Whistle Characteristics Research Articles

Evidence of geographic variation in the non-signature whistle repertoires of two isolated populations of New Zealand common bottlenose dolphin

ABSTRACT Dolphins are social animals that depend on sound to communicate, navigate, and find food. The aim of our research was to describe dolphin whistle characteristics and compare whistles between two isolated populations of bottlenose dolphins in New Zealand. We focused on three aspects of dolphin bioacoustics (1) classification of whistles; (2) comparison of the whistles of two discrete bottlenose dolphin populations; and (3) comparison of whistles of dolphin populations from around the world. At Great Barrier Island, the most common type of whistle was upsweep followed by sine, while at Doubtful Sound, convex and sine whistles were the most common whistle types. Random Forest models discriminated between whistles from the two populations with an estimated accuracy of 90%. The key differences were whistle-type contour, duration, and end frequency. Linear Mixed models show that whistle acoustic parameters were influenced by location but not by group size or the presence of immatures. Whistle parameters of both populations fell within the range of multiple populations from around the world. This studyprovides a benchmark for future research into drivers of geographic variation in dolphin vocal communication.

Vocal signatures affected by population identity and environmental sound levels.

Passive acoustic monitoring has improved our understanding of vocalizing organisms in remote habitats and during all weather conditions. Many vocally active species are highly mobile, and their populations overlap. However, distinct vocalizations allow the tracking and discrimination of individuals or populations. Using signature whistles, the individually distinct calls of bottlenose dolphins, we calculated a minimum abundance of individuals, characterized and compared signature whistles from five locations, and determined reoccurrences of individuals throughout the Mid-Atlantic Bight and Chesapeake Bay, USA. We identified 1,888 signature whistles in which the duration, number of extrema, start, end, and minimum frequencies of signature whistles varied significantly by site. All characteristics of signature whistles were deemed important for determining from which site the whistle originated and due to the distinct signature whistle characteristics and lack of spatial mixing of the dolphins detected at the Offshore site, we suspect that these dolphins are of a different population than those at the Coastal and Bay sites. Signature whistles were also found to be shorter when sound levels were higher. Using only the passively recorded vocalizations of this marine top predator, we obtained information about its population and how it is affected by ambient sound levels, which will increase as offshore wind energy is developed. In this rapidly developing area, these calls offer critical management insights for this protected species.

Raise your pitch! Changes in the acoustic emissions of resident bottlenose dolphins in the proximity of vessels

AbstractMaritime traffic is a major contributor of anthropogenic disturbance for cetaceans, especially for coastal populations, such as that of resident common bottlenose dolphins (Tursiops truncatus) in the Sado estuary (Portugal). Animals have been found to adjust their vocal behavior by changing vocal rates, or call frequency and/or duration, to overcome masking effects of underwater noise. To evaluate the potential impacts of boat traffic on the acoustic behavior of these dolphins, emission rates and acoustic characteristics of whistles and burst‐pulsed signals were analyzed with and without boats operating nearby. In this study, no significant differences were found for emission rates of each type of vocal element in the presence of vessels. However, significant differences were found in acoustic parameters, namely changes in frequency and duration, for whistles and for pulsed sounds (creaks, grunts, squeaks, and gulps). These changes, such as a shift in vocal frequencies and production of shorter signals, may represent behavioral strategies to compensate for the noisy environment. Although resident bottlenose dolphins in the Sado region seem to have developed some tolerance to vessel noise, continuous noise exposure and noise‐induced frequency shifts in vocal outputs could have indirect fitness costs for this population.

Signature whistle use and changes in whistle emission rate in a rehabilitated rough-toothed dolphin

Acoustic signals play a crucial role in communication among animals, particularly in dolphins. Signature whistles, one of their most extensively studied vocalizations, enable dolphins to convey their identity to conspecifics through individually distinct whistle contours. However, it remains unclear whether rough-toothed dolphins (Steno bredanensis) also produce signature whistles with individually identifying contours and, if so, whether they are associated with stress and poor health, such as in bottlenose dolphins. To bridge this knowledge gap, we recorded sounds emitted by a live-stranded rough-toothed dolphin during its rehabilitation in May 2017 at Isla Mujeres, Quintana Roo, Mexico. We assessed if the dolphin produced a signature whistle and whether whistle rate, inter-whistle interval, mean low and high frequencies, and blood chemistry measures, changed significantly over time. While isolated from conspecifics during rehabilitation, the dolphin generated a single, repeated, and stereotyped whistle contour that met the previously established SIGnature IDentification criteria for signature whistle emissions for bottlenose dolphins. Whistle characteristics varied over the 11 recording days: whistle rate and inter-whistle interval significantly decreased over time; the number of whistles with preceding echolocation click trains decreased over time; and mean low and high frequencies changed over recording days. We conclude that this rough-toothed dolphin possessed what resembles a signature whistle contour, and the emission of this contour underwent significant changes throughout the rehabilitation process. While our study presents evidence of a single rough-toothed dolphin producing a signature whistle, further research is necessary to determine whether this vocal behavior is prevalent across the species.

Short-beaked common dolphins of Dilek Peninsula National Park, South Aegean Sea: Preliminary results on group structure, spatial distribution, behavioural patterns, whistle characteristics and behavioural context

Basic knowledge about populations of short‐beaked common dolphins (Delphinus delphis) in the eastern part of the Mediterranean Sea is still lacking. Classified as ‘Endangered’ in an IUCN assessment released in 2022, it is necessary to gain baseline knowledge to understand the species’ population status and develop impactful conservation measures. The current study conducted boat surveys, employing visual and acoustic techniques between January 2019 and July 2022 in the Dilek Peninsula National Park, central Aegean Sea. Results revealed the continuous coastal presence of common dolphins, with subadults recorded during every sighting in low group sizes. Travelling‐related activities made up 58% of observed behaviour while resting comprised just 1%. Whistles were produced in the range of 1.83–48kHz with a peak frequency of 10.4kHz and median duration of 0.54s. Concave whistles were the most frequently recorded whistle type. Whistles were found to be generated when dolphins were interacting with boats and diving/travelling fast, forming 53% and 31% of whistle production respectively. The continuous presence of common dolphins, particularly subadults, emphasises the importance of the Dilek Peninsula. Whistle parameters show similarities with other common dolphin populations of the Mediterranean and Black Sea, albeit with shorter durations. The high whistle presence during boat interactions requires investigation to assess whether whistles carry avoidance or predatory messages, or whether there are potential consequences for the species’ energy budget. The current study presents preliminary information on the behavioural context of acoustic patterns. Further research is essential to understand the click and whistle characteristics within the biological, environmental and anthropogenic variables of their surroundings.

Evidence of signature whistles produced by Indian Ocean bottlenose dolphins (Tursiops aduncus) in Mozambique

ABSTRACT Individually distinctive signature whistles are used by common bottlenose dolphins (Tursiops truncatus) during social interactions and to facilitate group cohesion. There is evidence from a few populations that Indian Ocean bottlenose dolphins (T. aduncus) also use signature whistles. We investigated this from a coastal resident population of T. aduncus in Mozambique. Video-audio data were collected during recreational swim-with activities over 12 years, where potential signature whistles were identified from 28 individuals. Of the 405 whistles documented, 75% were produced in SIGID bouts. Visual assessment of individual signature whistle contours demonstrated temporal stability for up to 8 years. Overall, most signature whistle types had upsweep frequency modulation and were emitted as multiloop whistles. Comparing all whistle contours to each other indicated low discrimination between individuals, with contours frequently categorised together. However, sex differences in the frequency characteristics of whistles were identified, with females whistling at lower frequencies than males. Our results indicate either a shared whistle repertoire or identity encoding with subtle contour features, requiring acute auditory perception and discrimination to decipher caller identity. More widespread geographic investigation into signature whistle use may demonstrate variation in acoustic communication systems for bottlenose dolphins, which are thus far not well understood.

Initial results on the variation of whistle characteristics of bottlenose dolphins from two neighbouring regions of the Mediterranean Sea: Northern Ionian and Southern Adriatic Sea

Bottlenose dolphins have a complex vocal repertoire that varies depending on behavioral context, social structure, group composition, and anthropogenic pressures. This current study describes the whistle characteristics of bottlenose dolphins for the first time in the South Adriatic Sea while assessing the potential differences between whistle characteristics of geographically separated dolphins within neighbouring waters of the North Ionian Sea. The results show that whistle characteristics were similar between Taranto Gulf (Italy) and Boka Bay (Montenegro), despite their spatial differences. The mean peak frequency was 10kHz for each study location while the mean minimum and maximum frequency ranged from 7 to 14kHz. The average duration of whistles was 500 milliseconds. These results share similarities with previous literature, although several studies reported slightly different mean peak frequencies, ranging up to 15kHz in the neighbouring waters of Croatia and Italy. Further, harmonics were produced and formed in 40% of the whistles in Taranto Gulf and 30% of the whistles in Boka Bay. A high incidence of harmonics has previously been associated with behavioral states (i.e., travelling) and with certain types of marine traffic (i.e., fishing vessels). Therefore, it is important to collect simultaneous data on the visual behavior of the focal group as well as document the type and density of marine traffic within the proximity of the dolphins to have an in-depth understanding of vocal behavior. Despite the similarities of whistle characteristics of Taranto and Boka Bay, the whistle contours showed notable variations. Upsweep whistles were the most regularly produced whistle type in each location, which coincides with previous studies in the Mediterranean Sea. However, the least produced whistle had a concave contour in Taranto and was flat in Boka Bay. Previous studies have confirmed that flat whistles account for the least produced whistle contour in the Mediterranean Basin. Examining the whistle characteristics and the variation in whistle contours provides an in-depth understanding of the behavioral complexity as well as its plasticity in the presence of pressure. Therefore, future studies need to include behavior, group composition, noise levels, and human presence to enable an effective understanding of variation in whistle characteristics of bottlenose dolphins.

Machine learning models can predict bottlenose dolphin health status from whistle recordings

Recently, there have been incredible strides in applying machine and deep learning methods to predict the presence of disease states in humans from recordings of voice. Pattern analyses of dolphin whistles have historically depended on human operators visually and audibly differentiating between sound types. The training and implementation of machine and deep learning strategies for audio analyses reduce human bias and may improve feature detection. Our team at the Navy Marine Mammal Program in San Diego, CA has developed a substantial vocal catalog of whistle recordings for a group of focal dolphins. Additionally, an extensive health history database for each animal is maintained as part of a preventive medicine program. Together, these result in a unique, labeled dataset comprised of tens of thousands of whistles (input) emitted during differing health states (output) that have been leveraged for training machine and deep learning models to classify health status from whistles. We present preliminary results that suggest that health information may be encoded across dolphin whistle characteristics, similar to changes in the human voice. Further, we describe the applied goals for testing and implementing these innovative tools for early predicting changes in dolphin health status from non-invasive recordings.

Probable signature whistle production in Atlantic white‐sided (Lagenorhynchus acutus) and short‐beaked common (Delphinus delphis) dolphins near Cape Cod, Massachusetts

Probable signature whistle production in Atlantic white‐sided (<i>Lagenorhynchus acutus</i>) and short‐beaked common (<i>Delphinus delphis</i>) dolphins near Cape Cod, Massachusetts

Vocal correlates of arousal in bottlenose dolphins (Tursiops spp.) in human care.

Human-controlled regimes can entrain behavioural responses and may impact animal welfare. Therefore, understanding the influence of schedules on animal behaviour can be a valuable tool to improve welfare, however information on behaviour overnight and in the absence of husbandry staff remains rare. Bottlenose dolphins (Tursiops spp.) are highly social marine mammals and the most common cetacean found in captivity. They communicate using frequency modulated signature whistles, a whistle type that is individually distinctive and used as a contact call. We investigated the vocalisations of ten dolphins housed in three social groups at uShaka Sea World dolphinarium to determine how patterns in acoustic behaviour link to dolphinarium routines. Investigation focused on overnight behaviour, housing decisions, weekly patterns, and transitional periods between the presence and absence of husbandry staff. Recordings were made from 17h00 – 07h00 over 24 nights, spanning May to August 2018. Whistle (including signature whistle) presence and production rate decreased soon after husbandry staff left the facility, was low over night, and increased upon staff arrival. Results indicated elevated arousal states particularly associated with the morning feeding regime. Housing in the pool configuration that allowed observation of staff activities from all social groups was characterised by an increase in whistle presence and rates. Heightened arousal associated with staff presence was reflected in the structural characteristics of signature whistles, particularly maximum frequency, frequency range and number of whistle loops. We identified individual differences in both production rate and the structural modification of signature whistles under different contexts. Overall, these results revealed a link between scheduled activity and associated behavioural responses, which can be used as a baseline for future welfare monitoring where changes from normal behaviour may reflect shifts in welfare state.

Acoustic Characteristics of Bubblestream-Associated Whistles Produced by Atlantic Bottlenose Dolphins (Tursiops truncatus) During the First Thirty Days of Life

Acoustic Characteristics of Bubblestream-Associated Whistles Produced by Atlantic Bottlenose Dolphins (Tursiops truncatus) During the First Thirty Days of Life

Whistle characteristics of a newly recorded Indo‐Pacific humpback dolphin (Sousa chinensis) population in waters southwest of Hainan Island, China, differ from other humpback dolphin populations

AbstractIndo‐Pacific humpback dolphins (Sousa chinensis) use whistles to communicate with their conspecifics. Little is known about the acoustic repertoire of Indo‐Pacific humpback dolphins in waters southwest of Hainan Island, a newly recorded population in 2014. In this study, whistles of Hainan humpback dolphin population were collected by using autonomous acoustic recorders. The fundamental frequencies and durations of whistles were in ranges of 0.71–21.35 kHz and 0.06–2.22 s, respectively. Significant intraspecific differences in duration and frequency of whistles were found between the Hainan population and the other geographically neighboring populations (in Chinese waters) or the population in Malaysia waters. Compared with other Sousa species, significant interspecific differences were also observed. Based on clustering analysis, the whistle parameters of neighboring populations were likely similar to each other. Significant differences were found between humpback dolphins in waters southwest of Hainan Island and those dolphins in the neighboring areas, supporting the hypothesis that this population may be independent. Ambient noise measurements in waters of Hainan Island, Zhanjiang, and Sanniang Bay showed that humpback dolphin populations may use whistles with longer duration, lower frequency, and fewer inflection points for more effective communication to adapt to a noisier environment.

Intraspecific variation in short-beaked common dolphin’s whistle repertoire

ABSTRACT Dolphins’ whistles are strongly diversified mainly related to physiology and behaviour. We present the whistle characteristics and intraspecific variations of short-beaked common dolphins Delphinus delphis of the Western South Atlantic Ocean (WSAO). A total of 1630 whistles from six encounters were analysed considering group size, contours and acoustical parameters. No significant correlation was found (p > 0.05) between number of whistles and the group size. Upsweep, downsweep and constant were the most frequent contours. A positive relation between the downsweep contour and group size was detected (p < 0.05). Minimum, maximum, centre, peak, initial and final frequencies, frequency range, frequency gradient, duration and inflection points were extracted. The frequencies ranged from 1.34 to 35.23 kHz and the duration varied from 0.11 to 2.16 seconds. Comparing encounters between years through Bonferroni method, minimum frequency, duration, and frequency range were significantly different. C5.0 decision tree showed that whistles of 2014 and 2015 had correct classifications (67.0% and 54.0%, respectively). The nMDS grouped the Atlantic and Pacific oceans based on inflection points and duration, further corroborated through PERMANOVA (p < 0.05). Our results suggest the influence of certain parameters related to intraspecific variation. Whistles comparison with other localities corroborates with differences in geographical scale.

Whistle variation in Mediterranean common bottlenose dolphin: The role of geographical, anthropogenic, social, and behavioral factors

The studies on the variation of acoustic communication in different species have provided insight that genetics, geographic isolation, and adaptation to ecological and social conditions play important roles in the variability of acoustic signals. The dolphin whistles are communication signals that can vary significantly among and within populations. Although it is known that they are influenced by different environmental and social variables, the factors influencing the variation between populations have received scant attention. In the present study, we investigated the factors associated with the acoustic variability in the whistles of common bottlenose dolphin (Tursiops truncatus), inhabiting two Mediterranean areas (Sardinia and Croatia). We explored which factors, among (a) geographical isolation of populations, (b) different environments in terms of noise and boat presence, and (c) social factors (including group size, behavior, and presence of calves), were associated with whistle characteristics. We first applied a principal component analysis to reduce the number of collinear whistle frequency and temporal characteristics and then generalized linear mixed models on the first two principal components. The study revealed that both geographic distance/isolation and local environment are associated with whistle variations between localities. The prominent differences in the acoustic environments between the two areas, which contributed to the acoustic variability in the first principal component (PC1), were found. The calf's presence and foraging and social behavior were also found to be associated with dolphin whistle variation. The second principal component (PC2) was associated only with locality and group size, showing that longer and more complex tonal sound may facilitate individual recognition and cohesion in social groups. Thus, both social and behavioral context influenced significantly the structure of whistles, and they should be considered when investigating acoustic variability among distant dolphin populations to avoid confounding factors.

Whistle variability of the Mediterranean short beak common dolphin

Abstract The short‐beaked common dolphin is a highly vocal species, with a wide distribution in all oceans, including the Mediterranean and the Black Seas. In the Mediterranean Sea, the short‐beaked common dolphin inhabits both pelagic and neritic waters. Osteological collections and the literature show that short‐beaked common dolphins were widespread and abundant in much of the Mediterranean Sea until the late 1960s. During recent decades the species has declined in the whole basin, and, in 2003, it was listed as Endangered in the IUCN Red List. Genetic studies strongly suggest that the Mediterranean and the Eastern North Atlantic populations are isolated from each other. Genetic differentiation within the Mediterranean Sea, between the Eastern Mediterranean (Ionian Sea) and Western Mediterranean populations, is also reported. The aim of this study was to investigate the geographical variation in the characteristics of whistles of free‐ranging short‐beaked common dolphins living in the Mediterranean Sea, and to evaluate if whistle acoustic structure is the result of adaptation to local environment characteristics or of a possible genetic diversification. Recordings were collected from 1994 to 2012 throughout the basin, employing multiple platforms. Twenty‐six independent acoustic detections were made, and 704 whistles were extracted and considered for statistical analysis. Whistle analysis enabled the identification of distinct geographical units of short‐beaked common dolphin within the Mediterranean Sea. Genetic isolation is probably the major cause of the geographic variance of the Mediterranean short‐beaked common dolphin whistle structure, which may reflect some evolutionary adaptations to particular ecological conditions or may be the by‐product of morphological evolution. The results of the present study show that intra‐Mediterranean variability of whistle structure reflects the path of genetic studies, highlighting the possible use of acoustic data in combination with other sources of data (genetic, morphological, etc.) to identify geographic areas where discrete management units occur.

Whistle Classification of Sympatric False Killer Whale Populations in Hawaiian Waters Yields Low Accuracy Rates

Cetaceans are ecologically important marine predators, and designating individuals to distinct populations can be challenging. Passive acoustic monitoring provides an approach to classify cetaceans to populations using their vocalizations. In the Hawaiian Archipelago, three genetically distinct, sympatric false killer whale (Pseudorca crassidens) populations coexist: a broadly distributed pelagic population and two island-associated populations, an endangered main Hawaiian Islands (MHI) population and a Northwestern Hawaiian Islands (NWHI) population. The mechanisms that sustain the genetic separation between these overlapping populations are unknown but previous studies suggest that the acoustic diversity between populations may correspond to genetic differences. Here, we investigated whether false killer whale whistles could be correctly classified to population based on their characteristics to serve as a method of identifying populations when genetic or photographic-identification data are unavailable. Acoustic data were collected during line-transect surveys using towed hydrophone arrays. We measured 50 time and frequency parameters from whistles in 16 false killer whale encounters identified to population and used those measures to train and test random forest classification models. Random Forest models that included three populations correctly classified 42% of individual whistles overall and resulted in a low kappa coefficient, κ = 0.15, indicating low agreement between models and the true population. Whistles from the MHI population showed the highest correct classification rate (52%) compared to pelagic and NWHI whistles (42% and 36%, respectively). Pairwise random forest models classifying pelagic and MHI whistles proved slightly more accurate (62% accuracy, κ = 0.24), though a similar pelagic-NWHI model did not (56% accuracy, κ = 0.12). Results suggest that the time-frequency whistle characteristics are not suitable to confidently classify encounters to a specific false killer whale population, although certain features of whistles produced by the endangered MHI population allow for overall higher classification accuracy. Inclusion of other vocalization types, such as echolocation clicks, and alternative whistle variables may improve correct classification success for these sympatric populations.

Characteristics and microgeographic variation of whistles from the vocal repertoire of beluga whales (Delphinapterus leucas) from the White Sea.

The beluga whale (Delphinapterus leucas) produces a variety of sounds, including whistles as well as pulsed, noisy, and biphonic vocalizations. This study presents the fine-scale, microgeographic characteristics of beluga whistles from data collected in four locations across Onega Bay and Dvina Bay in the White Sea, Russia. Ten parameters were measured from 1232 whistles. The whistles had a fundamental frequency from 0.4 to 25.0 kHz and duration from 0.04 to 3.93 s. Although two distinct types could be recognized in the White Sea beluga's whistle repertoire, other whistles formed a graded continuum. Among them, "flat" whistle contours with no inflection points appear to be the most common (39.7%), to be followed by frequency-modulated whistles: ascending (27.1%) and descending (15.6%). Non-linear phenomena detected in the whistles included frequency jumps (23.1%), biphonations (13.2%), sidebands (5.2%), and subharmonics (0.5%). The whistles varied in frequency parameters and duration across the locations, while those recorded in the same location in different years showed minimal difference. Beluga whistles appear to be an extremely diverse class of vocalizations. This, together with the lack of clear correlations between the behavior of whales and whistle production suggests whistles may perform multiple functions within the beluga whale communication system.

Spatio-temporal patterns in underwater sound within an urban estuarine river

Underwater noise environments are increasingly being considered in marine spatial planning and habitat quality assessments, particularly with regard to acoustically specialised fauna. The Swan River in Western Australia flows through the state capital of Perth and consequently experiences a range of anthropogenic activities. However, the river is also extensively used by a resident community of bottlenose dolphins (Tursiops aduncus). This study aimed to describe underwater sound sources within the Swan River, examine spatial and temporal soundscape variability, and determine dolphin responses to noisy environments. Acoustic datasets collected from 2005 to 2015 indicated that the Swan River was comprised of multiple acoustic habitats, each with its own characteristic soundscape and temporal patterns in underwater noise. The anthropogenically “noisiest” site was the Fremantle Inner Harbour (mean broadband noise level: 106 dB re 1 μPa rms [10 Hz–11 kHz]); yet dolphins remained present in this area even at high vessel densities. However, fine-scale analyses indicated significant alterations to dolphin behavior at high vessel densities and to dolphin whistle characteristics in high broadband noise conditions. These results highlight the need to consider spatial and temporal patterns when assessing the composition of underwater soundscapes, and identify potential responses of coastal dolphins to busy, noisy environments.Underwater noise environments are increasingly being considered in marine spatial planning and habitat quality assessments, particularly with regard to acoustically specialised fauna. The Swan River in Western Australia flows through the state capital of Perth and consequently experiences a range of anthropogenic activities. However, the river is also extensively used by a resident community of bottlenose dolphins (Tursiops aduncus). This study aimed to describe underwater sound sources within the Swan River, examine spatial and temporal soundscape variability, and determine dolphin responses to noisy environments. Acoustic datasets collected from 2005 to 2015 indicated that the Swan River was comprised of multiple acoustic habitats, each with its own characteristic soundscape and temporal patterns in underwater noise. The anthropogenically “noisiest” site was the Fremantle Inner Harbour (mean broadband noise level: 106 dB re 1 μPa rms [10 Hz–11 kHz]); yet dolphins remained present in this area even at hi...

Description and classification of Indian Ocean humpback dolphin (Sousa plumbea) whistles recorded off the Sindhudurg coast of Maharashtra, India

AbstractThe Indian Ocean humpback dolphin (Sousa plumbea), a common cetacean species in India, has a diverse vocal repertoire, which to date has not been described in detail. This study focused on analyzing their whistle vocalizations. Humpback dolphins were recorded off the Sindhudurg coast of Maharashtra, India, and 2,260 whistles were analyzed for their acoustic characteristics. Whistles spanned a wide frequency range between 2.3 kHz and 33.0 kHz, with durations ranging from 0.01 s to 1.60 s. Whistles were categorized into seven contour classes based on their qualitative properties. A classification and regression tree (CART) analysis was used to quantify variability between the seven contour classes using the measured acoustic features. Based on the CART analysis, frequency gradient, minimum slope, maximum slope, beginning and ending slope accounted for most variability between whistle types. CART resulted in an overall classification accuracy of 89.5%. This study provides a detailed description of acoustic features and qualitative properties of humpback dolphin whistles from the northwestern coast of India. Further comparisons of acoustic data from Sousa populations along the Indian coast are necessary to determine possible geographic variations in whistle characteristics and whether the variations are driven by environmental or genetic factors or a combination of both.

Effects of vessel traffic and underwater noise on the movement, behaviour and vocalisations of bottlenose dolphins in an urbanised estuary

AbstarctThe potential disturbance of dolphins from tourism boats has been widely discussed in the literature, in terms of both physical vessel presence and associated underwater noise. However, less attention has been paid to the potential impact of non-tourism vessels, despite these being much more widespread and occurring in greater numbers throughout coastal dolphin habitats. The Indo-Pacific bottlenose dolphin (T. aduncus) community using the Fremantle Inner Harbour, Western Australia, is exposed to high levels of vessel traffic. To investigate whether behavioural responses could be occurring, a non-invasive combination of visual and acoustic monitoring was conducted using a theodolite and an autonomous acoustic logger. Dolphins significantly increased their average movement speeds in high vessel densities, but only for some activity states. Behavioural budgets also changed in the presence of vessels, with animals spending greater time travelling and less time resting or socialising. Finally, multiple whistle characteristics varied with rising levels of broadband noise, and other contextual variables. Despite being acoustically specialised for higher frequencies, dolphins had the strongest acoustic variation during low-frequency noise. This study highlights the complexity of disturbance responses in this species, confirming the need for consideration of both surface and acoustic behaviour alongside appropriate contextual data.