Orientation of blacktip sharks ( Carcharhinus limbatus ) to underwater sound

Abstract

Sharks respond to low frequency pulsed sounds but are presumed to lack the capacity to detect these sounds beyond the acoustic near field, where particle motion dominates. This study quantified the distance that blacktip sharks (Carcharhinus limbatus) oriented to sound stimuli and determined that responses could be consistently initiated from the acoustic far field. Using an aerial drone, C. limbatus were filmed responding to sound stimuli (100 – 200 Hz; 200 – 400 Hz; and 400 – 800 Hz) generated by an underwater speaker. Upon detection, C. limbatus elicited a sudden 20° – 160° turn away from the speaker and rapidly swam away. Sharks responded to all frequencies from at least 62 m, and 71.6% of all responses (n = 209) occurred in the far field. Sharks never responded to a high frequency (10 kHz) control stimulus of comparable volume. The sound pressure levels for all stimuli were measured in situ and used to model the propagation away from the source. This permitted direct calculation of the stimulus intensity for the point at which each shark initiated a response. A greater sound pressure level was required to elicit a response at higher frequencies, supporting earlier work that demonstrated greatest sensitivity to low frequencies. The ability of blacktip sharks to detect and orient away from a sound stimulus at distances that extend beyond the acoustic near field, suggests that they are detecting particle motion in the far field.