A passive acoustical survey method of finless porpoises

Abstract

The finless porpoises (Neophocaena phocaenoides) have narrow band and high frequency sonar signals, which are distinctive from background noises. Underwater sound monitoring with hydrophones placed along the sides of a research vessel, concurrent with visual observations was conducted in the Yangtze River from Wuhan to Poyang Lake in China. In a total of 774km cruise, 588 finless porpoises were sighted by visual observation and 44, 864 ultrasonic pulses were recorded by the acoustical observation system. The acoustic monitoring system could detect presence of the finless porpoises 88% of the time, 300m away from the hydrophone. False alarm in the system occurred with a frequency of 0.9%. Such a high detection performance of the present system was due to a large sound pressure level of sonar signals and frequent sound production. Over 160 dB off-axis sound pressure level and sonar signal production at every 3 to 4 s were observed by an acoustic and a behavioral data logger. The high frequency acoustical observation is suggested as an effective method for field surveys of small cetaceans, which produce sonar signals.