Whistle Characteristics of Indo-Pacific Bottlenose Dolphins (Tursiops

Acoust Aust DOI 10.1007/s40857-015-0041-4 ORIGINAL PAPER Whistle Characteristics of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) in the Fremantle Inner Harbour, Western Australia Rhianne Ward1 · Iain Parnum1 · Christine Erbe1 · Chandra Salgado-Kent1 Received: 15 October 2015 / Accepted: 22 December 2015 © Australian Acoustical Society 2016 Abstract Bottlenose dolphins use whistles to communicate with their conspecifics and maintain group cohesion. We recorded 477 whistles of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the Fremantle Inner Harbour, Western Australia, on nine occasions over a six-week period during May/June 2013. Over half (57%) of the whistles had complex contours exhibiting at least one local extremum, while 32% were straight upsweeps, 5% downsweeps and 6% constant-frequency. About 60% of whistles occurred in trains. Fundamental frequency ranged from 1.1 to 18.4kHz and whistle duration from 0.05 to 1.15s. The maximum numbers of local extrema and inflection points were 7 and 9, respectively. Whistle parameters compared well to those of measurements made from other T. aduncus populations around Australia. Observed differences might be due to ambient noise rather than geographic separation. Keywords Whistle characteristics · Bottlenose dolphin · Tursiops aduncus · Fremantle Inner Harbour 1 Introduction The Fremantle Inner Harbour, located in south-western Australia, is a year-round hotspot for about 45 individuals Bottlenose dolphins (Genus Tursiops) are found globally in from two resident communities of Indo-Pacific bottlenose tropical to temperate and coastal to offshore waters. Around dolphins (T. aduncus) [10–13]: one occurring mainly in Australia, morphological and genetic studies support the Cockburn Sound just west of Fremantle and in the open existence of three distinct species of Tursiops:(1)T. trun- water, and the other in the Swan-Canning River estuary. catus occurring offshore in deep water as well as in coastal The Fremantle Inner Harbour is located at the entrance of areas [1], (2) T. aduncus, a smaller inshore form found only the Swan-Canning River estuary and connects Cockburn in coastal and some estuarine waters of the Indian and West- Sound to the estuary, thus bordering the core home range ern Pacific Ocean [1,2] and (3) T. australis found in the areas of the two communities. Thus, the home ranges of coastal waters of southern Australia [3,4]. Bottlenose dol- some of the individuals from the Cockburn Sound commu- phins can occur in genetically segregated populations or nity overlap with individuals from the Swan-Canning River in sub-populations or communities with overlapping home estuary within the Fremantle Inner Harbour and proximal ranges (i.e. regularly used geographic areas [5]). Site fidelity locations. However, dolphins from the Cockburn Sound com- and home ranges of dolphins are thought to be influenced by munity do not appear to use the middle and upper reaches food availability, predation risk, social parameters within the of the Swan River, while dolphins from the Swan-Canning population and associated habitat preferences (e.g. topogra- River estuary community are regularly sighted many tens phy, ambient noise levels, etc.) [6–9]. of kilometres further up the river [12]. The Fremantle Inner Harbour is also the location of the Port of Freman- B Rhianne Ward tle, which is the largest and the busiest general cargo port [email protected] on the west coast of Australia. As such, the underwater acoustic environment of the Inner Harbour has contribu- 1 Centre for Marine Science and Technology, Curtin University, Bentley, WA 6102, Australia tions from various human activities including shipping, as 123 Acoust Aust well as biological sounds from snapping shrimp, fish and dolphins [14]. Bottlenose dolphins rely heavily on acoustic signals to maintain contact with their conspecifics and to actively sense their environment [15], even in relatively noisy environ- ments such as the Fremantle Inner Harbour. The sound repertoire of bottlenose dolphins is comprised of whistles, clicks, burst pulse sounds and low-frequency, narrow-band harmonic sounds [16–18]. Clicks primarily have a biosonar function and are used for sensing the environment, navigation and foraging [19]. Whistles and burst-pulse sounds serve for communication [20]. Specific vocalisations have been associated with specific behaviours in many dolphin species (e.g. [21–26]). Furthermore, bottlenose dolphins have been shown to develop individually distinctive signature whistles during the first few months of their lives and emit these when in Fig. 1 A satellite image of the study area, Fremantle Inner Harbour, Western Australia. The white X indicates the location where all obser- isolation and when meeting conspecifics as a way of broad- vational and acoustic data were collected. The white dot in the insert of casting identity; they also copy signature whistles of others Western Australia indicates the location of Fremantle. Source Google interpreted as ‘calling others’ (e.g. [27–33]). Earth The vocalisation characteristics described above have been found to vary among populations, possibly due to habi- deployed over the north side of the Port of Fremantle small tat differences, ambient noise, sympatry with other dolphin craft jetty (Fig. 1) at a depth of 1.5m below the surface of the species, etc. (e.g. [34–38]). In this article, the whistle charac- water. On one occasion (23rd May 2013), the hydrophone teristics of Indo-Pacific bottlenose dolphins measured in the was lowered over the west side of the jetty due to a strong Fremantle Inner Harbour are described, compared to mea- current, which could have resulted in flow noise. Record- surements from other populations in Australia, and correlated ings commenced once a group of dolphins was observed in to ambient noise to assess whether whistle features were asso- the study area and ended when dolphins could no longer be ciated with noise energy. sighted. A group of dolphins was defined as any aggregation of dolphins observed in an apparent association, frequently engaged in the same general activity (modified from Shane 2 Methods [39]). Counts of individuals were made several times during recordings and the minimum and best estimate recorded. 2.1 Study Area While groups were often very dynamic consisting of smaller social units such as mother–calf pairs or male alliances, they ◦ Data were collected in the Fremantle Inner Harbour (32 02 were considered to be part of the group if they were observed ◦ 31.23 S, 114 45 10.21 E) located 17km south-west of the in the same aggregation. Individuals within groups were not city of Perth on the west coast of Australia. The Port of Fre- identified, nor were they identified as belonging to either one mantle is situated at the entrance of the Swan River from the of the two communities occurring in the Fremantle Inner Har- Indian Ocean, and serves as a major port for importing and bour (i.e. the Swan-Canning River estuary or the Cockburn exporting products and natural resources in Western Aus- Sound community); therefore, groups may have no, some, tralia. Acoustic recordings were collected from a 50 m long or all common individuals from previous times and days of jetty (Fig. 1). recording. Because of this, we do not relate whistle parameters to individuals, or to the proportion of the total number 2.2 Data Collection of dolphins that use the Fremantle Inner Harbour. Rather, we focus our discussion on parameters of whistles produced Data were collected on nine occasions over a period of six by some of the dolphins occurring in the Fremantle Inner weeks (13th May 2013 to 21st June 2013), using a HTI- Harbour during the periods of recordings. 96-MIN hydrophone with a built-in preamplifier, sensitivity − µ of 163.9 dB re 1 V/ Pa, and a frequency response of 2.3 Data Analysis 2 Hz–30 kHz (±3dB). A Jammin Pro HR-5 recorder sampled sound at 96kHz and stored data as 24-bit WAV files. Prior Acoustic analysis focussed on the identification and descrip- to fieldwork, the recording system was calibrated with white tion of whistles emitted by bottlenose dolphins in Fremantle noise of known level. During recordings, the hydrophone was Inner Harbour. Whistles were identified aurally and visually 123 Acoust Aust fend, fmin and fmax agreed to within 1kHz, and if the order of local and absolute extrema was the same. This 1kHz toler- ance was to allow for the variability in frequency parameters which can be between 6 and 26 % in other T. aduncus populations [40]. The inter-whistle-interval was measured for all similar whistles, as the difference between the end time of one whistle and the start time of the next whistle [41,42]. If whistles were emitted in bouts of at least five whistles of similar shape, where four out of five whistles in the bout had inter-whistle-intervals of 1–10s, this whistle was considered a signature whistle, following the criterion of Gridley et al. [40]. Whistles of similar shape that occurred with inter- whistle-intervals of <1 s were considered to be part of a train. Ambient noise levels in the Inner Harbour were calculated for each recording when dolphins were present and hence Fig. 2 Example spectrogram showing several of the parameters mea- included dolphin vocalisations and clicks. The recordings sured from each whistle. (Color figure online) were Fourier transformed in 1/8s windows, and averaged over 60s. Power spectral density percentiles were computed over all 60s spectra. In addition, octave band levels (0.5– (from spectrograms) using Audition CS6 (Adobe Systems 1, 1–2, 2–4, 4–8, 8–16, and 16–32 kHz) of ambient noise Inc. 2013). To describe the whistles, nine parameters were were calculated for 10s prior to each whistle or whistle train measured from the fundamental contours in the spectro- in order to assess whether ambient noise levels determined grams: (1) start frequency (fstart); (2) end frequency (fend); whistle parameters.

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