Comparison between visual and passive acoustic detection of finless porpoises in the Yangtze river, China

Recently, sonar signals and other sounds produced by cetaceans have been us ed for acoustic detection of individuals and groups in the wild. However, t he detection probability ascertained by concomitant visual survey has not b een demonstrated extensively. The finless porpoises (Neophocaena phocaenoid es) have narrow band and high-frequency sonar signals, which are distinctiv e from background noises. Underwater sound monitoring with hydrophones (B&K 8103) placed along the sides of a research vessel, concurrent with visual o bservations was conducted in the Yangtze River from Wuhan to Poyang Lake in 1998 in China. The peak to peak detection threshold was set at 133 dB re 1 mu Pa. With this threshold level, porpoises could be detected reliably wit hin 300 m of the hydrophone. In a total of 774-km cruise, 588 finless porpo ises were sighted by visual observation and 44 864 ultrasonic pulses were r ecorded by the acoustical observation system. The acoustic monitoring syste m could detect the presence of the finless porpoises 82% of the time. A fal se alarm in the system occurred with a frequency of 0.9%. The high-frequenc y acoustical observation is suggested as an effective method for field surv eys of small cetaceans, which produce high-frequency sonar signals. (C) 200 1 Acoustical Society of America.