Tzu-Hao Lin

Seasonal Distribution of Indo-Pacific Humpback Dolphins at an Estuarine Habitat: Influences of Upstream Rainfall

River estuaries are dynamic regions that are influenced by the interactions between freshwater and seawater as well as seasonal variations in river runoffs. Studies focusing on the distribution of Indo-Pacific humpback dolphins (Sousa chinensis) have indicated their general tendency toward estuarine habitats. The seasonal activities of humpback dolphins are likely to synchronize with environmental fluctuations. This study investigated the effects of seasonal changes in river runoffs on the distribution gradient of humpback dolphins by deploying acoustic data loggers along the Xin Huwei River estuary, Western Taiwan, between July 2009 and September 2012. Seasonal shifts were observed in the areas with high detected duration of humpback dolphins, which mainly stayed near the river mouth during the dry seasons but moved seaward during rainy seasons and following heavy rainfall. In addition, the gradient of ambient ultrasonic pulses, dominated by snapping shrimp sounds, exhibited regional differences following heavy rainfall. The outward movements of the humpback dolphins and the snapping shrimp sounds in the estuary indicated a temporary trophic-system shift in response to local environmental changes resulting from high volumes of river runoffs. In the future, the seasonal variation in the distribution of humpback dolphins must be considered during the conservation management of this critically endangered population.

Dynamics of soundscape in a shallow water marine environment: a study of the habitat of the Indo-Pacific humpback dolphin.

The underwater acoustic field is an important ecological element for many aquatic animals. This research examines the soundscape of a critically endangered Indo-Pacific humpback dolphin population in the shallow water environment off the west coast of Taiwan. Underwater acoustic recordings were conducted between late spring and late fall in 2012 at Yunlin (YL), which is close to a shipping lane, and Waisanding (WS), which is relatively pristine. Site-specific analyses were performed on the dynamics of the temporal and spectral acoustic characteristics for both locations. The results highlight the dynamics of the soundscape in two major octave bands: 150-300 Hz and 1.2-2.4 kHz. The acoustic energy in the former frequency band is mainly associated with passing container vessels near YL, while the latter frequency band is from sonic fish chorus at nighttime in both recording sites. In addition, large variation of low frequency acoustic energy throughout the study period was noticed at WS, where the water depths ranged between 1.5 and 4.5 m depending on tidal cycle. This phenomenon suggests that besides certain sound sources in the environment, the coastal soundscape may also be influenced by its local bathymetry and the dynamics of the physical environment.

Passive acoustic monitoring of the temporal variability of odontocete tonal sounds from a long-term marine observatory.

The developments of marine observatories and automatic sound detection algorithms have facilitated the long-term monitoring of multiple species of odontocetes. Although classification remains difficult, information on tonal sound in odontocetes (i.e., toothed whales, including dolphins and porpoises) can provide insights into the species composition and group behavior of these species. However, the approach to measure whistle contour parameters for detecting the variability of odontocete vocal behavior may be biased when the signal-to-noise ratio is low. Thus, methods for analyzing the whistle usage of an entire group are necessary. In this study, a local-max detector was used to detect burst pulses and representative frequencies of whistles within 4.5–48 kHz. Whistle contours were extracted and classified using an unsupervised method. Whistle characteristics and usage pattern were quantified based on the distribution of representative frequencies and the composition of whistle repertoires. Based on the one year recordings collected from the Marine Cable Hosted Observatory off northeastern Taiwan, odontocete burst pulses and whistles were primarily detected during the nighttime, especially after sunset. Whistle usage during the nighttime was more complex, and whistles with higher frequency were mainly detected during summer and fall. According to the multivariate analysis, the diurnal variation of whistle usage was primarily related to the change of mode frequency, diversity of representative frequency, and sequence complexity. The seasonal variation of whistle usage involved the previous three parameters, in addition to the diversity of whistle clusters. Our results indicated that the species and behavioral composition of the local odontocete community may vary among seasonal and diurnal cycles. The current monitoring platform facilitates the evaluation of whistle usage based on group behavior and provides feature vectors for species and behavioral classification in future studies.

An automatic detection algorithm for extracting the representative frequency of cetacean tonal sounds

Most studies on tonal sounds extract contour parameters from fundamental frequencies. The presence of harmonics and the frequency distribution of multiple tonal sounds have not been well researched. To investigate the occurrence and frequency modulation of cetacean tonal sounds, the procedure of detecting the instantaneous frequency bandwidth of tonal spectral peaks was integrated within the local-max detector to extract adopted frequencies. The adopted frequencies, considered the representative frequencies of tonal sounds, are used to find the presence of harmonics and overlapping tonal sounds. The utility and detection performance are demonstrated on acoustic recordings of five species from two databases. The recordings of humpback dolphins showed a 75% detection rate with a 5% false detection rate, and recordings from the MobySound archive showed an 85% detection rate with a 5% false detection rate. These detections were achieved in signal-to-noise ratios of -12 to 21 dB. The parameters that measured the distribution of adopted frequency, as well as the prominence of harmonics and overlaps, indicate that the modulation of tonal sounds varied among different species and behaviors. This algorithm can be applied to studies on cetacean communication signals and long-term passive acoustic monitoring.

Automatic classification of delphinids based on the representative frequencies of whistles

Classification of odontocete species remains a challenging task for passive acoustic monitoring. Classifiers that have been developed use spectral features extracted from echolocation clicks and whistle contours. Most of these contour-based classifiers require complete contours to reduce measurement errors. Therefore, overlapping contours and partially detected contours in an automatic detection algorithm may increase the bias for contour-based classifiers. In this study, classification was conducted on each recording section without extracting individual contours. The local-max detector was used to extract representative frequencies of delphinid whistles and each section was divided into multiple non-overlapping fragments. Three acoustical parameters were measured from the distribution of representative frequencies in each fragment. By using the statistical features of the acoustical parameters and the percentage of overlapping whistles, correct classification rate of 70.3% was reached for the recordings of seven species (Tursiops truncatus, Delphinus delphis, Delphinus capensis, Peponocephala electra, Grampus griseus, Stenella longirostris longirostris, and Stenella attenuata) archived in MobySound.org. In addition, correct classification rate was not dramatically reduced in various simulated noise conditions. This algorithm can be employed in acoustic observatories to classify different delphinid species and facilitate future studies on the community ecology of odontocetes.

Automatic detection and classification of cetacean tonal sounds from a long-term marine observatory

Passive acoustic monitoring is effective for examining the temporal variation of cetacean occurrences through continuous recording. However, the interpretation of behavior variability remains difficult. Cetacean tonal sounds are believed to play an important role in communication. Changes in tonal sound usage can be indicative of changes in cetacean behavior. In this study, we applied a generalized tonal sound detector on recordings from the marine cable hosted observatory (MACHO) off Northeastern Taiwan. The tonal sound detector is a modified local-max detector, which examines the prominence of tonality in 2 ms time intervals. Detections were confirmed based on the short-time transient characteristic of tonal sounds. The burst pulse, fundamental frequency, and harmonic were separated during post-processing. The detection results can be used to analyze the frequency distribution and modulation of the tonal sounds. Following individual contour extractions, the tonal sounds were classified to examine the repertoire composition through duration and frequency normalization. An evident nighttime occurrence was identified for both the burst pulse and tonal sound. The mode frequency of tonal sound did not show a significant difference, but the percentage of overlapping tonal sounds, Shannon entropy of frequency distribution, and repertoire complexity varied significantly among diurnal cycles. The current framework of quantification on tonal sound modulation and repertoire complexity provides intuitive parameters for the variability of cetacean behavior, which can be used to examine the temporal variations of cetacean habitat use. The precision in distinguishing different species and behaviors may be increased by comparing the repertoire established through automatic classification.

Detection on the presence and frequency use pattern of cetacean tonal sound

Passive acoustic monitoring (PAM) had already been proved to assess the presence of many cetacean species successfully. However, the continuous recording makes the manual data analysis difficult. In the present study, an automatic detection algorithm was developed for tonal sounds produced by Indo-Pacific humpback dolphins. The algorithm included a tonal sound detector which found the local spectral peaks and sampled the dominant frequencies every 5.3 ms. A noise exclusion process was used to exclude the spectral peaks with wide -3 dB bandwidth. After filtering the isolated frequency points within specific duration and frequency range, the adopted frequencies of tonal sounds could be obtained. The result showed the algorithm had 70% correct detection and 2.8% false positive based on each 1 sec time bin in 10 field recordings. The first to third quartile of adopted frequencies showed significant difference with those extracted manually, but the differences were only 245-489 Hz in average. The current algor...

Temporal and spatial patterns of marine soundscape in a coastal shallow water environment

Underwater acoustic recordings were made at two coastal shallow water locations, Yunlin (YL) and Waishanding (WS), off Taiwan between June and December 2012. The purpose of the study was to establish soundscape baselines and characterize the acoustic habitat of the critically endangered Eastern Taiwan Strait Chinese white dolphin by investigating: (1) major contributing sources that dominant the soundscape, (2) temporal, spatial, and spectral patterns of the soundscape, and (3) correlations of known sources and their potential effects on dolphins. Results show that choruses from croaker fish (family Sciaenidae) were dominant sound sources in the 1.2–2.4 kHz frequency band for both locations at night, and noises from container ships in the 150–300 Hz frequency band define the relative higher broadband sound levels at YL. In addition, extreme temporal variation in the 150–300 Hz frequency band were observed at WS, which was shows to be linked to the tidal cycle and current velocity. Furthermore, croaker cho...

Passive acoustic monitoring on the seasonal species composition of cetaceans from a marine observatory

Information on the species diversity of cetaceans can help us to understand the community ecology of marine top predators. Passive acoustic monitoring has been widely applied in the cetacean research, however, species identification based on tonal sounds remains challenging. In order to examine the seasonal changing pattern of species diversity, we applied an automatic detection and classification algorithm on acoustic recordings collected from the marine cable hosted observatory off the northeastern Taiwan. Representative frequencies of cetacean tonal sounds were detected. Statistical features were extracted based on the distribution of representative frequency and were used to classify four cetacean groups. The correct classification rate was 72.2% based on the field recordings collected from onboard surveys. Analysis on one-year recordings revealed that the species diversity was highest in winter and spring. Short finned pilot whales and Risso’s dolphins were the most common species, they mainly occurr...

Variations of soundscape in a shallow water marine environment for the Chinese white dolphin

For acoustically oriented animals, sound field can either provide or mask critical information for their well-being and survival. In addition, understanding the variations of the soundscape in the Chinese white dolphin habitat is important to monitoring the relationship between human activities, calling fish, and dolphins, thus assist in coastal conservation and management. Here, we examined the soundscape of a critically endangered Chinese white dolphin population in two shallow water areas next to western coast of Taiwan. Two recording stations were established at Yunlin, which is close to an industrial harbor, and Waisanding, which is nearby a fishing village, in summer 2012. Site specific analyses were performed on variations of the temporal and spectral acoustic characteristics for both locations. The results show different soundscapes for the two sites from different recurring human activities. At Yunlin, high acoustic energy was usually dominated by cargo ships producing noise below 1 kHz. At Waisa...