Detection probability of vocalizing dugongs during playback of conspecific calls

Abstract

Dugongs (Dugong dugon) were monitored using simultaneous passive acoustic methods and visual observations in Thai waters during January 2008. Chirp and trill calls were detected by a towed stereo hydrophone array system. Two teams of experienced observers conducted standard visual observations on the same boat. Comparisons of detection probabilities of acoustic and visual monitoring between two independent observers were calculated. Acoustic and visual detection probabilities were 15.1% and 15.7%, respectively, employing a 300 s matching time interval. When conspecific chirp calls were broadcast from an underwater speaker deployed on the side of the observation boat, the detection probability of acoustic monitoring rose to 19.2%. The visual detection probability was 12.5%. Vocal hot spots characterized by frequent acoustic detection of calls were suggested by dispersion analysis, while dugongs were visually observed constantly throughout the focal area (p<0.001). Passive acoustic monitoring assisted the survey since detection performance similar to that of experienced visual observers was shown. Playback of conspecific chirps appeared to increase the detection probability, which could be beneficial for future field surveys using passive acoustics in order to ensure the attendance of dugongs in the focal area.

Callback response of dugongs to conspecific chirp playbacks

“…4) suggests that dugongs exchange vocalizations. The number of vocalizing dugongs also increased when a sequence of chirps and a trill was broadcast repeatedly during towed acoustic surveys (Ichikawa et al, 2009). The chirp ratio, or the repetition rate of chirps, was significantly higher after playback of any stimuli than that after a silence.…”

“…However, at least two wild dugongs vocalized almost concurrently on four occasions during the experiment. Furthermore, Ichikawa et al (2009) demonstrated that dugongs in the focal area vocally respond to playbacks of a chirp and trill sequence. These observations support the hypothesis that dugongs in the focal area exchange vocalizations.…”

“…2). Dugong vocalizations have been captured mostly in this area around Talibong Island (Ichikawa et al, 2009). The sea floor in the study area is flat and scarcely covered with seagrass vegetation (Nakanishi et al, 2005).…”

“…Most of the dugongs were sighted in or around seagrass beds (Nakanishi et al, 2005;Hines et al, 2005). The AUSOMS-D devices are represented by a star and were deployed on the sea floor at a depth of approximately 5 m. Dugong vocalizations around Talibong Island are mostly observed in the focal area (Ichikawa et al, 2009). system to eliminate human bias from the operation. The system consisted of a laptop computer, a D/A converter, a 12-V battery, and an underwater speaker (US300, FOSTEX, Tokyo, Japan).…”

Acoustic characterisation of bycatch mitigation pingers on shark control nets in Queensland, Australia

“…Ichikawa et al (2009) demonstrated for wild dugongs in Thailand that playbacks of sounds of actual or synthetic dugong calls attracted dugongs to within 10 and 19 m, respectively, from within a 250 m radius. Playback of 3.5 kHz tones of 141 dB re 1 µPa at 1 m SL resulted in a stand-off distance of 100 m.…”