Apparent source level of free-ranging humpback dolphin,Sousa chinensis, in the South China Sea

Abstract

The acoustic performance and behaviour of free-ranging cetaceans requires investigation under natural conditions to understand how wild animals use sound. This is also useful to develop quantitative evaluation techniques for passive acoustic monitoring. There have been limited studies on the acoustics of the Indo-Pacific humpback dolphin; nevertheless, this species is of particular concern because of the anthropogenic activity in the coastal habitats. In the present study, we used a four-hydrophone array to estimate the apparent source levels (ASLs) of biosonar sequences (click trains), of this species in San-Niang Bay, China. As the dolphins approached the array, 173 click trains were found to meet the criteria of on-axis sounds produced within 60 m of the equipment. In total, 121 unclipped click trains were used for the ASL estimation. The qualified click trains contained 36.3 ± 32.5 clicks, lasting for 1.5 ± 1.5 s, with average inter-click intervals (ICIs) of 51.2 ± 38.3 ms. Average ICIs showed a bimodal distribution, with a cut-off at 20 ms. Short-range click trains, with short ICIs of <20 ms on average, were characterized by smaller ASLs, relatively stable ICIs and a shorter click train duration. The mean back-calculated ASL for humpback dolphins with an approximately maximum body size of 2.5 m was 181.7 ± 7.0 dB re 1 μPa at a distance of 1.6–57.2 m. This value was comparable to that recorded for other dolphins of similar body size, although the ASL estimates obtained in this study might be conservative.

Echolocation signals of free-ranging Indo-Pacific humpback dolphins (Sousa chinensis) in Sanniang Bay, China

“…The mean value of the click SL _pp of free-ranging IndoPacific humpback dolphins was 187.7 dB, which is nearly 6 dB higher than the value estimated by Kimura et al (2014) using an A-tag recording system. The difference between the two studies could simply be due to the different recording systems used.…”

“…Although the sounds of Indo-Pacific humpback dolphins have been investigated in a number of studies (Schultz and Corkeron, 1994;Van Parijs and Corkeron, 2001a,b;Goold and Jefferson, 2004;Nanayakkara et al, 2007;Li et al, 2012;Sims et al, 2012;Li et al, 2013;Lin et al, 2013;Wang et al, 2013;Kimura et al, 2014), the present study represents the first documentation of characteristics of the onaxis clicks of Indo-Pacific humpback dolphins in their natural habitat, obtained using a broadband and high-frequency acoustic sampling system. The five-hydrophone array recording system made it possible to determine which clicks were on axis.…”

“…Though the sounds of the Indo-Pacific humpback dolphin have been described in a number of papers (Schultz and Corkeron, 1994;Van Parijs and Corkeron, 2001a,b;Goold and Jefferson, 2004;Nanayakkara et al, 2007;Sims et al, 2012;Lin et al, 2013;Wang et al, 2013;Kimura et al, 2014), most of these studies were performed with sounds recorded either within a 30-kHz frequency range (Schultz and Corkeron, 1994;Van Parijs and Corkeron, 2001a,b;Nanayakkara et al, 2007;Sims et al, 2012;Wang et al, 2013) or by a miniature stereo acoustic event data-logger a) Also at: University of Chinese Academy of Sciences, Beijing, 100039, People's Republic of China. b) Electronic mail: wangd@ihb.ac.cn (A-tag), which only records high-frequency phonating events and peak pressures, but not waveforms of the high-frequency sounds Kimura et al, 2014).…”

“…b) Electronic mail: wangd@ihb.ac.cn (A-tag), which only records high-frequency phonating events and peak pressures, but not waveforms of the high-frequency sounds Kimura et al, 2014). Ultrasound recordings of Indo-Pacific humpback dolphins with frequencies higher than 100 kHz have only been obtained from a single hydrophone system under opportunistic and uncontrolled recording conditions in the field (Goold and Jefferson, 2004) or in rescue scenarios of few stranded individuals in captivity (Li et al, 2012;Li et al, 2013) by high speed digital sampling.…”

Echolocation parameters of Australian humpback dolphins (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the wild

“…$182 6 7 dB re 1 lPa (pp), which is lower than the minimum source levels reported here. However, Kimura et al (2014) had technical limitations including a limited frequency range and low clipping level. This may have resulted in clicks with high source levels being excluded and, conversely, the inclusion of low source level buzz clicks in the analysis.…”