[Show abstract][Hide abstract] ABSTRACT: Sedentary and territorial rockfish of the genus Sebastes exhibit distinctive homing ability and can travel back to an original location after displacements of metres or even kilometres. However, little is known about the behavioural and sensory mechanisms involved in homing. Although our previous study demonstrated that nocturnal black rockfish Sebastes cheni predominantly use their olfactory sense for homing from an unfamiliar area, the possibility of using landmarks in a familiar area cannot be discounted; i.e. site-specific fish are likely to use three-dimensional spatial memory for navigation and orientation. Using high-resolution acoustic telemetry, we investigated whether S. cheni exhibit distinctive homing paths. Results show that all of the eight rockfish increased their effort within a small area of an unfamiliar region around the release site just after displacement, suggesting that the rockfish probably searched for the homeward direction. The rockfish showed the search movement in the upstream and/or downstream direction, which did not lead home. Finally, after returning to their familiar area, the rockfish exhibited more directed movements with faster speeds at a shallower depth, which was similar to the depth utilised in daily life as well as that of the fish capture.
Full-text · Article · Aug 2012 · Journal of Experimental Biology
[Show abstract][Hide abstract] ABSTRACT: Previous studies have demonstrated that vocal signals facilitate acoustic communication of dugongs. We recorded wild dugong calls from around Talibong Island, Thailand (n = 586) and in Moreton Bay, Australia (n = 331). We also recorded vocalizations of a newborn calf (n = 315) kept at Phuket Marine Biological Center, Thailand, a 19 year old female (n = 73) at Toba Aquarium, Japan, and a 7 year old female (n = 203) at Underwater World, Singapore. Dominant frequency, duration and coefficient of frequency modulation were compared across populations and age. Statistical differences were found for almost all pairwise comparisons (p < 0.05) except between the captive dugongs kept in Japan and also between wild dugongs in Thailand and in Australia. A negative correlation was found between variance of the dominant frequency and dugong age, and a positive correlation was found between variance of the duration and age. The average dominant frequency of wild dugong calls collected in Thailand and in Australia were 5205.4 and 5760.2 Hz, respectively. These acoustic characteristics ranged between those of the 7 and 19 year old female. Our results suggest that dugongs change their vocal repertoire as they grow.
No preview · Article · Apr 2012 · The Journal of the Acoustical Society of America
[Show abstract][Hide abstract] ABSTRACT: Distribution pattern of dugongs is a key component for space-based managements. Vocal interaction of dugongs may result in a distinctive distribution pattern. This study described the distribution patterns of vocalising dugongs, solitary and cow-calf pairs of dugongs. Total of 31 hours and 24 minutes of aerial surveys over southern Thai waters were conducted to observe distribution of the dugongs in 2006, 2008 and 2010. We also conducted towed acoustic surveys to observe the distribution of vocalising dugongs. Total of 473 adult dugongs and 122 calves and 223 vocalizations were found. The distribution of vocalising dugongs was clumped with the range of about 1 km 2. Groups with cow-calf pairs (9 animals on average) were also clumped. Their distribution range was about 3 km2 and did not overlap that of vocalising dugongs. Average number of individuals in groups without cow-calf pairs was about 1, indicating that the most of the group members were solitary. They distributed widely throughout the focal area with the distribution range of about 41 km2.
[Show abstract][Hide abstract] ABSTRACT: Migrational patterns of mangrove jack Lutjanus argentimaculatus were studied in a mangrove estuary in Trang province, Thailand, using ultrasonic telemetry. Ultrasonic coded transmitters were surgically implanted in 18 fish and 16 of them were subsequently monitored by nine fixed receivers installed along Sikao Creek estuary in June and November 2006. Due to technical limitations all of the individuals were released in the middle of the creek. Their movements were monitored for a period of up to 1 month, the data being used to describe short term migration of juvenile Lutjanus argentimaculatus in the creek and to find possible environmental cues for the observed movements. All of the individuals showed a tide related movement pattern, suggesting foraging in the small mangrove channels and/or mangrove forest during high tides. 50% of the fish left the study area for the open coast area within a short time following release, indicating that a part of juvenile L. argentimaculatus may move in between estuarine habitats instead of being site attached. As the fish were reared in fish cages for a certain period of time before the study this behavior could partly be explained by the time spent in captivity. It was found that L. argentimaculatus showed higher movement activity during night high tides possibly explained by an increased availability of the sough after food items.
No preview · Article · Jun 2011 · Environmental Biology of Fishes
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Jun 2011 · The Journal of the Acoustical Society of America
[Show abstract][Hide abstract] 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.
No preview · Article · Oct 2009 · The Journal of the Acoustical Society of America
[Show abstract][Hide abstract] ABSTRACT: December 15-17, 2007 Royal Phuket City Hotel, Phuket, Thailand An acoustical approach for research on marine mammals has been a very active research method in recent years. Dugong (Dugong dugon) is one of the highly endangered species, which are strictly-marine herbivorous and mainly inhabit coastal areas. In order to detect dugong calls from recorded data, several algorithms have been adapted by researchers in the analyzing process. However, the number of misses in the detection is still non-zero. The sound of snapping shrimp recorded in a wide range (2-300 kHz) is one of the main background noises that makes the detection of dugong calls difficult in warm shallow waters. Impulse elimination was employed in the system to get rid of the snapping shrimp noise. In order to improve the performance of the detection system by increasing the detection rate and decreasing the number of misses, two new algorithms were tested in the experiment. The experimental results for the new algorithms including impulse elimination and the cepstrum method are presented in this paper.
[Show abstract][Hide abstract] ABSTRACT: 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 en-
vironment 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.
[Show abstract][Hide abstract] ABSTRACT: 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 (4510 Hz) were above the cutoff frequency of Lloyd’s mirror
effect in shallow waters of the present study. Frequency‐modulated narrow‐band sounds like chirps travel longer and enable accurate measurements of source directions by binaural receivers. These suggest that chirps could be used for conspecific recognition in noisy acoustic scenes in tropical shallow waters.
No preview · Article · Nov 2006 · The Journal of the Acoustical Society of America
[Show abstract][Hide abstract] ABSTRACT: 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.
Preview · Article · Oct 2006 · The Journal of the Acoustical Society of America
[Show abstract][Hide abstract] ABSTRACT: 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.
Preview · Article · Jul 2006 · The Journal of the Acoustical Society of America
[Show abstract][Hide abstract] ABSTRACT: December 13-14, 2006 Siam City Hotel, Bangkok, Thailand Dugongs (Dugong dugon) vocalize several calls in a row. Previous reports showed that dugongs have two kinds of calls: long duration calls (trills) and short duration calls (chirp-squeaks hereinafter called chirps). Vocalization pattern of the dugong were classified and the differences and the similarities in the phonetic structure among different dugong populations are discussed in this paper. We recorded the dugong calls from the southern part of Talibong Island, Thailand (2222 calls in total), Toba aquarium, Japan (694 calls), and Underwater World, Singapore (203 calls). Short duration calls with less than 300 milliseconds were defined as chirps and trills ware defined as a call lasting over 300 milliseconds. The end of a call sequence was defined by a silence over 1 second. Within a call sequence, chirp-to-chirp transition was the most frequent among the 3 groups. Trills appeared at the end of a call sequence. The position of the trill did not differ among the 3 groups. The average of the dominant frequency component of dugong calls collected in Thai waters was 4179.8 + 1630.6 Hz (S.D.), 2567.2 + 1371.3 Hz (S.D.) in Toba aquarium, and 7362 + 1745.6 Hz (S.D.) in Underwater world. This study provided detailed information on the vocalization pattern of the dugong, although the functional use of the calls in the context of acoustic communication is yet to be clarified.
[Show abstract][Hide abstract] ABSTRACT: February 23-25, 2010, Phuket, Thailand Ocean noise has a possibility to interfere with acoustic communication of marine mammals by masking biological signals. Some species of marine mammals change their vocalization (e.g., increasing amplitude, duration, repetition and shifting frequency) with elevated ambient noise level to avoid acoustic interference. We studied the dial pattern of ambient noise and dugong calls. We used the underwater sound data recorded off the south coast of Talibong Island, Trang, Thailand, by Automatic Underwater SOund Monitoring System for Dugong (AUSOMS-D). We used the data recorded from 10:00h on November 16 to 9:00h November 23, 2006. To quantify the dial pattern of the ambient noise, hourly ambient noise levels were analyzed. Then, we counted the number of dugong calls and analyzed their acoustic characteristics. Finally, we tested the relationship between the peak frequency of calls and the ambient noise level to investigate whether and how dugongs change their vocalizations. We found high ambient noise levels in the morning through noon (6:00-12:00h) and evening (18:00-21:00h). Dugongs calls were frequently observed at night as described in previous studies. Dugongs call showed the high frequency (6-9 kHz) with elevated level of the ambient noise, while they usually use 3-6 kHz call. Dugongs may adapt their vocalization in relation to the continuous ambient noise.
[Show abstract][Hide abstract] ABSTRACT: December 13-14, 2006 Siam City Hotel, Bangkok, Thailand Effects of man-made, low-frequency sounds on the behavior of the dugong are discussed in this paper. We developed a monitoring system of power-driven vessel to assess the impact of man-made noise on dugongs. Ship navigation was monitored by questionnaire for boaters and visual observations from an anchored vessel. We used automatic under water sound monitoring systems for dugongs (AUSOMS-D) to record under water sound and to track ship navigations acoustically. The visual observations were performed for a total of 10 hours and 20 minutes and 72 ships were detected. The acoustic monitoring was conducted for over 81 hours and detected 258 ships. Shortest distance between the visual-observation platform and the power-driven vessels ranged from 18 to 500 m or more. We calculated the monitoring range of the system by comparing the result of the visual observation and the acoustic survey. The system detected 51.4 % of noise-making ships within 500 meters from the observation platform, and 78.1 %, 89.5 %, and 100 % within 300, 200, and 100 meters, respectively. The ship navigation showed bimodal occurrence during 6:00-7:00 and during 15:00-17:00. We could position the sound source of ship sounds and draw the pathway of a ship by using AUSOMS-D. Based on this result, we calculated the position fix accuracy of ship sound, which was 17.1±8.71 m. This study provided information on detailed techniques for tracking the noise-making vessels and will lead to tracking the vocalizing animal, such as the dugong.