Results of low-frequency playback of M-sequence noise to humpback whales, Megaptera novaeangliae, in Hawai'i

The behavior of humpback whales, Megaptera novaeangliae, exposed to playback of low-frequency sounds, was examined to test whether animals would respond to this signal when received sound levels exceeded 120 dB re: 1 mu Pa. The source signal was an M-sequence (essentially a sine wave with a series of phase reversals) centered at 75 Hz with a 30-Hz bandwidth. Behavior and movements of whales were described before, during, and after playback. Eighty-five trials were conducted, of which 50 had an experimental condition of M-sequence playback. Thirty-four were no-sound controls and a single trial used a playback of the Alaska humpback whale feeding call. The received playback sound level at the whales ranged from ambient level (approximate to 90 dB) to 130 dB re: 1 mu Pa (60-90 Hz). A comparison revealed no difference in whale tracks and bearings between control and playback conditions. Behavior rates were examined statistically using independent variables describing pod composition, nearby vessels, and playback sound level. Natural variables of pod composition were the most important factors predicting behavior rates. Vessels had a larger impact and affected more behavioral variables than playback. A slight increase in the duration and distance between successive surfacings was found as the received playback sound level increased. Eleven playbacks in which whales passed within the 120-dB isopleth yielded only three potential responses (one movement away and two toward the sound source) and eight nonresponses. Overall, subtle responses to M-sequence playbacks could only be detected statistically, but the biological significance of these responses is uncertain.