title: Apparent source levels and active communication space of whistles of free-ranging Indo-Pacific humpback dolphins (Sousa chinensis) in the Pearl River Estuary and Beibu Gulf, China.
abstract: Background. Knowledge of species-specific vocalization characteristics and their associated active communication space, the effective range over which a communication signal can be detected by a conspecific, is critical for understanding the impacts of underwater acoustic pollution, as well as other threats. Methods. We used a two-dimensional cross-shaped hydrophone array system to record the whistles of free-ranging Indo-Pacific humpback dolphins (Sousa chinensis) in shallow-water environments of the Pearl River Estuary (PRE) and Beibu Gulf (BG), China. Using hyperbolic position fixing, which exploits time differences of arrival of a signal between pairs of hydrophone receivers, we obtained source location estimates for whistles with good signal-to-noise ratio (SNR ≥10 dB) and not polluted by other sounds and back-calculated their apparent source levels (ASL). Combining with the masking levels (including simultaneous noise levels, masking tonal threshold, and the Sousa auditory threshold) and the custom made site-specific sound propagation models, we further estimated their active communication space (ACS). Results. Humpback dolphins produced whistles with average root-mean-square ASL of 138.5 ± 6.8 (mean ± standard deviation) and 137.2 ± 7.0 dB re 1 µPa in PRE (N = 33) and BG (N = 209), respectively. We found statistically significant differences in ASLs among different whistle contour types. The mean and maximum ACS of whistles were estimated to be 14.7 ± 2.6 (median ± quartile deviation) and 17.1± 3.5 m in PRE, and 34.2 ± 9.5 and 43.5 ± 12.2 m in BG. Using just the auditory threshold as the masking level produced the mean and maximum ACSat of 24.3 ± 4.8 and 35.7 ± 4.6 m for PRE, and 60.7 ± 18.1 and 74.3 ± 25.3 m for BG. The small ACSs were due to the high ambient noise level. Significant differences in ACSs were also observed among different whistle contour types. Discussion. Besides shedding some light for evaluating appropriate noise exposure levels and information for the regulation of underwater acoustic pollution, these baseline data can also be used for aiding the passive acoustic monitoring of dolphin populations, defining the boundaries of separate groups in a more biologically meaningful way during field surveys, and guiding the appropriate approach distance for local dolphin-watching boats and research boat during focal group following.
authors.affiliation: The Key Laboratory of Aquatic Biodiversity and Conservation of the Chinese Academy of Sciences, Institute of Hydrobiology of the Chinese Academy of Sciences, Wuhan, Hubei, China; University of Chinese Academy of Sciences, Beijing, China; Marine Mammal Research Program, Hawaii Institute of Marine Biology, University of Hawaii, Hawaii, HI, United States of America; Current affiliation: Division of Marine Science and Conservation, Nicholas School of the Environment, Duke University, Beaufort, NC, United States of America.
authors.forename: Zhi-Tao
authors.initials: ZT
authors.lastname: Wang
authors.affiliation: Marine Mammal Research Program, Hawaii Institute of Marine Biology, University of Hawaii , Hawaii, HI , United States of America.
authors.forename: Whitlow
authors.initials: W
authors.lastname: W L Au
authors.affiliation: Sea Mammal Research Unit, School of Biology, University of St. Andrews , Fife , United Kingdom.
authors.forename: Luke
authors.initials: L
authors.lastname: Rendell
authors.affiliation: The Key Laboratory of Aquatic Biodiversity and Conservation of the Chinese Academy of Sciences, Institute of Hydrobiology of the Chinese Academy of Sciences , Wuhan, Hubei , China.
authors.forename: Ke-Xiong
authors.initials: KX
authors.lastname: Wang
authors.affiliation: School of Marine Sciences, Qinzhou University , Guangxi , China.
authors.forename: Hai-Ping
authors.initials: HP
authors.lastname: Wu
authors.affiliation: School of Marine Sciences, Sun Yat-Sen University , Guangzhou , China.
authors.forename: Yu-Ping
authors.initials: YP
authors.lastname: Wu
authors.affiliation: Transport Planning and Research Institute, Ministry of Transport , Guangzhou , China.
authors.forename: Jian-Chang
authors.initials: JC
authors.lastname: Liu
authors.affiliation: Hongkong-Zhuhai-Macao Bridge Authority , Guangzhou , China.
authors.forename: Guo-Qin
authors.initials: GQ
authors.lastname: Duan
authors.affiliation: Hongkong-Zhuhai-Macao Bridge Authority , Guangzhou , China.
authors.forename: Han-Jiang
authors.initials: HJ
authors.lastname: Cao
authors.affiliation: The Key Laboratory of Aquatic Biodiversity and Conservation of the Chinese Academy of Sciences, Institute of Hydrobiology of the Chinese Academy of Sciences , Wuhan, Hubei , China.
authors.forename: Ding
authors.initials: D
authors.lastname: Wang
keywords: Active communication space; Apparent source level; Beibu Gulf; Hydrophone arrays; Indo-Pacific Humpback dolphins; Pearl River Estuary; Sound propagation model; Sousa chinensis; Whistles