Passive acoustical counting of odontocetes using towed and stationed platforms.

Abstract

Counting the number of animals is essential for monitoring and management of marine mammals. Autonomous stereo acoustic event recorder (A-tag) to identify each sound source enabled to count phonating odontocetes in the wild. Finless porpoises were surveyed by towed A-tags in 1100 km stretch of Yangtze River from Wuhan to Shanghai. The calculated detection probability using acoustic method was approximately twice that for visual detection. The difference in detection probabilities between the two methods was caused by the large number of single individuals that were missed by visual observers. However, acoustics tended to underestimate group size due to the limited resolution of sound source bearing angles. The detection performance regarding stationary acoustic monitoring of this species was also examined. Three stereo acoustic data loggers were placed at different locations near the confluence of Poyang Lake and the Yangtze River. Acoustic monitoring confirmed the low density bottle neck of the population between the lake and the river that was consistent with visual observations. The passive pulse event monitoring was proved to be a practical and useful alternative to visual observations, especially for long-term and range-wide monitoring.