DOI: 10.5281/zenodo.15658353
title: Dataset of Acoustic Recordings of Bottlenose Dolphins Collected During Boat-Based Encounters Using HydroMoth Devices, Event 006 (2024-12-01)

description: Acoustic recordings of bottlenose dolphins collected during boat-based encounters using HydroMoth devices and analysed with PAMGuard

This dataset comprises 8 minutes of underwater acoustic recordings of a group of bottlenose dolphins (Tursiops truncatus). 

The vocalisations were recorded on 1 December 2024, approximately 5.3 miles off the coast of Vilanova i la Geltrú (Catalonia, Spain), in the northwestern Mediterranean Sea. The recordings were collected during boat-based cetacean encounters using a HydroMoth recorder (AudioMoth Firmware Basic v1.11.0), with the support of team members and volunteers from the Institut de Ciències del Mar (CSIC) and Associació Cetàcea.

The navigation spanned approximately 9 hours, from 2024-12-01T07:07:12 (UTC) to 2024-12-01T16:51:00 (UTC). The first visual encounter with the group of Bottlenose dolphins occurred around 09:34:00 (UTC+1). The motor was turned off after the encounter.

The resulting recording were analysed using the open-source software PAMGuard (v2.02.16; DOI: 10.5281/zenodo.15056259). Whistles were classified with the ROCCA (Real-time Odontocete Call Classification Algorithm) delphinid whistle module, integrated into PAMGuard (https://github.com/ProjectROCCA/ROCCA). Clicks were manually categorized in the PAMGuard Viewer as echolocation clicks, burst-pulse sounds, or click trains. This dataset and its analysis includes comprehensive data covering the full recording session, structured according to the Darwin Core terms SamplingEvent, Occurrence, ExtendedMeasurementsAndFacts, and SimpleMultimedia.

This dataset is also available through GBIF at: https://doi.org/10.15470/77bhey 
description: Recording Configuration

The HydroMoth device was configured to capture high-frequency sounds at a sampling rate of 250 kHz, with medium gain. A band-pass filter ranging from 0.5 kHz to 125 kHz was applied. No threshold settings were enabled during the recording session.

Deployment at sea

Upon sighting cetaceans and approaching them, the vessel’s engine was turned off, and the acoustic recorder was deployed at depths of approximately 5 to 7 metres. The device remained submerged until the encounter was considered concluded—typically after several minutes without visual or acoustic detections, or when the animals were observed to be moving away or did not return. The recorder was then retrieved.

Sample preparation

Due to the high data volume generated by the recording settings (approximately 143 MB per 5 minutes at a 250 kHz sampling rate), the audio files were segmented into 1-minute clips prior to analysis. This segmentation facilitated more detailed processing and ensured compatibility with public platforms linked to citizen science initiatives, many of which impose a file size limit of 20 MB.

PAMGuard analysis

All acoustic data recorded during the deployments were analysed both aurally and visually using the open-source software PAMGuard Version 2.02.16 (DOI: 10.5281/zenodo.15056259) by biologists from the EMBIMOS research group at ICM-CSIC.

The Whistle and Moan Detector and Click Detector modules were used to detect tonal and pulsed vocalisations, respectively. The ROCCA module was employed to measure whistle contours, which were manually extracted using the contour-extraction graphical interface (see Oswald & Oswald, 2013, ROCCA User’s Manual).

Spectrograms were visualised with the following settings: FFT length = 2048, hop size = 1024, and a Hann window with 50% overlap, yielding a temporal resolution of approximately 8.19 ms. Additional modules and visualisation tools were configured to assist in the inspection of low-frequency acoustic events. For a complete description of the analysis workflow, refer to the PAMGuard Configuration Overview included with the dataset.

Following configuration, PAMGuard was used to process the acoustic data. Outputs were reviewed and interpreted using the PAMGuard Viewer. Click data were examined in detail using the Time-Domain (TD) Display and the Click Detector Label function available in the Bearing Display. Three types of pulsed sounds—echolocation clicks, click trains, and burst pulses—were manually annotated based on waveform structure and amplitude profiles.

Quality control

The organizations collaborating in this participatory project were trained to use HydroMoth devices with configurations optimized for detecting the expected species, based on local ecological knowledge and seasonal occurrence. A recording protocol was developed, and multiple training sessions were conducted for all participants involved in the data collection process. 

The resulting acoustic data were analysed by an experienced biologist from the EMBIMOS research group at ICM-CSIC. Quality control included careful configuration of PAMGuard modules to ensure accurate automated detection, followed by manual annotation and curation of the data using the PAMGuard Viewer mode.
description: DATA INTERPRETATION

Certain parameters in the ExtendedMeasurementsAndFacts metadata file—particularly those generated by the ROCCA module within PAMGuard—should be interpreted with caution. This applies especially to the whistle-based species classification results and their associated confidence scores.

In several audio files, the species with the highest encounter-level vote was consistent with the species visually confirmed during the boat-based encounter, particularly when high-quality whistle signals were present. However, in recordings where whistle signals were weak or degraded, classifier performance diminished, often leading to species misclassification.

Several factors may contribute to this issue, including low signal-to-noise ratio (SNR), background noise, or suboptimal configuration settings in PAMGuard or the ROCCA module, among others. Users are advised to consider these limitations when interpreting species classification data derived from acoustic recordings.

With regard to the number of pulsed tones, the results of automatic classification should also be interpreted with caution. To improve data accuracy, click-type classifications were further refined through manual review and annotation using the PAMGuard Viewer interface. This additional step enhanced the precision and consistency of click-related measurements. However, results should still be interpreted carefully due to potential limitations such as signal strength variability, differences in detection thresholds, manual misclassification, the gain settings, among other configuration parameters used during data collection and/or data analysis. 

The resulting metadata, formatted according to the Darwin Core standard, is included in this record, along with the PAMGuard configuration files in both .txt and .psfx formats.