing evidence that underwater anthropogenic noise has negative consequences on baleen whale hearing and behavior, there are no direct data available on mysticete hearing in noise. Here, a behavioral response study design is used to quantify humpback whale hearing in natural ocean noise. A suite of tonal signals, at frequencies ranging from 62 Hz to 22 kHz, will be used as the stimuli. A change in humpback group behavior is assumed to indicate that the whales heard, and responded to, the signal. The intended sample size is between eight and ten independent humpback whale groups per frequency. Individual whale and group behavior will be quantified using both land-based data (where a team will track the group from a land-based station) and acoustic tag data (deployed on individual whales to record fine-scale 3D movement underwater). The signal-to-noise ratio will be estimated at the response position of the whale, assuming that this will be consistent between groups for each frequency regardless of the level of background noise. Preliminary results confirm that humpback whales are likely to respond to signals at noise at levels comparable to other marine mammals and suggest these whales can hear the 16 kHz signals. Not only does this provide early data on hearing to high frequencies in a mysticete, but results indicate their ability to hear signals in noise is likely better than expected.