Response of gray whales to low-frequency sounds

Abstract

One transducer from the U.S. Navy SURTASS-LFA source was used during January 1998 to expose migrating gray whales off the California coast to low-frequency sound. These transmissions were 42 s in duration, repeating every 6 min, and 160–330 Hz. The whales were observed from shore stations north and south of the source location. Each pod of whales’ closest point of approach (CPA) to the sound source was estimated from the shore observations. The received level (RL) at the CPA was estimated using measured transmission loss curves and the known source level. The probability of observing whale pods at each RL is computed under both playback and control conditions. The difference between these probabilities as a function of RL provides a measure of avoidance for the animals. Previous work [Malme et al. (1984)] found a 50% change in these probabilities at a RL of 120 dB re:1μPa. For an inshore source location, the current work found 50% avoidance occurred at a RL of 141 dB, with 95% confidence bounds of ±3 dB derived by bootstrap statistical methods. For the offshore source location, 50% avoidance was not achieved for any observed RL. [Work supported by ONR.]