ORCID

Abstract

Passive acoustic monitoring techniques are useful for studying vocally active marine species, particularly in remote and difficult to access areas. In this study, three months of acoustic recordings were collected off Egmont Atoll, Chagos Archipelago in the central Indian Ocean to detect, characterise, and investigate temporal variability of delphinid whistles. A subsample (3.6% of the total dataset) was used to manually annotate whistles and test automated whistle detectors. Higher frequency whistles (6-25 kHz, n = 126) were manually classified into six categories: upsweep (56%), convex (19%), downsweep (13%), concave (5%), sine (4%), and constant frequency (3%). An automated whistle detector was evaluated under five detection thresholds (measured in dB): 5.5, 6.0, 6.5, 7.0, and 7.5 above background noise. The 6.5 dB threshold demonstrated the best balance between precision (0.9) and recall (0.5). The whistles were detected on 78% of days, with a bimodal diel pattern, where whistles peaked after sunrise (0800-1000 h) and before sunset (1600-1800 h), with fewer detections in the middle of the day and at night. This study highlights the value of passive acoustic monitoring techniques to better understand the delphinid occurrence in remote and understudied areas.

Publication Date

2025-10-21

Publication Title

The Journal of the Acoustical Society of America

Volume

158

Issue

4

ISSN

0001-4966

Acceptance Date

2025-10-06

Deposit Date

2026-01-30

Funding

This study was supported by generous funding from the Garfield Weston Foundation and the Bertarelli Foundation, contributing to the Bertarelli Programme in Marine Science. The research was conducted under permits issued by the British Indian Ocean Territory Administration (BIOTA) (Permit Nos. 0008SE19 and 0006SE20). We extend our heartfelt thanks to Dr. Phil Hosegood and all the research vessel (Tethys Supporter) crew who provided invaluable support in the field. We thank Dr. Charles Anderson for generously sharing his knowledge throughout the project. We also thank Professor Liz Ferguson (Ocean Science Analytics) who generously donated a PAMGUARD training course and Naomi Perera (Oceanswell) who extended her open-handed support in scientific writing in English.

Keywords

Vocalization, Animal/classification, Acoustics/instrumentation, Sound Spectrography, Time Factors, Animals, Dolphins/physiology, Signal Processing, Computer-Assisted, Indian Ocean, Environmental Monitoring/methods

First Page

3226

Last Page

3238

Download

Share

COinS