Peter L. Tyack
Woods Hole Oceanogr. Inst., Woods Hole, MA 02543
Playback experiments of low-frequency noise to two species of cetacean were linked to measurements of transmission loss in order to relate responses to received sound level. Coastal observation sites were selected to allow visual tracking of animals using theodolites. The Tursiops playbacks involved a population resident near Sarasota, FL, using a 800--900 Hz M-code stimulus with a source level of 170 dB re: 1 (mu)Pa at 1 m. The identification of individual dolphins of known sex and age with predictable movement patterns permitted repeat playbacks to monitor habituation. The Eschrichtius playbacks were performed as the whales migrated past Big Sur, CA; each whale was only exposed once during its migration. A single airgun (source level=226 dB) and several continuous (source levels 151--163 dB) stimuli associated with offshore oil industry were used. Observation of >3500 migrating whales showed significant responses. Whales slowed down and altered course at ranges of 1--3 km, some near 0 dB SNR. Half of the whales avoided exposure to continuous stimuli at levels >117--123 dB, to airgun pulses at levels >170 dB. Bottlenosed dolphins exposed to continuous M-code at levels near 120 dB showed no such avoidance response.