Low-frequency sampling rates are effective to record bottlenose dolphins

Abstract

Acoustic monitoring in cetacean studies is an effective but expensive approach. This is partly because of the high sampling rate required by acoustic devices when recording high-frequency echolocation clicks. However, the proportion of echolocation clicks recorded at different frequencies is unknown for many species, including bottlenose dolphins. Here, we investigated the echolocation clicks of two subspecies of bottlenose dolphins in the western South Atlantic Ocean. The possibility of recording echolocation clicks at 24 and 48 kHz was assessed by two approaches. First, we considered the clicks in the frequency range up to 96 kHz. We found a loss of 0.95–13.90% of echolocation clicks in the frequency range below 24 kHz, and 0.01–0.42% below 48 kHz, to each subspecies. Then, we evaluated these recordings downsampled at 48 and 96 kHz and confirmed that echolocation clicks are recorded at these lower frequencies, with some loss. Therefore, despite reaching high frequencies, the clicks can also be recorded at lower frequencies because echolocation clicks from bottlenose dolphins are broadband. We concluded that ecological studies based on the presence–absence data are still effective for bottlenose dolphins when acoustic devices with a limited sampling rate are used.

The Development of a Low-Cost Hydrophone for Passive Acoustic Monitoring of Dolphin’s Vocalizations

“…Outside the beam, the low-frequency components suffer less attenuation and appear in the frequency spectrum [35]. Thus, low-frequency components are expected to be predominant when free-ranging dolphin vocalizations are recorded since they are mainly off-axis of the transmitting beam [36]. It is worth recalling that the CoPiDi hydrophone was developed with just the main aim of measuring free-ranging dolphin vocalizations.…”

“…It is worth recalling that the CoPiDi hydrophone was developed with just the main aim of measuring free-ranging dolphin vocalizations. Moreover, based on the abovementioned analyses, a study by Romeu et al showed that a sampling frequency of 48 kHz (and thus recorded bandwidth = 24 kHz) may be effective for recording the entire acoustic repertoire of Turpsiops truncatus when employed in studies analyzing and quantifying the presence/absence of the single animal or a population of cetaceans in a specific location [36]. For all the situations described above, the proposed hydrophone appears to be a suitable tool for recording dolphin's vocalization, especially considering its low cost.…”

A WAV file dataset of bottlenose dolphin whistles, clicks, and pulse sounds during trawling interactions

“…Outside of the beam, the low-frequency components experience less attenuation and appear in the frequency spectrum 35 . Therefore, low-frequency components are expected to be predominant when free-ranging dolphin vocalizations are recorded since they are mainly off-axis of the transmitting beam 36 . In the current study, bottlenose dolphin vocalizations were recorded with no regard to specific distance or orientation from the hydrophone.…”

“…In the current study, bottlenose dolphin vocalizations were recorded with no regard to specific distance or orientation from the hydrophone. Based on these considerations, it was determined that the frequency of 48 kHz could be effective for recording the entire acoustic repertoire of Tursiops truncatus , including echolocation signals, in studies analyzing the occurrence or distribution of individuals in a population or at a specific location when the presence or absence data are sufficient 36 . These observations indicate that the present database recorded at 192 kHz could be suitable for the majority of dolphin vocalization analyses.…”