Description and seasonal detection of two potential whale calls recorded in the Indian Ocean

Distribution and Seasonality of the Omura's Whale (Balaenoptera omurai) in Australia Based on Passive Acoustic Recordings

The Omura's whale (Balaenoptera omurai) is one of the most recently described species of baleen whale. Initially known only from stranding and whaling specimens, it has now been identified in all ocean basins excluding the central and eastern Pacific. Unlike most baleen whales that migrate between the poles and the equator seasonally, the Omura's whale is known to inhabit tropical to sub-tropical waters year-round. In Australian waters, there remain fewer than 30 confirmed visual sightings over the past decade. However, based on acoustic records, the Omura's whale has been detected off areas of the northwest coast of Australia year-round. This study utilises passive acoustic recordings from 41 locations around Australia from 2005 to 2023 to assess the distribution and seasonality of the Omura's whale. The seasonal presence of Omura's whale vocalisations varied by location, with higher presence at lower latitudes. Vocalisations were detected year-round in the Joseph Bonaparte Gulf in the Timor Sea, and near Browse Island and Scott Reef, in the Kimberley region. In the Pilbara region, acoustic presence mostly peaked from February to April and no acoustic presence was consistently observed from July to September across all sites. The most southerly occurrence of Omura's whale vocalisations was recorded off the North West Cape in the Gascoyne region. Vocalisations similar but not identical to those of the Omura's whale were detected in the Great Barrier Reef. The identified seasonal distribution provides valuable information to assess environmental and anthropogenic pressures on the Omura's whale and to aid in creating management and conservation policies for the species in Australia.

BASSA: New software tool reveals hidden details in visualisation of low-frequency animal sounds

The study of animal sounds in biology and ecology relies heavily upon time-frequency (TF) visualisation, most commonly using the short-time Fourier transform (STFT) spectrogram. This method, however, has inherent bias towards either temporal or spectral details that can lead to misinterpretation of complex animal sounds. An ideal TF visualisation should accurately convey the structure of the sound in terms of both frequency and time, however, the STFT often cannot meet this requirement. We evaluate the accuracy of four TF visualisation methods (superlet transform [SLT], continuous wavelet transform [CWT] and two STFTs) using a synthetic test signal. We then apply these methods to visualise sounds of the Chagos blue whale, Asian elephant, southern cassowary, eastern whipbird, mulloway fish and the American crocodile. We show that the SLT visualises the test signal with 18.48%-28.08% less error than the other methods. A comparison between our visualisations of animal sounds and their literature descriptions indicates that the STFT's bias may have caused misinterpretations in describing pygmy blue whale songs and elephant rumbles. We suggest that use of the SLT to visualise low-frequency animal sounds may prevent such misinterpretations. Finally, we employ the SLT to develop 'BASSA', an open-source, GUI software application that offers a no-code, user-friendly tool for analysing short-duration recordings of low-frequency animal sounds for the Windows platform. The SLT visualises low-frequency animal sounds with improved accuracy, in a user-friendly format, minimising the risk of misinterpretation while requiring less technical expertise than the STFT. Using this method could propel advances in acoustics-driven studies of animal communication, vocal production methods, phonation and species identification.

Hydroacoustic study of fin whales around the Southern Wake Island: Type, vocal behavior, and temporal evolution from 2010 to 2022

The progress of fin whale study is hindered by the debate about whether the two typical type-A and type-B calls (characterized by central source frequencies of 17-20 Hz and 20-30 Hz, respectively) originate from a single fin whale or two individual fin whales. Here, hydroacoustic data is employed to study the type, vocal behavior, and temporal evolution of fin whale calls around the Southern Wake Island from 2010 to 2022. It is identified that (1) type-A and type-B calls come from two individuals based on the large source separation of the two calls through high-precision determination of source location; (2) type-A fin whales exhibit vocal influence on type-B fin whales, where type-B fin whales become paired with type-A calls and vocalize regularly when type-A fin whales appear, and type-A fin whales always lead the call sequences; and (3) some type-A fin whales stop calling when another type-A fin whale approaches at a distance of about 1.6 km. During 2010-2022, type-A calls occur every year, whereas type-B calls are prevalent only after November 2018. A culture transmission is proposed from type-A fin whales to type-B fin whales and/or a population increase of type-B fin whales in the region after November 2018.

Natural and Anthropogenic Sources of Seismic, Hydroacoustic, and Infrasonic Waves: Waveforms and Spectral Characteristics (and Their Applicability for Sensor Calibration)

The record of seismic, hydroacoustic, and infrasonic waves is essential to detect, identify, and localize sources of both natural and anthropogenic origin. To guarantee traceability and inter-station comparability, as well as an estimation of the measurement uncertainties leading to a better monitoring of natural disasters and environmental aspects, suitable measurement standards and reliable calibration procedures of sensors, especially in the low-frequency range down to 0.01 Hz, are required. Most of all with regard to the design goal of the Comprehensive Nuclear-Test-Ban Treaty Organisation's International Monitoring System, which requires the stations to be operational nearly 100% of the time, the on-site calibration during operation is of special importance. The purpose of this paper is to identify suitable excitation sources and elaborate necessary requirements for on-site calibrations. We give an extensive literature review of a large variety of anthropogenic and natural sources of seismic, hydroacoustic, and infrasonic waves, describe their most prominent features regarding signal and spectral characteristics, explicitly highlight some source examples, and evaluate the reviewed sources with respect to requirements for on-site calibrations such as frequency bandwidth, signal properties as well as the applicability in terms of cost-benefit. According to our assessment, earthquakes stand out across all three waveform technologies as a good natural excitation signal meeting the majority of the requirements. Furthermore, microseisms and microbaroms allow a calibration at very low frequencies. We also find that in each waveform technique man-made controlled sources such as drop weights or air guns are in good agreement with the required properties, although limitations may arise regarding the practicability. Using these sources, procedures will be established allowing calibration without record interrupting, thereby improving data quality and the identification of treaty-related events.

Multiple pygmy blue whale acoustic populations in the Indian Ocean: whale song identifies a possible new population

Blue whales were brought to the edge of extinction by commercial whaling in the twentieth century and their recovery rate in the Southern Hemisphere has been slow; they remain endangered. Blue whales, although the largest animals on Earth, are difficult to study in the Southern Hemisphere, thus their population structure, distribution and migration remain poorly known. Fortunately, blue whales produce powerful and stereotyped songs, which prove an effective clue for monitoring their different ‘acoustic populations.’ The DGD-Chagos song has been previously reported in the central Indian Ocean. A comparison of this song with the pygmy blue and Omura’s whale songs shows that the Chagos song are likely produced by a distinct previously unknown pygmy blue whale population. These songs are a large part of the underwater soundscape in the tropical Indian Ocean and have been so for nearly two decades. Seasonal differences in song detections among our six recording sites suggest that the Chagos whales migrate from the eastern to western central Indian Ocean, around the Chagos Archipelago, then further east, up to the north of Western Australia, and possibly further north, as far as Sri Lanka. The Indian Ocean holds a greater diversity of blue whale populations than thought previously.

Long-term frequency changes of a potential baleen whale call from the central Indian Ocean during 2002-2019

The multiple baleen whales of the central Indian Ocean use distinct calls, with their acoustic signatures marking their respective geographic distributions. This paper uses observations from Diego Garcia to track long-term changes (2002-2019) in calls produced by an unidentified whale. The calls around 20-45 Hz consist of closely spaced frequency tones that resemble a comb, followed by a downsweep. The observations show that while the average comb frequencies steadily increase, the downsweep portion decreases. Some frequencies disappear, while new ones appear. These frequency-observations are different from similar studies of other baleen whales in the region, which mostly show a decrease.

Occurrence of Omura’s whale, Balaenoptera omurai (Cetacea: Balaenopteridae), in the Equatorial Atlantic Ocean based on Passive Acoustic Monitoring

The current known distribution of Omura’s whale includes the tropical and warm temperate waters of the western Pacific, Indian, and Atlantic Oceans. Evidence of their presence in the Atlantic Ocean is based on beach cast specimens found on the coasts of Mauritania (North Atlantic) and Northeastern Brazil (South Atlantic). The present study characterizes the occurrence of this species in the São Pedro and São Paulo Archipelago (SPSPA), on the mid-Atlantic ridge between South America and Africa, based on autonomous recording systems. Acoustic signals were similar, but not identical, to B. omurai vocalizations recorded off the coast of Madagascar. Although these signals were recorded for only 11 months, there are peaks in vocal activity between May and June in the vicinities of SPSPA, suggesting either a shift in distribution within the Atlantic equatorial waters or seasonality in the species’ vocal behavior in this region. The first acoustic records of Omura’s whales in the Equatorial Atlantic suggest that these animals may also use deep-water habitats, in addition to the shallow-water habitat use observed in other regions.

Identification of two potential whale calls in the southern Indian Ocean, and their geographic and seasonal occurrence

Since passive acoustic monitoring is widely used, unidentified acoustic signals from marine mammals are commonly reported. The signal characteristics and emission patterns are the main clues to identify the possible sources. In this study, the authors describe two previously unidentified sounds, recorded at up to five widely-spaced sites (30 × 30 degree area) in the southern Indian Ocean, in 2007 and between 2010 and 2015. The first reported signal (M-call) consists of a single tonal unit near 22 Hz and lasting about 10 s, repeated with an interval longer than 2 min. This signal is only detected in 2007. The second signal (P-call) is also a tonal unit of 10 s, repeated every 160 s, but at a frequency near 27 Hz. Its yearly number increased greatly between 2007 and 2010, and moderately since then. Based on their characteristics and seasonal patterns, this study shows that both signals are clearly distinct from any known calls of blue whale subspecies and populations dwelling in the southern Indian Ocean. However, they display similarities with blue whale vocalizations. More particularly, the P-call can be mistaken for the first tonal unit of the Antarctic blue whale Z-call.

Observations and Bayesian location methodology of transient acoustic signals (likely blue whales) in the Indian Ocean, using a hydrophone triplet

A notable sequence of calls was encountered, spanning several days in January 2003, in the central part of the Indian Ocean on a hydrophone triplet recording acoustic data at a 250 Hz sampling rate. This paper presents signal processing methods applied to the waveform data to detect, group, extract amplitude and bearing estimates for the recorded signals. An approximate location for the source of the sequence of calls is inferred from extracting the features from the waveform. As the source approaches the hydrophone triplet, the source level (SL) of the calls is estimated at 187 ± 6 dB re: 1 μPa-1 m in the 15-60 Hz frequency range. The calls are attributed to a subgroup of blue whales, Balaenoptera musculus, with a characteristic acoustic signature. A Bayesian location method using probabilistic models for bearing and amplitude is demonstrated on the calls sequence. The method is applied to the case of detection at a single triad of hydrophones and results in a probability distribution map for the origin of the calls. It can be extended to detections at multiple triads and because of the Bayesian formulation, additional modeling complexity can be built-in as needed.