Conference presentation: 21st Biennial Conference on the Biology of Marine Mammals

13-18 Dec 2015

A noisy dinner? Passive acoustic monitoring on the predator-prey interactions between Indo-Pacific humpback dolphins and croakers

Tzu-Hao Lin, Wen-Ching Lien, Chih-Kai Yang, and Lien-Siang Chou
Institute of Ecology and Evolutionary Biology, National Taiwan University

Shane Guan
Office of Protected Resources, National Marine Fisheries Service, Silver Spring, MD, USA

The spatio-temporal dynamics of prey resources have been considered as important factors for shaping the distribution and behavior of odontocetes. Indo-Pacific humpback dolphin (Sousa chinensis) is a costal species, which primary feeds on benthic croakers. It has been hypothesized that the distribution pattern and periodic occurrence of humpback dolphins are results of their prey movement. However, the interactions between humpback dolphins and croakers remain unclear. During May 2013 and November 2014, underwater sound recordings were collected in western Taiwan waters. Croaker choruses and humpback dolphin echolocation clicks were automatically detected using custom developed algorithms. Both croaker choruses and dolphin clicks were frequently detected in shallow estuarine waters during spring and summer. In addition, shorter inter-click intervals were detected with higher frequencies in these areas, indicating more likely foraging behavior. Current results suggest that the core habitats of humpback dolphins show an agreement with the areas of prominent croaker chorus. Diurnal cycle analysis showed that croaker choruses were most evident after sunset to until approximately 4 A.M. In estuarine waters, humpback dolphin clicks were most frequently detected during the nighttime, with reduced detection rates after 8 A.M. This suggests that the diurnal behavior of humpback dolphins could be associated with the calling behavior of croakers. Although whether the position of a calling croaker could be passively localized by a dolphin remains unknown, our results indicate that the foraging probability of humpback dolphins may be elevated during the nighttime chorus of croakers. Information regarding the spatio-temporal dynamics of croaker chorus can be important for the conservation management of humpback dolphins. Further details on the predator-prey interactions between humpback dolphins and croakers can be investigated by using hydrophone arrays.

Poster (pdf)

聽海洋的聲音? 從聲音訊號多樣性來探討魚類群聚的行為

生活在競爭激烈的社會中,大家都知道要去尋找『藍海』,而不是拼命地往『紅海』裡鑽。同樣的,各種動物為了減少彼此競爭資源的衝突,也在長期演化過程中發展出偏好不同資源的趨勢,在生態系統中佔據著不同區位。但大家可曾想過,『聲音空間』是否也是一種資源?

其實對於仰賴聲音求偶、競爭領域的動物來說,可供作溝通的領域、時間甚至頻率範圍都是一種另類的生存資源。如果『聲音空間』被別種動物佔據、或是被噪音干擾而無法和同種動物傳遞訊息,可能就會降低尋找同伴、交配的機會,甚至提高了和別種動物衝突的可能。就像待在一個吵鬧且缺乏光線的演唱會,台上的歌手用喇叭佔據了大部分的聲音空間。在缺乏可見度的狀況下,我們只能大聲喊叫,或是開啟假聲模式提高自己的發聲音頻,讓同伴在歌手所佔據的音頻範圍之外察覺到我們。為了避免陷入這種慘況,許多發聲昆蟲、蛙類、鳥類還有哺乳動物都會在聲音溝通中區隔出所偏好發聲的時間、音頻範圍。

但是對於共處一海域的魚類群聚來說,如何分享、利用所處的音響環境,並且能夠在吵雜的狀況下正確分辨同類聲音仍沒有明確結論。直到Ruppé等人於2013年春天,在南非Sodwana Bay外海約120公尺水深的海底峽谷收錄聲音。並將2793筆聲音訊號分為17大類,雖然沒有同步的錄影監視發聲動物,但根據聲音特徵幾乎可以確認其中有16大類屬於魚類叫聲,另一類則可能是齒鯨的脈衝聲波。

從各類聲音頻繁出現的時間,可再區分為白天和晚上出現的兩大類群。有趣的是,白天出現的聲音類群在聲學特徵上(脈衝波重複率和峰值頻率)較為相似,反而是在晚上出現的聲音類群中,各類聲音的特徵區分相當明顯。換句話說,比起白天的魚類聲音,晚上的魚類聲音多樣性較高。作者推測這是因為在白天活動的魚類主要依賴視覺展示溝通、求偶,聲音只是輔助的媒介。然而,晚上活動的魚類無法透過視覺觀察,必須仰賴聲音進行溝通。如果夜晚活動的各種魚類都使用極為相似的聲音,則可能會降低同種之間的溝通效率。這種在聲音使用上的限制,可能是夜間活動的魚類聲音具有豐富多樣性的原因。

Hastings 和Širović在2015年 PNAS 上的文章中也指出,聲學監測系統的研發在近年來大幅增加了我們對海洋聲景的了解,一旦能夠掌握各種魚類在不同行為的聲音特徵,即可透過海裡的聲音了解各種魚類的豐度與行為。即使現在還沒有辦法建立起完整的魚類聲音資料庫,也可以進行和Ruppé等人相似的研究,從水下錄音中透過聲音特徵在時間與空間上的變動趨勢,發掘出和當地魚類群聚相關的資訊。此外,在水下噪音汙染日漸增加在情況下,長期的水下錄音也可以協助我們了解魚類群聚的長期變化趨勢是否受到噪音污染的影響。

石首魚夏季鳴唱

石首魚冬季鳴唱

在台灣,雖然對於魚類群聚聲音的日夜變化還沒有詳盡的研究,但從不同季節之間的水下錄音也可以聽到石首魚在夏季和冬季之間的叫聲改變。這樣的改變是因為魚種的不同呢? 還是行為的改變? 還有待我們去發掘。

其實,透過水下錄音我們可以獲取非常大量的『聲態資訊』,生態學、海洋物理和資訊學門的跨領域合作,透過訊號偵測、大量資料分析等技術,將能夠進一步從海洋動物的聲音、海洋環境的聲景了解海洋生態與環境的變化。雖然目前這類研究在台灣學術界還非常的小眾,但在歐美已經是一個逐漸興盛的領域,並且廣泛的被應用在海洋魚類與海洋哺乳動物的生態調查、海洋工程的環境影響評估、甚至是海洋生態系的長期變遷研究之中。未來隨著台灣海洋再生能源的開發,相信這類的研究也會慢慢的在台灣擴展開來。

參考資料:

1. Philip A. Hastings and Ana Širović (2015) Soundscapes offer unique opportunities for studies of fish communities. PNAS, 112: 5866-5867.
2. Laëtitia Ruppé, Gaël Clément, Anthony Herrel, Laurent Ballesta, Thierry Décamps, Loïc Kéver, and Eric Parmentier (2015) Environmental constraints drive the partitioning of the soundscape in fishes. PNAS, 112: 6092-6097.

延伸閱讀1:【看啥小魚可以吃】有錢吃鮸,沒錢免吃!

延伸閱讀2:研究海洋生態保育魚類聲音藏玄機

International Conference on Biodiversity, Ecology and Conservation of Marine Ecosystems 2015 @ Hong Kong

1-4 June 2015

Seasonal changes in habitat use of Indo-Pacific humpback dolphins at an estuary

Tzu-Hao Lin, Chia-Yun Lee, Lien-Siang Chou

Institute of Ecology and Evolutionary Biology, National Taiwan University

Tomonari Akamatsu

National Research Institute of Fisheries Engineering, Fisheries Research Agency

River estuaries are ecotone environments where freshwater and seawater mix together. Seasonal rainfall is likely to influence the salinity, turbidity, and development of estuarine fronts, thus alter the distribution of aquatic animals at an estuary. Indo-Pacific humpback dolphin is a coastal species that use estuaries as their core habitat. According to previous studies, the distribution of humpback dolphins in their estuarine habitat moved seaward during wet seasons. In addition, circling movement associated with the hunt for epipelagic fish increased during flooding tides. However, it remains unclear how seasonal rainfall influences the estuarine habitat use of humpback dolphins. During July 2009 and October 2014, acoustic data loggers were deployed at the Xin Huwei River estuary, Taiwan to record ultrasonic pulsed sounds. Biosonar clicks of humpback dolphins were detected using an automatic detection algorithm. The temporal variations of humpback dolphin behavior were investigated in terms of detection rate, occurrence pattern within the tidal cycle, echolocation behavior. The behavior of humpback dolphins significantly varied among the four monitoring sections and two periods (wet and dry seasons). The tide related occurrence was evident at the entire monitoring area during wet seasons, however, the similar occurrence pattern was only observed at the inner and outer estuary during dry seasons. In addition, long distance biosonars were much frequently detected at the inshore and offshore sections. During drought periods, the inshore and offshore sections are less likely to be influenced by the mixtures between freshwater and seawater. Our results suggest the river runoff may play an important factor in shaping the estuarine habitat use of humpback dolphins. Therefore, it is necessary to consider the interception of river runoff in the conservation management of humpback dolphins in an estuarine habitat.

New article online: Passive acoustic monitoring of the temporal variability of odontocete tonal sounds from a long-term marine observatory

A new article regarding the usage of long-term marine observatory to monitor the odontocete behavior was published on PLOS One.

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

PLOS One (2015). DOI: 10.1371/journal.pone.0123943

Tzu-Hao Lin, Hsin-Yi Yu, Lien-Siang Chou

Institute of Ecology and Evolutionary Biology, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan (R.O.C.)

Chi-Fang Chen

Department of Engineering Science and Ocean Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan (R.O.C.)

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.

Using local-max detector to detect tonal sounds

Lots of animals produce tonal calls. The acoustic characteristics of tonal sounds may help us to identify the species and behavior of calling animals. However, some animals like cetaceans, have a highly complex repertoire of tonal sounds. This elevates the difficulty of using automatic detection method in the passive acoustic monitoring.

In terms of this, I developed this program to help people use passive acoustic monitoring to study the animals’ tonal sounds. The purpose of this program is to decrease the labor work of detecting animals’ vocalizations by passive acoustic monitoring. The current program can detect multiple types of tonal sound without training or using sound template. The detection is based on the prominent of tonal appearance on the spectrogram.

The detection target of this program primary focus on “tonal sounds”, but it is also possible to detect “burst-pulses” and other “tonal noise” with strong tonal appearance. This program aims to work for everyone. It not only helps user to detect the occurrence of calls, but also provide information on their acoustic features so that user can use those information for further analysis.

The following figure is a demonstration of my algorithm.

whistle detector

The detection process include three main steps: 1. remove ambient noise, 2. extract tonal spectral peaks, and 3. noise filtering.

  1. Spectrograms of sound recordings are produced using fast Fourier transform (FFT) with the Hamming window. Ambient noise is removed by pre-whitening spectrograms. Spectrograms are further smoothed using a Gaussian kernel.
  2. Tonal sounds are detected by applying two thresholds (SNR and tonality). If the instantaneous frequency bandwidth of tonal sound is suitable, the peak frequency is extracted by finding the local maximum in the power spectrum.
  3. A noise filter is employed to exclude broadband noise and isolated narrowband noise. The tonal spectral peaks are claimed as adopted frequencies of animals’ tonal sound after noise filtering.

Detail of the local-max detector for tonal sound is available in the following publications:
Lin, Tzu-Hao, Chou, Lien-Siang, Akamatsu, Tomonari, Chan, Hsiang-Chih, Chen, Chi-Fang. (2013) An automatic detection algorithm for extracting the representative frequency of cetacean tonal sounds. Journal of the Acoustical Society America, 134: 2477-2485.

If you are interested, please feel free to contact me. This program is free of charge and we can cooperate together! Please also take a look at the operation manual of this program so that you can understand the system requirement and the possible application.

Operation manual of Local-max detector for tonal sounds

E-mail: schonkopf@gmail.com

Tzu-Hao (Harry) Lin

鯨豚聲音的行為生態與野外監測之應用 -「第十三屆鯨豚生態與保育-賞鯨永續發展」研討會演講摘要

2013.6.30 (日) @ 蘭陽博物館

鯨豚聲音的行為生態與野外監測之應用

林子皓  國立台灣大學生態演化所博士

受到懸浮物質以及深度的影響,水生動物的視覺能力較陸域動物受到限制。鯨豚在適應海洋環境的過程中,演化出許多利用聲音的生活方式。齒鯨會利用回聲定位來探測環境,鼠海豚之外的鯨豚也會利用哨叫聲來與其他個體溝通、交換訊息,鯨豚動物更可以利用竊聽其食餌生物所發出的聲音來覓食。與陸地哺乳動物的發聲機制不同,鯨豚並非利用聲帶震動發出聲波。以齒鯨為例,齒鯨的發聲器官位於噴氣孔下方的喉唇,透過空氣在聲門兩側的壓力差使喉唇震動發聲,聲波經過頭頂的額隆匯聚後傳遞入水中。回聲從被偵測到的物體反射回來後,透過下頷骨內的油脂通道將聲波傳到耳骨與大腦 (Tyack and Miller 2002)。

鯨豚的聲音主要可以分為三大類:寬頻的答聲、寬頻脈衝聲以及窄頻的哨叫聲。答聲與脈衝聲目前已知都具有回聲定位的功能,齒鯨在發出答聲後,可透過接收到回聲的時間推斷反射回聲的物體距離,再發出下一個答聲持續了解偵測物體的位置。當捕捉獵物時,為了在近距離精準的追蹤獵物移動的路徑,齒鯨會以很高的頻度發出答聲連續不斷的更新獵物位置。此時一連串的答聲由於其發聲間隔極小而形成了脈衝聲。目前已知所有的齒鯨都是利用類似的機制來搜索、捕捉獵物,因此脈衝聲的出現往往可以做為齒鯨覓食行為的代表性聲音 (Au 1993)。

鯨豚的哨叫聲是一種窄頻的聲音,不同類型的哨叫聲在頻譜圖上具有不同的音頻變化特徵(曲調),每一種鯨豚的哨叫聲曲譜皆可由數種哨叫聲類別所組成。鯨豚使用哨叫聲的模式具有相當大程度的變異,其複雜度可與人類語言相比擬 (Ferrer-i-Cancho and McCowan 2009)。雖然哨叫聲被認為是鯨豚個體之間維持聯繫、互相溝通的聲音,但目前對於鯨豚如何使用哨叫聲溝通仍然沒有一個清楚的定論,主要是因為哨叫聲的使用模式可能在不同的個體或是群體行為之間改變。例如瓶鼻海豚被認為會發出個體所特有的簽名哨聲,以和同類互相保持連繫。當瓶鼻海豚在進行覓食行為時,母親可能會暫時離開其仔豚,母子對會透過簽名哨聲來確認彼此的位置,以在短暫分離過後重新團聚 (Smolker et al. 1993)。此外,中華白海豚群體在移動旅行時所發出的哨叫聲曲譜較為單純,但是在進行社交活動時,除了會利用移動旅行所發出的哨叫聲之外,也會使用較為複雜的哨叫聲曲譜組成,但並沒有哪一類哨叫聲與特定的行為事件相關(Lin et al. 2013a)。

不同種類的鯨豚哨叫聲在音頻特徵上可能不盡相同,例如大型鬚鯨僅能發出低頻的聲音,且曲譜組成較為單純;相對的,體型小的海豚聲音頻率則可達到超音波範圍。因此鯨豚聲音的音頻特徵,也可以作為鯨豚種類辨識的依據之一。以宜蘭外海所收錄到的6種鯨豚聲音為例,體型較大的偽虎鯨哨叫聲的峰值頻率最低,曲調最為平緩;熱帶斑海豚哨叫聲的峰值頻率最高,曲調的音頻變化最大。透過起始頻率、結束頻率、最小頻率、最大頻率、峰值頻率、音頻變化係數等曲調參數來進行分類,僅有偽虎鯨的聲音能夠百分之百被正確辨識,其餘種類的聲音辨識率皆低於60%,弗氏海豚更僅有6.8%,顯示宜蘭外海常見的小型海豚在聲音頻率的使用範圍上極為相似。除了音頻特徵上的差異之外,不同種鯨豚在哨叫聲曲譜的組成上也略有差異,如偽虎鯨以恆定頻率的類型為主,瑞氏海豚以正弦波和凸型為主,飛旋海豚和熱帶斑海豚的哨聲曲譜中則上升頻率類型為主(林等人 2012)。雖然目前仍未確認這些曲譜組成的種間差異是否受到每種鯨豚慣用哨叫聲類型的影響,抑或是受到錄音當時的鯨豚群體行為影響,但仍可作為利用水下聲學來辨識鯨豚種類的重要資料。

利用鯨豚經常發出聲音的特性,我們可以透過偵測鯨豚聲音來監聽鯨豚是否出現。被動式聲學監測是一種利用水下麥克風來被動傾聽水中聲音的技術,在近年來被廣泛地採用作為監測鯨豚類活動的方式(Mellinger et al. 2007)。被動式聲學監測可以輔助目視觀察的不足,在能見度低的時間也能夠收集到鯨豚活動的資料。甚至透過某些特殊的聲音,還能夠辨識出鯨豚當時的活動。以中央氣象局在宜蘭外海所建置的台灣東部海域海纜觀測系統為例,其所配備的水下麥克風可以連續不間斷地收錄當地的水下聲音,並即時傳輸回岸上的資料處理中心。透過哨叫聲偵測器,可以自動化的把各種鯨豚的哨叫聲出現時間偵測出來,同時透過曲調追蹤演算法計算哨叫聲的音頻特徵,並透過電腦自我判斷的方法來分出各類哨叫聲。透過此一自動化的平台,目前發現在監測海域的鯨豚類活動以小型海豚為主,且其主要的活動時間以夜間為活動高峰。此外,在日落之後偵測到許多脈衝聲,代表著這些海豚可能在日落之後較常進行覓食行為 (Lin et al. 2013b)。

除了鯨豚聲音之外,水下的許多聲音也可能來自於地殼運動、海上風浪或是各種海洋生物的活動,因此被動式聲學監測也可以用來了解當地海域的氣候與海洋生態。此外,不同季節的海域生態以及氣象狀態也可能會改變不同季節的環境噪音特性。除了自然界所產生的聲音,海洋中的背景噪音在近年來隨著長途貨輪運輸的成長也逐年提升。在淺海的人為開發,更伴隨著填海、疏濬、水下打樁等工程產生的高強度噪音,這些噪音除了會增加當地海域環境的吵雜程度,也可能會傷害海洋生物的聽力(Richardson et al. 1995)。未來透過被動式聲學方法,將可協助相關單位掌握台灣鄰近海域的環境狀況,進而提供保育海洋生態、維持人為開發與保育平衡的重要監測平台。

參考文獻:

林思瑩、余欣怡、林子皓、周蓮香、宜蘭縣立蘭陽博物館 (2012) 臺灣東部宜蘭海域六種鯨豚哨音的種間變異研究. 第三屆兩岸三地鲸類研究和保護交流研討會,南京。

Au WWL (1993) The sonar of dolphins. Springer, New York

Ferrer-i-Cancho R, McCowan B (2009) A law of word meaning in dolphin whistle types. Entropy 11:688–701

Lin TH (2013) The application of passive acoustic monitoring for studying Indo-Pacific humpback dolphin behavior and habitat use off western Taiwan. Ph.D. dissertation, National Taiwan University

Lin TH, Yu HY, Chen CF, Chou LS (2013) Automatic detection and classification of cetacean tonal sounds from a long-term marine observatory. Proceedings of Symposium on Underwater Technology 2013.

Mellinger DK, Stafford KM, Moore SE, Dziak RP, Matsumoto H (2007) An overview of fixed passive acoustic observation methods for cetaceans. Oceanography 20:36–45

Richardson WJ, Greene CR, Malme CI, Thompson DH (1995) Marine mammals and noise. Academic Press, San Diego.

Smolker RA, Mann J, Smuts BB (1993) Use of signature whistles during separations and reunions by wild bottle dolphin mothers and infants. Behavioral Ecology and Sociobiology 33: 393–402

Tyack PL,Miller EH (2002) Vocal anatomy, acoustic communication and echolocation. In: Rus Hoelzel A (ed) Marine mammal biology: an evolutionary approach. Blackwell, UK, pp 142–184