Preliminary analysis of echolocation signals produced by fleeing Irrawaddy dolphins (Orcaella brevirostris) Ying Jiang 1, 4 , Xuelei Zhang 1, 2 , Zhiguo Yang 3 *, Saifullah Arifin Jaaman 5 , Qinzeng Xu 1, 2 , Azmi Marzuki Muda 5 , Hairul Masrini Muhamad 6 1 First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China 2 Laboratory for Marine Ecology and Environmental Science, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China 3 China National Deep Sea Center, Ministry of Natural Resources, Qingdao 266237, China 4 Laboratory for Regional Oceanography and Numerical Modeling, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China 5 Institute of Oceanography & Environment, University Malaysia Terengganu, Kuala Terengganu 21030, Malaysia 6 Key Laboratory of Underwater Acoustics and Marine Information Technology, Department of Applied Marine Physics & Engineering, College of Ocean and Earth, Xiamen University, Xiamen 361102, China Received 7 May 2018; accepted 29 June 2018 © Chinese Society for Oceanography and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract In this study, echolocation signals were recorded from a wild Irrawaddy dolphin (Orcaella brevirostris) in shallow water in the Bay of Brunei. During sound recording, a small fishing boat engine startled a nearby Irrawaddy dolphin and began chasing it on two occasions. Variations in the acoustic parameters were detected. When the Irrawaddy dolphin was startled and chased, the sound pressure level, number of click trains per minute, pulse number, and average inter-pulse interval (PI) per click train were all affected. The PI increased and exhibited a slight downward trend during the chase. The increase in PI indicated an increase in the inspection distance as the dolphin escaped. Thus, Irrawaddy dolphins may adapt their echolocation signals to stand out from ambient noise in the wild and to improve their search efforts in potentially risky situations. Appropriate management of the burst noise around the dolphins is important. Key words: click train, echolocation signal, Irrawaddy dolphin, signal change Citation: Jiang Ying, Zhang Xuelei, Yang Zhiguo, Jaaman Saifullah Arifin, Xu Qinzeng, Muda Azmi Marzuki, Muhamad Hairul Masrini. 2019. Preliminary analysis of echolocation signals produced by fleeing Irrawaddy dolphins (Orcaella brevirostris). Acta Oceanologica Sinica, 38(1): 85–89, doi: 10.1007/s13131-019-1373-y 1  Introduction The Irrawaddy dolphin (Orcaella brevirostris) is a small ceta- cean found in Southeast Asia and the Bay of Bengal (Beasley et al., 2013). The Irrawaddy dolphin is on the IUCN Red List of Threatened Species (Reeves et al., 2008). This dolphin is some- times called the Irrawaddy river dolphin but it is not a true river dolphin; instead, it is an euryhaline dolphin that lives in brackish water near coasts, river mouth and estuaries. The Irrawaddy dol- phin has established subpopulations in freshwater rivers, includ- ing the Ganges and Mekong, as well as the Irrawaddy River, after which it was named. The total population appears to comprise over 7 000 and over 90% occur in Bangladesh. Irrawaddy dol- phins are often seen in estuaries and bays in Borneo, and sight- ings range from Sandakan in Sabah to Sarawak, Malaysia, and most parts of Brunei. Few studies have investigated the biosonar signals and beha- vioral responses of this endangered species. Jensen et al. (2013) reported details of the signals produced by Irrawaddy dolphins in the Bangladesh part of the Sundarban mangrove, which were re- corded in 2010. They demonstrated that these animals use con- sistently lower source levels and higher repetition rates than oceanic delphinids as an adaptation to the shallow, acoustically complex habitat with high amounts of reverberation and acous- tic clutter. In this study, we performed a joint visual–acoustics survey in the shallow water of the Bay of Brunei. During sound recordings, a small fishing boat engine startled a nearby Irrawaddy dolphin and another boat began chasing it on two occasions. In the fol- lowing, we present and discuss our preliminary analysis of the echolocation signals (click trains). 2  Materials and methods Echolocation signal recordings were obtained of the Ir- rawaddy dolphin (Orcaella brevirostris) in the shallow waters of the Bay of Brunei, Sarawak, Malaysia. This bay is near a main habitat of the Irrawaddy dolphin (Fig. 1). The data set was collec-   Foundation item: The National Key R&D Program of China under contract No. 2017YFC1405100; the Key Laboratory of Marine Ecology and Environmental Science and Engineering, Ministry of Natural Resources under contract No. MESE-2016-05; the National Natural Science Foundation of China-Shandong Joint Fund for Marine Ecology and Environmental Sciences under contract No. U1406403; the China-ASEAN Maritime Cooperation Fund. *Corresponding author, E-mail: zg.yang@163.com   Acta Oceanol. Sin., 2019, Vol. 38, No. 1, P. 85–89 https://doi.org/10.1007/s13131-019-1373-y http://www.hyxb.org.cn E-mail: hyxbe@263.net