Kotaro Ichikawa

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.

Short-range homing in a site-specific fish: search and directed movements

SUMMARY 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.

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.

The influence of body size on the intermittent locomotion of a pelagic schooling fish.

There is a potential trade-off between grouping and the optimizing of the energetic efficiency of individual locomotion. Although intermittent locomotion, e.g. glide and upward swimming (GAU), can reduce the cost of locomotion at the individual level, the link between the optimization of individual intermittent locomotion and the behavioural synchronization in a group, especially among members with different sizes, is unknown. Here, we continuously monitored the schooling behaviour of a negatively buoyant fish, Pacific bluefin tuna (N = 10; 21.0 ∼ 24.5 cm), for 24 h in an open-sea net cage using accelerometry. All the fish repeated GAU during the recording periods. Although the GAU synchrony was maintained at high levels (overall mean = 0.62 for the cross-correlation coefficient of the GAU timings), larger fish glided for a longer duration per glide and more frequently than smaller fish. Similar-sized pairs showed significantly higher GAU synchrony than differently sized pairs. Our accelerometry results and the simulation based on hydrodynamic theory indicated that the advantage of intermittent locomotion in energy savings may not be fully optimized for smaller animals in a group when faced with the maintenance of group cohesion, suggesting that size assortative shoaling would be advantageous.

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.

Intraspecific variation in vocal repertoire among dugong populations

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 5...

Simultaneous observation of intermittent locomotion of multiple fish by fine-scale spatiotemporal three-dimensional positioning

Recent advances in biotelemetry techniques, especially positioning methods, have revealed the detailed behaviour and movement of aquatic organisms. Behavioural intermittence in animal locomotion, such as the Lévy walk, is a popular topic in the field of movement ecology. Previous attempts to describe intermittent locomotion quantitatively have been constrained by the spatial and temporal resolution possible with conventional biotelemetry systems. This study developed a fine-scale spatiotemporal three-dimensional positioning method using a new biotelemetry system with a positional precision of <10 cm and positioning interval of <10 s. Using this proposed positioning method, the intermittent stop-and-go locomotion of Siebold’s wrasses (Pseudolabrus sieboldi) was observed during travel from an unsuitable to a suitable location following displacement. The fish displayed behavioural intermittence in relocating to a suitable location. Initially, their movement halted for reorientation, after which they moved intermittently yet in a straight line to the suitable location. To test the positioning ability of the proposed method, data sets were resampled at intervals of 5, 10, 30, 60, and 300 s. Longer sampling intervals failed to identify reorientations and underestimated the number of stops, distance travelled, and speed. Overall, the results highlighted the adequacy and ability of the proposed positioning method to observe the intermittent locomotion of fish, such as stop-and-go behaviour, in a natural environment.

Evaluation of multipath effects on depth measurements provided by acoustic transmitters in shallow water

Measurement errors caused by multipath effects are one of the problems of acoustic biotelemetry, especially in shallow waters because both the sea surface and bottom can be major boundaries reflecting ultrasonic pulses. We conducted an experiment using transmitters with a pressure sensor to examine occurrence probabilities of multipath effects in a shallow sea area. We deployed seven transmitters with a 1.28-s signal transmitting interval in the same location, and two receivers located 18.5 m (R1) and 38.0 m (R2) horizontally from the transmitters. In order to solve the problems derived from multipath effects, we applied a state-space model to the depth data to estimate the correct values. Then we compared data availabilities, which were percentages of the number of correct values to the number of actual transmissions, between the acquired data and the model estimation. In 15 min recording, the mean proportions of the correct values for all transmitters were 88.4% for R1 and 86.3% for R2. The data availabilities became significantly larger from 65.7% to 96.7% with R1, and from 64.6% to 93.3% with R2 by applying the model. This suggests that we can reduce the measurement errors caused by multipath effects by post processing.