# First Report on the Acoustic Signals of Lahille’s Bottlenose Dolphins in Argentina

## TL;DR

This study is the first to describe the sounds made by Lahille’s bottlenose dolphins in Argentina, helping to monitor and protect this endangered species.

## Contribution

The first detailed acoustic characterization of Lahille’s bottlenose dolphins in Argentina using passive acoustic monitoring.

## Key findings

- Dolphin activity is mainly diurnal and synchronized with the tidal cycle, especially during the ebbing tide.
- The acoustic repertoire includes echolocation clicks, whistles, bray-calls, and chirps with specific frequency ranges.
- Whistle frequencies overlap with other regional populations, while echolocation clicks are broadband with a peak around 40 kHz.

## Abstract

Lahille’s bottlenose dolphin is the most endangered small cetacean in Argentina. While it was once common along the northern coast of Buenos Aires, the population collapsed in the 1980s and 1990s due to overfishing and pollutants, leaving only small, isolated groups today. These fragmented and small populations are difficult to study using traditional visual studies. In this study, we used passive acoustic monitoring—underwater sound recorders—to detect dolphins in the turbid inner channels of the Bahía Blanca Estuary. We provide the first detailed description of the species’ acoustic signals in Argentina, including echolocation clicks and communication whistles. Our results reveal that dolphin activity is predominantly diurnal and strongly synchronized with the tidal cycle, particularly during the ebbing tide. These findings suggest that dolphins coordinate their movements with water flow to optimize foraging in a complex environment. These findings establish a preliminary baseline for non-invasive monitoring and support the development of long-term conservation strategies for this vulnerable population in environments heavily influenced by human activities.

This study provides the first characterization of the acoustic signals emitted by Lahille’s bottlenose dolphins (Tursiops gephyreus) in Argentine waters, establishing a baseline for the species’ southern distribution limit. Passive acoustic monitoring was conducted in the inner channels of the Bahía Blanca Estuary using a broadband hydrophone and F-POD detectors. We documented a diverse acoustic repertoire consisting of echolocation clicks, whistles, bray-calls, and chirps. Acoustic presence was predominantly diurnal and showed a marked synchronization with the tidal cycle, with peak activity occurring during the ebbing tide. Whistles had a mean center frequency of 8.1 kHz, showing spectral overlap with other regional populations in the Southwestern Atlantic. Echolocation clicks were broadband, with peak frequencies ranging from 18 to 127 kHz and a mode around 40 kHz. These results suggest that dolphin movement in the estuary’s internal channels is structured by hydrodynamics, likely reflecting foraging opportunities associated with tidal flow. Given the endangered status of this species, these data establish a technical framework for non-invasive monitoring and habitat-use assessments in complex estuarine environments.