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

Abstract

Recently, sonar signals and other sounds produced by cetaceans have been used for acoustic detection of individuals and groups in the wild. However, the detection probability ascertained by concomitant visual survey has not been demonstrated extensively. The finless porpoises ͑Neophocaena phocaenoides͒ have narrow band and high-frequency sonar signals, which are distinctive from background noises. Underwater sound monitoring with hydrophones ͑B&K8103͒ placed along the sides of a research vessel, concurrent with visual observations 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 Pa. With this threshold level, porpoises could be detected reliably within 300 m of the hydrophone. In a total of 774-km 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 the presence of the finless porpoises 82% of the time. A 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.

Population status, threats and conservation of the Yangtze finless porpoise

“…Their maximal swim speed may be nearly 4 m/s, and their maximal dive duration could be as long as 3 min. This means that they are capable of traveling underwater several hundred meters without breaking the water surface [21,22] . Even when they breathe, only a small part of their tiny body is exposed above the water surface; and because of their dark body color, it is difficult to make them out against the backdrop of the dark water.…”

“…These unique acoustical properties make it possible to identify which species is vocalizing by recording them in the wild. As early as in 1998, we developed the first acoustical survey system using two hydrophones positioned 6.3 m apart from each other and deployed at a depth of 0.8 m on both sides of the survey vessel [21] . We subsequently developed three different towed systems equipped with hydrophones, or so-called REVIEW HYDROBIOLOGY data loggers (i.e., A-tags; ML200-AS2, Marine Micro Technology, Saitama, Japan, see ref.…”

Comparison of echolocation behaviour between coastal and riverine porpoises

“…The reliability of detection can be quite high as shown for finless porpoises (Neophocaena phocaenoides). For example, 80% of visually detected finless porpoises were also detected acoustically with false alarm rates as low as 1% (Akamatsu et al, 2001;Wang et al, 2005).…”

Passive acoustic survey of Yangtze finless porpoises using a cargo ship as a moving platform

“…The sound signal trains which contain less than five successive click events were eliminated from our data analysis conservatively, even if their interclick interval and sound pressure changed gradually. Any irregular successive clicks greater than twice or less than half the previous interclick intervals were also considered to be noises and excluded (Akamatsu et al, 1998(Akamatsu et al, , 2001. In addition, the sound source of the five pulses should come from same direction to make sure all pulses originated from a single sound source (reviewed in Li et al, 2010).…”