Acoustic Monitoring of Echolocating Porpoises

Underwater ultrasonic sound of finless porpoises Neophocaena phocaenoides were monitored concurrent with visual observations in the Yangtze River, China. In a total of 774 km cruise, 588 finless porpoises were sighted by visual observation and 44,864 ultrasonic pulses were recorded. The acoustic monitoring system could detect presence of the finless porpoises 82% of the time. False alarm in the system occurred with a frequency of 0.9%. The high frequency acoustical observation is suggested as an effective method for field surveys of small cetaceans, which produce high frequency sonar signals.