Gray whales hear and respond to signals from a 21–25 kHz active sonar

Shore‐based theodolite tracking of eastern gray whale ( Eschrichtius robustus ) movements was conducted to test for potential whale responses to a high‐frequency sonar system. Southbound migrating whales were observed from two California shore observation stations as the whales swam past the source vessel that was moored in their migration path. The sonar transducer was deployed from the vessel during all observations, broadcasting 21–25 kHz sweeps for half of each day, the other half remaining silent. The order of control and experimental periods was randomized. No readily apparent response to sonar transmissions was observed in the field or in the visual data. Statistical analysis of tracking data indicates that, compared to control data, gray whales deflected inshore at ranges of 1–2 km from the vessel during sonar transmissions at a received sound pressure level of approximately 148 dB re 1 μPa 2 (134 dB re 1 μPa 2 s). These data suggest that the functional hearing sensitivity of gray whales extends to at least 21 kHz.