Tomio Shinke

Dugong (Dugong dugon) vocalization patterns recorded by automatic underwater sound monitoring systems

To quantitatively examine the diurnal, or tidal, effects on dugong behavior, we employed passive acoustic observation techniques to monitor the animals. Automatic underwater sound monitoring systems for dugongs (AUSOMS-D) were deployed on the sea floor at depths of about 5 m south of Talibong Island, Thailand. The AUSOMS-D recorded underwater sound in stereo at a sampling frequency of 44.1 kHz for more than 116 consecutive hours. Dugong calls were automatically detected by newly developed software with a detection rate of 36.1% and a false alarm rate of 2.9%. In total, 3453 calls were detected during the 164 h of recording. The autocorrelation of the call rate indicated an attendance cycle of about 24 or 25 h, and the most frequent vocalizations were observed from 0300 to 0500 h. The calculated bearings of the sound sources, i.e., dugongs, were used as an indicator to track the relative numbers of dugongs during the monitoring periods.

Feeding behavior of wild dugongs monitored by a passive acoustical method

Little is known about feeding behavior of wild dugongs (Dugong dugon) because direct measurements of feeding events in the water were scarcely feasible. In this study, the authors achieved the first successful feeding sound monitoring in a seagrass area using a full-band underwater recording system (called automatic underwater sound monitoring system for dugong: AUSOMS-D). In total, 175 feeding sounds were identified in 205 h of recording. Feeding sounds were only detected at night, implying diurnal differences in the feeding behavior of the studied dugong population. Differences in periodicity of feeding sounds suggested that two or more individuals were in the acoustically observable area. Furthermore, a feeding position monitored by two AUSOMS-Ds was used to calculate source levels of dugong feeding sounds. Assuming spherical_propagation, source levels were measured between 70.6 and 79.0 dB rms re 1 microPa/square root of Hz.

Detection probability of vocalizing dugongs during playback of conspecific calls.

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

Dugongs (Dugong dugon) produce bird-like calls such as chirps and trills. The vocal responses of dugongs to playbacks of several acoustic stimuli were investigated. Animals were exposed to four different playback stimuli: a recorded chirp from a wild dugong, a synthesized down-sweep sound, a synthesized constant-frequency sound, and silence. Wild dugongs vocalized more frequently after playback of broadcast chirps than that after constant-frequency sounds or silence. The down-sweep sound also elicited more vocal responses than did silence. No significant difference was found between the broadcast chirps and the down-sweep sound. The ratio of wild dugong chirps to all calls and the dominant frequencies of the wild dugong calls were significantly higher during playbacks of broadcast chirps, down-sweep sounds, and constant-frequency sounds than during those of silence. The source level and duration of dugong chirps increased significantly as signaling distance increased. No significant correlation was found between signaling distance and the source level of trills. These results show that dugongs vocalize to playbacks of frequency-modulated signals and suggest that the source level of dugong chirps may be manipulated to compensate for transmission loss between the source and receiver. This study provides the first behavioral observations revealing the function of dugong chirps.

Stability of Call Sequence in Dugongs' Vocalization

Dugongs (Dugong dugon) produce different types of vocalization such as chirp, trill, and barks. Previous reports showed that dugongs have two kinds of phonemes: long duration calls (trill) and short duration calls (chirp-squeaks hereinafter called chirp). Especially, the chirp and trill calls were widely reported in different populations. However, characteristics of call patterns in dugongs have never been reported. Moreover, the function of these calls was not revealed. The objective of this study is to classify the vocalization patterns of dugong calls and discuss the stability call sequences within and across local populations of dugongs. We recorded the underwater sound at the off Talibong Island, Trang, Thailand in 2004 and 2005 for 120 hours by an automatic underwater sound recording system (AUSOMS-D, System Intech, Tokyo). The AUSOMS-D is the water resistant stand-alone recording system and developed for passive acoustical monitoring targeting human audible range. The AUSOMS-D consisted of a pair of hydrophones located 2 m apart for calculating the bearings of the sound sources. The electric circuits and batteries were housed in a pressure-resistant case, and the hydrophones were connected to a stereo preamplifier. The sound signals were fed into a 1-kHz high pass filter to eliminate low frequency background noise. Digitized signals were recorded on an 80 GB removable hard disk drive by uncompressed format with a time stamp. The power supply system with batteries and DC/DC converter were also housed inside the case. The sampling frequency of the A/D converter was 44.1 kHz and the dynamic range was 74-120 dB (re 1 muPa) with a 16-bit resolution. Each hydrophone had flat frequency responses within 2 dB between 1 and 10 kHz. For the comparison purpose, we made recording of a captive dugong in Toba Aquarium, Japan, which was introduced from Philippine waters. We analyzed the underwater sound data set that was obtained in both animals in different environment and populations. Short duration calls with less than 300 milliseconds were defined as chirp and trill was defined as a call lasting over 300 milliseconds. The end of a call sequence was defined at the silence over 3 seconds. Total of 1174 audible calls were detected from total of 12 hours data set (from 3:50 to 6:50 1-4 March 2004). Chirp were observed more than trill calls (567 chirps and 67 trills). Chirp-to-chirp transitions were most frequency observed (81.68%), whereas trill-to-trill transitions were the least (4.27%). Transitions between the two types of calls were also observed (6.98%, 7.07%). Trill appeared in the middle and the end of a call sequence. The position of the trill did not differ between wild individual in Thailand and a captive individual from off the Philippines that are considered to be separated populations. The stability of the sequence of each type of calls in a call sequence is investigated. Unlike the song of birds or baleen whales, the call sequence pattern of the dugong suggests small differences across the populations. The call sequence analysis as well as the behavioral context observation will provide the key to interpret the function of dugong calls.

Wild dugongs’ vocal responses to conspecific calls

Wild dugongs were found to call back more to conspecific calls than to artificially synthesized sounds. The population was exposed to four different playback stimulii a wild dugong chirp, a synthesized down‐sweep sound similar to the dugong chirp, a synthesized constant‐frequency sound, and no sound as a control. Vocalizing dugongs were localized using an array of stereo‐underwater‐recording systems. Wild dugongs vocalized more frequently after the playback of dugong chirps (2.8 calls/min) than those of constant‐frequency (0.55 calls/min) and control (0.2 calls/min), (p>0.01, Kruskal‐Wallis test). Dominant frequencies of response calls were 4810 Hz to dugong chirps and 4470 Hz to down‐sweep sounds. These were higher than those to other stimuli (3794 and 4044 Hz). Distances of calling‐back dugongs from the playback speaker were significantly shorter for dugong chirps (10.19 m) and down‐sweep (19.02 m) than that for constant frequency (105.84 m) (p>0.001). The observed dominant frequencies of response calls...