Changes in the acoustic behavior of gray whales Eschrichtius robustus in response to noise

Abstract

Gray whales Eschrichtius robustus, while engaged in underwater signaling, circumvented noise in their environment by altering the structure and timing of their calls. Acoustic responses of whales to both naturally occurring and artificially increased levels of noise were documented during sound playback experimentation in San Ignacio Lagoon, Baja California Sur, Mexico. Nine acoustic parameters were examined and compared between experimental con - ditions: calling rates, call types, frequency range of calls (Hz), call peak frequencies (Hz), call received levels (dB re 1 μPa), call duration (s), percentage of calls showing frequency modulation, number of pulses per call, and call repetition rates (number of pulses s-1). Multiple acoustic strategies were employed by whales which enabled them to minimize the detrimental effect that noise had on their underwater signaling. When different sources of noise were added to their habitat, a corresponding increase was observed in calling rates, call received levels, frequency-modulated signals, number of pulses per call, and call repetition rates. Our results show that gray whales vary their calling behavior dependent upon the noise source, duration, and presentation. Acoustic responses to noise may also differ based on the behavioral activity of the whale (e.g. breeding, migrating, feeding) and on the habitat the whale is occupying (shallow lagoons, coastal or pelagic waters). Background noise (both natural and man-made) has a profound effect on the acoustic behavior of this coastal species and calling is modified to optimize signal transmission and reception. Whether these modifications ensure that effective communication takes place in higher noise situations, without causing detrimental effects to individuals in the long term, remains to be tested.