# High-resolution acoustic recordings of wild free-ranging short-beaked common dolphins for etho-acoustical and repertoire studies

Loïc Lehnhoff, Hervé Glotin, Yves Le Gall, Eric Menut, Hélène Peltier, Jérôme Spitz, Olivier Van Canneyt, Bastien Mérigot

► To cite this version:

Loïc Lehnhoff, Hervé Glotin, Yves Le Gall, Eric Menut, Hélène Peltier, et al.. High-resolution acous- tic recordings of wild free-ranging short-beaked common dolphins for etho-acoustical and repertoire studies. Earth System Science Data, 2025, 17 (9), pp.4495-4511. 10.5194/essd-17-4495-2025 . hal- 05300981

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# High-resolution acoustic recordings of wild free-ranging short-beaked common dolphins for etho-acoustical and repertoire studies

Loïc Lehnhoff<sup>1,2,3,★</sup>, Hervé Glotin<sup>2,3,★</sup>, Yves Le Gall<sup>4</sup>, Eric Menut<sup>4</sup>, Hélène Peltier<sup>5</sup>, Jérôme Spitz<sup>5</sup>, Olivier Van Canneyt<sup>5</sup>, and Bastien Mérigot<sup>1,3,★</sup>

**Correspondence:** Loïc Lehnhoff (loic.lehnhoff@gmail.com)

Received: 3 April 2025 – Discussion started: 11 April 2025

Revised: 18 June 2025 – Accepted: 22 June 2025 – Published: 10 September 2025

**Abstract.** Dolphins are highly vocal cetaceans with a complex acoustic repertoire. These marine mammals rely heavily on sound for critical activities: echolocation clicks for navigation and prey detection, whistles for social communication, and pulsed sounds for less well-documented purposes. Understanding their acoustic behaviour is essential for insights into their ecology, social structure, and responses to anthropogenic noise. However, to date, there has been a lack of open-access datasets of acoustic recordings of wild free-ranging short-beaked common dolphins (*Delphinus delphis*) coupled with observational data. Here, we present a new dataset (https://doi.org/10.5281/zenodo.14637674, Lehnhoff, 2025a) of high-resolution acoustic recordings of (*D. delphis*) observed in various behavioural states, including foraging, travelling, socializing, milling, and attraction to the boat. The dataset was collected in the northern Bay of Biscay, France, in the summers of 2020 to 2022 during surveys conducted as part of the DOLPHINFREE project. Audio recordings were made during opportunistic encounters using two devices: a single high-quality hydrophone (sampling rate 512 kHz and bit depth 32 bits) and a compact array of four hydrophones (256 to 512 kHz and 16 to 24 bits) for localization purposes. The dataset comprises over 400 min of unedited audio recordings of *D. delphis* accompanied by visual observations. In total, we identified about 68 000 echolocation clicks, 4600 whistle contours, and more than 350 pulsed sounds. This comprehensive resource is invaluable for detailed studies of the acoustic repertoire of (*D. delphis*), coupled with behavioural studies or analyses of the directionality of their acoustic emissions.

## 1 Introduction

This data descriptor makes available a bio-acoustic dataset of audio recordings of wild free-ranging short-beaked common dolphins (*Delphinus delphis*) collected at sea. These recordings are accompanied by visual observations, providing a comprehensive contextual framework for each file. Comparable audio datasets are available for other delphinid species

(e.g. Di Nardo et al., 2023). However, to date, there has been a lack of open-access datasets of acoustic recordings of *D. delphis*.

Short-beaked common dolphins emit sounds that are similar to those of other delphinid species. These acoustic signals can usually be divided into three main categories: echolocation clicks, whistles, and rapid sequences of clicks (or pulsed

<footnote n="1"><sup>1</sup>MARBEC, Univ. Montpellier, CNRS, IFREMER, IRD, Sète, France</footnote>

<footnote n="2"><sup>2</sup>Université de Toulon, Aix Marseille Univ., CNRS, LIS DYNI, Toulon, France</footnote>

<footnote n="3"><sup>3</sup>Centre international d'IA en Acoustique Naturelle (CIAN), Toulon, France</footnote>

<footnote n="4"><sup>4</sup>French Research Institute for Exploitation of the Sea (IFREMER), Centre Bretagne, Plouzané, France</footnote>

<footnote n="5"><sup>5</sup>Observatoire Pelagis, La Rochelle Université, CNRS, La Rochelle, France</footnote>

<footnote>★These authors contributed equally to this work.</footnote>
sounds, often referred to as buzzes and/or burst pulses; Jones
et al., 2020).

Echolocation clicks are short (typically < 100 µs), broad-
band (ranging from 20 kHz to over 150 kHz, Henderson
et al., 2012) pulses used primarily for navigation, prey de-
tection, and object recognition (Au, 1993; Tyack, 1986; Nor-
ris et al., 1961). For *D. delphis*, these clicks have been the
most frequently observed when the animals are travelling
(e.g. Henderson et al., 2012).

Whistles are narrowband, frequency-modulated tonal
sounds (usually between 5 and 25 kHz) that are thought to
be used for inter-individual communication (Caldwell and
Caldwell, 1968; Au and Hastings, 2008), individual identi-
fication (signature whistles, Sayigh, 1992), and coordination
of group movements (Lammers and Au, 2003; Branstetter
et al., 2012). The whistle repertoire of *D. delphis* is well-
described (e.g. Ansmann et al., 2007; Pagliani et al., 2022).
However, there have been recent advances with formal iden-
tification of signature whistles for this species (Fearey et al.,
2019; Cones et al., 2023; Agafonov et al., 2024), showing the
importance of these signals in social interactions.

Pulsed sounds (Herman and Tavolga, 1980), including
burst pulses and buzzes, are rapid sequences of clicks whose
function remains a subject of debate (Ridgway et al., 2015).
The classification of these sounds can be complex, as they
form a graded continuum between clicks and whistles (Mur-
ray et al., 1998). Usually, their categorization is based on
an inter-click interval (ICI) (Martin et al., 2019). Buzzes are
characterized by very short ICIs and high repetition rates and
are typically associated with the terminal phase of prey cap-
ture (Wisniewska et al., 2014; Ridgway et al., 2015), while
burst pulses are more variable and are frequently observed
during close-range social interactions such as agonistic or in-
timate encounters (Overstrom, 1983; Lammers et al., 2006).

The dataset that we provide is extracted from audio record-
ings collected during surveys at sea made for the DOLPHIN-
FREE project off the coast of Penmarc'h, Brittany, France,
in 2020, 2021, and 2022. The aim of this project was to cre-
ate a bio-inspired acoustic signal to reduce the risk of fishery
by-catch of *D. delphis* (Lehnhoff et al., 2022). To that end,
surveys were conducted in order to visually and acoustically
assess the behavioural responses of wild free-ranging com-
mon dolphins to the bio-inspired acoustic signal (co-created
by the University of Montpellier and IFREMER) emitted
by version 1 of the CETASAVER-DOLPHINFREE beacon
(OCTech).

These surveys generated a large number of acoustic
recordings made with and without the activation of the bea-
con, with the presence or absence of a fishing net, and with
dependence on the behavioural state of the dolphins (i.e. for-
aging, travelling, socializing, milling, or being attracted to
the boat), in order to assess the acoustic activities of the ani-
mals under different conditions. Only the acoustic recordings
collected before the beacon was turned on are made avail-
able, as (i) the DOLPHINFREE signals are protected by in-

tellectual property rights (property of the University of Mont-
pellier and IFREMER) and, (ii) in order to deliver data that
are not dependent on the activation of the device, the study
of the natural behaviour of the dolphins during the different
states cited above is enabled.

The study of the sounds emitted by cetaceans represents
a field of research with numerous applications, including the
investigation of social interactions, behavioural patterns, lo-
calization (including trajectory reconstruction), and the iden-
tification of these animals (e.g. Caldwell and Caldwell, 1968;
Overstrom, 1983; Poupard et al., 2019; Halkias et al., 2013).
With this dataset, we hope to provide high-quality audio
recordings of free-ranging short-beaked common dolphins,
which should be of interest for the study of the acoustic reper-
toire of these animals, their acoustic behaviours, and the di-
rectionality of their acoustic emissions.

2 Material and surveys

2.1 Study area

Non-systematic scientific surveys were conducted at sea
from semi-rigid pneumatic boats carrying observers off the
coast of Penmarc’h, Brittany, France (see Fig. 1). The area
was chosen as it is locally known for its frequent sightings
of free-ranging dolphins, which have been observed just a
few nautical miles off the coast, thereby facilitating oppor-
tunistic encounters. In addition, the occurrence of dolphin
strandings in the area (Peltier et al., 2020) contributed to the
selection of this site. Surveys were performed during the pe-
riods 11–17 July 2020, 9–18 July 2021, and 16–24 July 2022.
They were only conducted when the weather conditions al-
lowed sightings of the dolphins and identification of their be-
haviours under windy conditions ≤ 10 kn and with a swell
≤ 1 m.

2.2 Acoustic devices

Two types of devices were used to make acoustic record-
ings: (i) a single Ocean Sonics icListen HF hydrophone and
(ii) a custom-made compact four-hydrophone tetrahedral ar-
ray (TETRA). The objective of the audio monitoring using
the HF hydrophone was to record the acoustic signals emit-
ted by dolphins at close range from the observation boat.
These signals could then be correlated directly with visual
observation of the behaviours of the dolphins. The record-
ings made using TETRA were dedicated to the study of the
angular movements of the dolphins during the surveys. As
these devices were not designed for the same purposes, they
were often deployed during different recording sequences.

2.2.1 Main hydrophone

The Ocean Sonics icListen HF hydrophone (see Ap-
pendix B2 for the technical details) was used during sur-
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Map of dolphins encountered during the 2020, 2021, and 2022 DOLPHINFREE surveys conducted at sea off the coast of Penmarc'h, Brittany, France.</figcaption></figure>

veys at sea in order to record short-beaked common dolphin acoustic signals. This high-frequency hydrophone was used to record sounds in one channel, with a sampling rate of 512 kHz and an audio bit depth of 32 bits. When the observation boat was stationary (engine off), this hydrophone was deployed from one side of the boat and positioned at –3 m under water. These procedures allow for high-quality sampling of the raw acoustic signals in the vicinity of the boat.

### 2.2.2 Compact hydrophone array

A custom-made prototype of a compact array of four hydrophones was deployed during the surveys. The device (TETRA, Fig. 2) has a tetrahedral shape, with one hydrophone at each of its apexes: one CR3 spherical hydrophone with a linear frequency range of up to 180 kHz and three SQ26 cylindrical hydrophones with satisfactory responses up to 50 kHz (see Appendices B3 and B4 for the technical details). The array is made of PVC tubes joined by 3D-printed parts and connected to a QHB motherboard made by the SMIoT laboratory (Barchasz et al., 2020) at the University of Toulon, France (technical specifications are available online at https://sabiod.lis-lab.fr/pub/QHB.pdf, last access: 1 April 2025).

This array was used in 2021 and 2022 to record audio in four channels at 256 to 512 kHz with a bit depth of 16 to 24 bits, depending on the survey sessions. Its materials and size (about 90 cm in length) make it a compact, portable, and practical device. Once in the water, it is left to drift freely under a buoy at –3 m under water and is monitored by observers on board. Consequently, when both recording devices were deployed, audio recordings made with TETRA might differ from those from the icListen hydrophone due to the distance between the two devices. It should also be noted that this version of TETRA was an initial prototype, which

has since been improved in subsequent versions (see Glotin et al., 2024).

## 2.3 Survey design

During the surveys, two to three people were present on board to navigate, to deploy the acoustic equipment, and to take notes on the visual observations made of the observed groups of dolphins (see Fig. 3). A group of dolphins was defined as any number of the animals observed within five body lengths of another that was conspecific, moving, and behaving in the same manner (Shane, 1990; Stockin et al., 2008; Filby et al., 2013). The distances of each group of dolphins from the boat were initially estimated using rangefinder binoculars (Bushnell Fusion 10 × 42) that enabled visual observers to get accustomed to estimating distances at sea. Once the observers were used to estimating distances, the binoculars were set aside as this allowed for quicker visual observations, which was a more pragmatic approach.

The DOLPHINFREE surveys required a group of dolphins to show a constant behavioural state (defined in Table 1) for at least 1 min of observation, meaning that the observers waited until the dolphins presented a homogeneous behavioural state before starting to record. As soon as this condition was met, the boat was stopped and the engine and sonar were switched off. Then, the recording devices were set: the icListen hydrophone at –3 m from one of the sides of the boat and the TETRA antenna at –3 m under a buoy left to drift from the other side of the boat. However, it should be noted that the TETRA antenna was only deployed when drift conditions enabled the observers on board to maintain visual contact with the buoy.

As stated in the Introduction section, only the recordings made before the emission of signals by the DOLPHINFREE acoustic beacon are available here. However, other signals
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Annotated photo of the TETRA antenna used during the 2021 and 2022 surveys of the DOLPHINFREE project. TETRA's sides are ≈ 90 cm in length. Photograph taken by Loïc Lehnhoff before the surveys.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Experimental layout during the DOLPHINFREE surveys. Image captured by a DJI Phantom drone piloted by Bastien Mérigot.</figcaption></figure>

were tested during part of the surveys: classical music pieces and whistles of orcas. The audio recordings of these sequences are fully available. In addition, during some surveys, a fishing net was set under water to simulate the conditions under which by-catch occurs. The distribution of audio files according to the behaviour observed and the presence or absence of a fishing net is presented in Fig. 4. Four fishing nets were utilized in the course of the surveys. While they do not constitute an exhaustive sample of all the existing nets deployed by fishers, they still represent a diverse sample of fishing gear. The technical specifications of these nets are provided in Appendix B1.

# 3 Data and methods

This section describes the files and data types that will be found in the dataset (https://doi.org/10.5281/zenodo.14637674, Lehnhoff, 2025a) that is made available.

## 3.1 Observation notes

Observation data were collected manually during the surveys, following a custom-made data table (see Appendix A). Then, notes were standardized and associated with each audio file independently. These visual observations are available as .xlxs files, distinct for each year and acoustic recording device. Each file contains the values described in Table 2. Animals within a group often exhibited the same behavioural state (as defined in Table 1). Behavioural states were relatively easy to identify and distinguish from each other. However, in some cases, the dolphins may have changed their behaviour within the observational window, exhibited different behavioural states within the same group, or displayed two different states simultaneously (notably attraction to the boat and socializing during our surveys). To take these cases into account, the behavioural states were recorded as a percentage of the behaviours observed each minute. In addition, each observation was associated with the name of the observer to help identify potential biases.
<table><caption>**Table 1.** Definitions of the behavioural states of common dolphins recorded in 2020, 2021, and 2022, according to Van Canneyt et al. (2006), Berg Soto et al. (2013), and Filby et al. (2013) from Lehnhoff et al. (2022).</caption><thead><tr><td>State</td><td>Definition</td></tr></thead><tbody><tr><td>Foraging</td><td>Dolphins were involved in any effort to pursue, capture, and/or consume prey, as defined by observations of two or more of the following: fish chasing, erratic movements at the surface, multi-directional diving, coordinated deep diving, and rapid circle swimming. Prey and hunting birds were often observed at the surface.</td></tr><tr><td>Travelling</td><td>Dolphins engaged in persistent, directional movement, making noticeable headway along a specific compass bearing. Group spacing varied and individuals swam with short (&lt; 20 s), relatively constant dive intervals.</td></tr><tr><td>Socializing</td><td>The animals were involved in active surface behaviour (frequent surfacing and breaching) that included physical interactions among group members and sometimes aerial behaviour.</td></tr><tr><td>Milling</td><td>Dolphins showed little movement, tended to remain in the same place, and either spent time floating at the surface or surfaced asynchronously.</td></tr><tr><td>Attraction</td><td>Dolphins came towards the boat and swam at a distance of a few metres along it, following its direction.</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Diagram illustrating the distribution of data collected as part of the DOLPHINFREE surveys, with a focus on the data made available depending on the presence of a fishing net and the observed behavioural state of the dolphins.</figcaption></figure>
<table><caption>**Table 2.** Descriptions of the column headers in the visual observation data (XLSX) files.</caption><thead><tr><th>Column name</th><th>Meaning</th><th>Value format</th></tr></thead><tbody><tr><td>audio_file</td><td>Name of the .wav file</td><td>[date]_[time].wav</td></tr><tr><td>datetime_utf</td><td>Start date and time of recording</td><td>YYYY-MM-DD HH:MM:SS</td></tr><tr><td>observers</td><td>Names of observers taking notes</td><td>firstname_lastname</td></tr><tr><td>ID_group</td><td>Identification number of a group of dolphins</td><td>Integer</td></tr><tr><td>ID_sequence</td><td>Identification number of each survey sequence</td><td>Integer</td></tr><tr><td>group_size</td><td>Number of individuals in a group</td><td>Integer</td></tr><tr><td>fishing_net</td><td>Presence or absence of a fishing net</td><td>"present" or "absent"</td></tr><tr><td>fishing_net_type</td><td>When present, the type of fishing net deployed</td><td>"type_of_net"</td></tr><tr><td>percent_[behaviour]</td><td>Percentage of observed behaviour as defined in Table 1</td><td>Integer</td></tr><tr><td>behavioural_event</td><td>Special behavioural events</td><td>jumps, spy-hopping, etc.</td></tr><tr><td>distance</td><td>Distance from the boat to the closest visible dolphin</td><td>Integer</td></tr><tr><td>group_clustering</td><td>Aspect of the spacing between the individuals</td><td>"compact" or "scattered"</td></tr><tr><td>direction</td><td>General direction in which the group is heading</td><td>"continuous" or "variable"</td></tr><tr><td>speed</td><td>Average speed of the animals (slow/fast threshold: ≈ 10 kn)</td><td>"slow", "variable", or "fast".</td></tr><tr><td>diving_time</td><td>Average diving time of the animals (normal/long threshold: 2 min)</td><td>"normal", "variable", or "long"</td></tr><tr><td>activation_sequence</td><td>Experimental treatment (relative to the emission of signals)</td><td>"before", "during", "after", or "control"</td></tr><tr><td>signal</td><td>Type of signal loaded into the emitter</td><td>See the signal_codes.txt file.</td></tr><tr><td>sonar.noise</td><td>Indicates whether a sonar was unintentionally recorded</td><td>0 (no) or 1 (yes)</td></tr><tr><td>special_observations</td><td>Any additional observation</td><td>Notes</td></tr></tbody></table>

## 3.2 Audio files

These are raw files obtained from the recording of wild free-ranging short-beaked common dolphins during the DOLPHINFREE surveys. There are 275 unedited 1 min files of audio recordings made using the icListen hydrophone. These files have a sampling rate of 512 kHz with 32-bit resolution in one channel.

As TETRA is a custom-made device, we experimented with different configurations of its QHB motherboard, leading to audio recordings made with varying parameters. Overall, 117 files are available, which in total add up to 162 min of audio data. Files obtained from the TETRA antenna were recorded in four channels (corresponding to its four hydrophones) and have varying sampling rates (256 or 512 kHz), bit-depth resolutions (16 to 24 bits), and durations (15 to 120 s).

For the acoustic data recorded by both recording devices, we recommend the application of a high-pass filter for the study of echolocation clicks in order to avoid background noises (such as waves and boat engines), as both devices were deployed only a few metres below the water surface. Visual observations confirmed the presence of dolphins in all of the recordings provided. These data contain a wide panorama of the signals that short-beaked common dolphins can produce: echolocation clicks, whistles, pulsed sounds, and probable bi-phonations (Jones et al., 2020).

## 3.3 Whistle contour annotation

The shapes of whistles can be linked to specific behaviours and contexts (Lehnhoff et al., 2022) or directly to individuals

(i.e. signature whistles; Caldwell and Caldwell, 1965). However, the extraction of whistle contours represents a challenging task for which a variety of automated methods have been developed, e.g. using modelling (Halkias and Ellis, 2006; Roch et al., 2011), pitch-tracking (Baumgartner and Musso-line, 2011), or deep-learning (Conant et al., 2022; Li et al., 2023) techniques.

We used a semi-automated method (Lehnhoff et al., 2025a) to annotate the contour of whistles from audio recordings made using the IcListen hydrophone. The spectrograms used for the annotations were generated using the following parameters: a 96 000 Hz sampling rate, a 1024-sample (11 ms) frame size, and a 512-sample (5 ms) hop length on a linear frequency scale. These annotations were then manually verified and corrected using https://gitlab.lis-lab.fr/loic.lehnhoff/PyAVA (last access: 1 April 2025) (Lehnhoff, 2022), a custom-made annotation tool for whistle contours. These results are made available as .json files in the Whistle_annotations folder within the dataset.

## 3.4 Other data

A README.md file is available in the parent folder of the dataset and describes the structure of the dataset, with usage notes and links to academic papers produced using its data.

The Tetra\Hydro_coordinates folder contains the .csv files giving the measured coordinates of the four hydrophones of the tetrahedral antenna in 2021 and 2022. These coordinates are needed to determine the angle of arrival of the signals detected using the hydrophones.
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Spectrogram (a) and waveform (b) extracted from audio recording "SCW1807_20200713_064400.wav". (a) Arrows point to the representative signals. (b) Segments show the time span of click-like sounds. The annotations indicate only some of the signals emitted by short-beaked common dolphins that are visible in the spectrogram.</figcaption></figure>

# 4 Results and output data

Some analyses have already been conducted of this dataset and been published, reviewed, or submitted as three different scientific papers. These papers show an increase in the acoustic activity of common dolphins in response to the DOLPHINFREE bio-inspired acoustic signal (Lehnhoff et al., 2022), characterize the features of whistles (Lehnhoff et al., 2025a), and identify signature whistles in the whistling repertoire of these animals (Lehnhoff et al., 2025b). However, it should be noted that the dataset made available here is slightly different than the one used in our previous analyses (due to the exclusion of audio recordings containing DOLPHINFREE signals).

From these works, the technical quality of the dataset is supported by manual and automatic detections of dolphin signals in audio recordings. A standard Teager-Kaiser click detector coupled with a data projection to exclude false-positive detections (Lehnhoff et al., 2022) was used to confirm the presence of echolocation clicks in most files. A semi-automatic detector of whistles was also applied to extract contour coordinates of whistles (Lehnhoff et al., 2025a) with manual validation of the detections. In total, we confirm the presence of about 68 000 echolocation clicks, 4600 whistles, and 350 pulsed sounds in the provided audio data recorded with the icListen hydrophone (Lehnhoff et al., 2022, 2025a).

The distribution of dolphin signals across the recordings is quite heterogeneous. Figure 5 shows the different signals that

can be found in the audio files made available. In addition, we show the magnitude spectrum of a sample of echolocation clicks recorded by the icListen hydrophone at 512 kHz, revealing the broadband nature of these clicks (see Fig. 6).

## 4.1 Two-dimensional localization

TETRA's recordings (four channels; see Fig. 7) can be used to determine the time difference of arrival (TDoA) of echolocation clicks at the hydrophones of the antenna, also enabling the estimation of the angle of arrival (AoA) of these clicks. A validation sequence test to evaluate the errors made in the estimation of the AoAs was conducted. DOLPHINFREE signals were emitted from the boat and recorded by the TETRA antenna at different angles around it.

The position of the antenna relative to the observation boat was determined by two GPS devices: one located on the observation boat and the other attached to the TETRA antenna buoy. Then, with the audio recordings, the DOLPHINFREE signals were used to estimate AoAs from TDoAs. A comparison of the values estimated by these two methods is shown in Fig. 8. It highlights that there are only a few deviations in the estimation of AoAs using TETRA compared to GPS measurements (on average, 0.15 radians (8.6°) for the azimuths and 0.08 radians (4.4°) for the elevation angles). However, since calibrations were performed by recording the DOLPHINFREE signal, the corresponding audio recordings are not publicly available.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Waveforms and magnitude spectra of three echolocation clicks selected randomly in audio file "SCW1807_20200713_064400.wav".</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Spectrogram extracted from audio recording "20210709_135634UTC+2_V12.wav" collected with the TETRA antenna. Hydrophone 0 is a CR3, and the other hydrophones are SQ26.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Angular comparison of azimuths (a) and elevation (b) between GPS references and TETRA estimates during a calibration experiment. The emitter's elevation is interpolated from the immersion depth of the TETRA antenna.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Screenshot showing the whistle contour annotations of audio file "SCW1807_20200712_090400.wav".</figcaption></figure>

## 4.2 Whistle contours

Whistle contours were determined using DYOC (Lehnhoff et al., 2025a), a deep-learning tool made for whistle contour annotation and developed using parts of this dataset. Then, contours were cleaned manually using PyAVA at https://gitlab.lis-lab.fr/loic.lehnhoff/PyAVA (last access: 1 April 2025), a custom-made annotation tool in Python. An example of the results is shown in Fig. 9.

In total, 4637 annotated whistle contours were verified and are provided as .json files (dictionaries) with the audio recordings. In each file, different keys correspond to different whistle contours, with points represented as lists of time-frequency coordinates. This dataset provides ground truths that could be used to study the short-beaked common dolphin whistle repertoire and to train and/or test performances of models for the extraction of whistle contours.

## 5 Code and data availability

The data described in this paper can be accessed at https://zenodo.org/records/14637675 (last access: 1 April 2025) under https://doi.org/10.5281/zenodo.14637674 (Lehnhoff, 2025a). Reuse of this dataset is facilitated by several scripts available at https://gitlab.lis-lab.fr/dolphinfree-experiments (Lehnhoff, 2025b).

The decision to publish only a part of the collected data was prompted by the presence of signals protected by intellectual property.

## 6 Conclusions

Overall, this open-access dataset is the first to provide high-quality recordings of free-ranging short-beaked common dolphins (*Delphinus delphis*) across multiple observed behavioural states. It comprises over 400 min of acoustic data collected from wild animals at sea, including approximately 68 000 echolocation clicks, 4600 whistles, and 350 pulsed
# Appendix B: Technical details

## B1 Fishing nets

During the DOLPHINFREE surveys, four nets were utilized to mimic the setting of a fishing net under water by professional fishers. They were all set from the surface and weighted with lead weights. These nets are described in Table B1.

<table><caption>**Table B1.** Descriptions of the fishing nets used during the DOLPHINFREE surveys.</caption><thead><tr><th>Name</th><th>Net type</th><th>Material</th><th>Mesh size</th><th>Net size<br>(H × W)</th></tr></thead><tbody><tr><td>trammel</td><td>Monkfish trammel net</td><td>Nylon</td><td>220 mm</td><td>2 × 4 m</td></tr><tr><td>white_trawl</td><td>Trawl netting</td><td>Reinforced nylon</td><td>12 mm</td><td>2 × 4 m</td></tr><tr><td>green_trawl</td><td>Trawl netting</td><td>Polyethylene</td><td>40 mm</td><td>2 × 4 m</td></tr><tr><td>large_net</td><td>Entangling net</td><td>Nylon</td><td>136 mm</td><td>3 × 20 m</td></tr></tbody></table>

## B2 icListen HF hydrophone

The icListen HF hydrophone is produced by © Ocean Sonics. Its specifications are available on their website (https://oceansonics.com/documents/icListen_FULL specsheet.pdf?_t=1731942062, last access: 1 April 2025). In the following figures, we report the hydrophone specifications provided by the manufacturer.

<figure><img src="image_12.png" /><figcaption>**Figure B1.** Noise spectrum levels of the different icListen HF hydrophones, provided by © Ocean Sonics. The SJ model (purple) was the one used in this study.</figcaption></figure>
<table><thead><tr><th></th><th>SJ</th><th>RB</th><th>SK</th><th>Units</th></tr></thead><tbody><tr><td colspan="5"><strong>SIGNAL PERFORMANCE</strong></td></tr><tr><td>Low Frequency Cutoff</td><td>10</td><td>10</td><td>10</td><td>Hz</td></tr><tr><td>±3 dB Bandwidth</td><td>200</td><td>200</td><td>12.8</td><td>kHz</td></tr><tr><td>Usable Bandwidth</td><td>200</td><td>200</td><td>12.8</td><td>kHz</td></tr><tr><td>Maximum Data Sample Rate</td><td>512</td><td>512</td><td>32</td><td>ksps</td></tr><tr><td>Minimum Data Sample Rate</td><td>1</td><td>1</td><td>1</td><td>ksps</td></tr><tr><td>Resolution</td><td>24</td><td>24</td><td>24</td><td>bits</td></tr><tr><td>Minimum Self Noise</td><td>32</td><td>36</td><td>17</td><td>dB re µPa²/Hz</td></tr><tr><td>Peak Input Level (µPa)</td><td>180</td><td>184</td><td>168</td><td>dB re µPa</td></tr><tr><td>Peak Input Level (Volts)</td><td>6</td><td>6</td><td>6</td><td>dBV</td></tr><tr><td>Voltage Sensitivity</td><td>-173</td><td>-177</td><td>-162</td><td>dB re V/µPa</td></tr></tbody></table>

**Figure B2.** Signal performance table of the different icListen HF hydrophones, provided by © Ocean Sonics. The SJ model (purple) was the one used in this study.

## B3 CR3 hydrophone

The CR3 hydrophone is produced by Cetacean Research™. Its specifications are available on their website (https://www.cetaceanresearch.com/hydrophones/cr3-hydrophone/index.html, last access: 1 April 2025). In the following figures and tables, we report the hydrophone specifications provided by the manufacturer.

<table><caption>**Table B2.** CR3 hydrophone specifications, provided by Cetacean Research™.</caption><thead><tr><th>Parameter</th><th>Value</th></tr></thead><tbody><tr><td>Linear frequency range (±3 dB)</td><td>0.0004 to 180 kHz</td></tr><tr><td>Usable frequency range (+3/ - 12 dB)</td><td>0.0001 to 240 kHz</td></tr><tr><td>Transducer sensitivity</td><td>-207 dB re 1 V µPa<sup>-1</sup></td></tr><tr><td>Sound pressure level (SPL)-equivalent self-noise at 1 kHz</td><td>54 dB re 1 µPa/√Hz</td></tr><tr><td>Maximum operating depth</td><td>980 m</td></tr><tr><td>Operating temperature range</td><td>-40 to 90 °C</td></tr><tr><td>Capacitance</td><td>6.7 nF</td></tr><tr><td>Dimensions</td><td>50 mm L × 18 mm diameter</td></tr></tbody></table>
<figure><img src="image_13.png" /><figcaption>**Figure B3.** Horizontal (left panel) and vertical (right panel) beam patterns of the CR3 hydrophone, provided by Cetacean Research™.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure B4.** Frequency response of the CR3 hydrophone, provided by Cetacean Research™.</figcaption></figure>
## B4 SQ26 hydrophone

The SQ26 hydrophone is provided by Cetacean Research™. Its specifications are available on their website (https://www.cetaceanresearch.com/hydrophones/sq26-01-hydrophone/index.html, last access: 1 April 2025). In the following figures and tables, we report the hydrophone specifications provided by the manufacturer.

<figure><img src="image_15.png" /><figcaption>**Figure B5.** Horizontal beam pattern at 5 kHz (left panel), 10 kHz (centre panel), and 35 kHz (left panel) of the SQ26 hydrophone, provided by Cetacean Research™.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure B6.** Frequency response of the SQ26 hydrophone, provided by Cetacean Research™.</figcaption></figure>
<table><caption>**Table B3.** SQ26 hydrophone specifications, provided by Cetacean Research™.</caption><thead><tr><td>Parameter</td><td>Value</td></tr></thead><tbody><tr><td>Linear frequency range (±1 dB)</td><td>0.001 to 28 kHz</td></tr><tr><td>Usable frequency range (+3/-12 dB)</td><td>0.001 to > 50 kHz</td></tr><tr><td>Transducer sensitivity</td><td>-193.5 dB re 1 V µPa<sup>-1</sup></td></tr><tr><td>SPL-equivalent self-noise at 1 kHz</td><td>54 dB re 1 µPa/√Hz</td></tr><tr><td>Maximum operating depth</td><td>Down to 2000 m</td></tr><tr><td>Operating temperature range</td><td>-30 to 60 °C</td></tr><tr><td>Capacitance</td><td>1.4 nF</td></tr><tr><td>Dimensions</td><td>25.4 mm L × 25.4 mm diameter</td></tr></tbody></table>

**Author contributions.** Conceptualization: BM, HG, and LL. Methodology: BM, HG, LL, YLG, EM, and OVC. TETRA design: HG. Code and formal analysis: LL, HG, and BM. Validation and supervision: BM and HG. Data curation: BM and LL. Writing – original draft preparation: LL. Figures: LL and OVC. Data acquisition and writing – review and editing: all of the authors. Project administration: BM. Project funding acquisition: BM and HG. All of the authors reviewed the manuscript.

**Competing interests.** The contact author has declared that none of the authors has any competing interests.

**Disclaimer.** The DOLPHINFREE project has been approved by agreement no. 0-12520-2021/PREMAR_ATLANT/AEM/NP of the French Maritime Prefecture of the Atlantic “to conduct a survey for monitoring groups of common dolphins by means of scientific instruments off the south Finistère coast, following Décret no. 2017-956 of the scientific marine research”. The DOLPHINFREE project obtained a favourable opinion from the Ethical Committee for Animal Experimentation of Languedoc-Roussillon (CEEA-LR) for request no. 26568.

Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors.

**Acknowledgements.** We thank Paul Best, Sophie Boyer, and Eleonore Meheust for their assistance in collecting some of the data made available with this paper. We thank Michael Paul for improving the English of the paper.

**Financial support.** The DOLPHINFREE project coordinated by Bastien Mérigot was funded by the European Maritime and Fisheries Fund (EMFF) and France Filière Pêche (FFP). Loïc Lehnhoff’s PhD grant was provided by Montpellier University. Hervé Glotin is supported by ANR-18-CE40-0014 SMILES, and his na-

tional Chair in Artificial Intelligence for bioacoustics is funded by ADSIL ANR-20-CHIA-0014-01 DGA and AID.

**Review statement.** This paper was edited by Sebastiaan van de Velde and reviewed by Will Rayment and one anonymous referee.

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