Behavioral Responses of a Harbor Porpoise (Phocoena phocoena) to 25.5- to 24.5-kHz Sonar Down-Sweeps With and Without Side Bands

Abstract: Signals used in naval and long-range fish detec­tion sonar often contain harmonics which may influence the behavior of harbor porpoises (Phocoena phocoena) more than the lower fun­damental frequencies, so the behavioral effects of 25-kHz FM signals with and without high-fre­quency side bands (71 and 121 kHz) on a harbor porpoise were quantified in a pool. Sequences of sonar signals were transmitted at four average received sound pressure levels ( to deter­mine the dose-response relationship. Sequences, lasting 30 min, consisted of 50 ms, 25.5- to 24.5-kHz down-sweeps (2-s pulse interval), with and without side bands, with identical SPLs at the fundamental frequency. Behavioral effects were quantified as the harbor porpoise’s distance from the transducer, respiration rate, number of jumps, and relative swimming speed. The distance to the transducer changed little in response to the sounds. Respiration rate increased with increasing SPL to maximum respiration rate increases of ~39%, accompanied by jumps, for sweeps with side bands at an of 148 dB re 1 μPa. At simi­lar broadband SPLs, signals with side bands had a greater effect on the harbor porpoise’s behavior than signals without. Side bands may influence behavioral responses, both by making sounds more audible and by affecting the way sounds are perceived by harbor porpoises. Mitigation of active sonar impact could be achieved by reducing the level of side bands (or harmonics). This would benefit species, such as the harbor porpoise, that are likely to have lower hearing thresholds for the frequencies of the harmonics than for the funda­mental frequency.

Key Words: fisheries sonar, naval sonar, noise impact, behavior, mitigation, odontocete

Document Type: Research Article


Page Numbers: 400-411

$12.00Add to cart