Passive acoustical monitoring systems were used to monitor local migrations of Yangtze finless porpoises in China. We stationed 3 acoustic data loggers (W20-ASII) at 3 different places in the confluence area of Poyang Lake and the Yangtze River on 27-29 April 2006. Acoustic data loggers recorded ultrasonic signals from porpoises swimming in a 300m radius approximate range. Visual observations were conducted simultaneously at two stations. Acoustical and visual observation could detect the presence of porpoises at 73.9 ± 20.9 % and 7.0 ± 6.5 % at all observation times, respectively. Visually-counted group sizes were underestimated compared with acoustical observations for groups of up to 5 individuals, whereas the opposite was true when the group size was more than 6. In summary, a passive acoustical method was proved to be effective for the long term monitoring of finless porpoises swimming underwater. The limitation of group size estimations is thought to be caused by a less accurate measurement of sound source direction due to the short baseline of W20-ASII. It can be improved by employing longer baseline stereo hydrophones.
