Passive acoustic monitoring of bottlenose dolphin and harbour porpoise, in Cardigan Bay, Wales, with implications for habitat use and partitioning

Abstract

Knowledge about harbour porpoise and bottlenose dolphin occurrence in Cardigan Bay Special Area of Conservation (SAC), Wales, is limited to daylight hours during summer, when conditions are suitable for traditional visual surveys. T-PODs are autonomous instruments programmed to log time-cues of species-specific echolocation signals for long periods of time. Here we investigated bottlenose dolphin and harbour porpoise habitat use and partitioning by deploying ten calibrated T-PODs in Cardigan Bay SAC for one year. The T-PODs detected both species all year round with a peak of detections in April–October for dolphins and in October–March for porpoise, revealing a previously unknown importance of the place to harbour porpoise during winter. Though the two species are sympatric, simultaneous detections of both species were rare and indication of temporal habitat partitioning between the two species in some parts of the SAC was observed. The one location where simultaneous detections were not as rare was close to the stretch of shoreline where stranding of porpoises killed by dolphins are most common, suggesting that the observed spatiotemporal overlap leads to inter-specific interactions, in some cases fatal for the porpoise.

Using Static Acoustic Monitoring to Describe Echolocation Behaviour of Heaviside’s Dolphins (Cephalorhynchus heavisidii) in Namibia

“…The highest detection rate (number of clicks detected by the T-POD per day) was seen in scans with a target frequency of 130 kHz, with declining detection rates in scans with other settings where the frequency of detected clicks was either less or greater than the target frequency in those scans. The T-POD was then set as shown in Table 1, with three scans/min to detect the high-frequency Heaviside's dolphin clicks (target frequency: 130 kHz; reference frequency: 90 kHz) and three scans set to allow for detection of other dolphin species with lower-frequency, non-NBHF clicks (target frequency: 50 kHz; reference frequency: 90 kHz) such as bottlenose dolphins (Philpott et al, 2007;Simon et al, 2010). Upon retrieval, data were downloaded and processed using the instrument-specific software (TPOD.…”

“…Clicks from this species are bimodal and have energy both in a lower (50 to 70 kHz) frequency range and around the same higher frequency as Heaviside's dolphins (Au, 1993;dos Santos & Almada, 2003), which can make species distinction difficult. Simon et al (2010) highlight the fact that on-axis clicks from bottlenose dolphins can have their peak energy at 100 kHz or higher and thus presumably could be detected on T-PODs with "porpoise" settings (target filter at 130 kHz) but might fail to be detected by a T-POD with "dolphin" settings (where the target filter is set at 50 kHz). However, off-axis bottlenose dolphin clicks can have peak energy at around 50 kHz and are therefore likely to be detected with "dolphin" settings.…”

“…Passive acoustic monitoring (PAM) techniques involve the detection of cetacean vocalizations from either towed or static hydrophones, and this method is increasingly being used to collect data on cetacean habitat use (e.g., Rayment et al, 2009b;Simon et al, 2010), behaviour (e.g., Leeney et al, 2007;Van Parijs et al, 2009;Akamatsu et al, 2010;Clausen et al, 2010;Kyhn et al, 2010), and even to estimate abundance (e.g., Marques et al, 2009;Whitehead, 2009). Static acoustic monitoring (SAM), using moored equipment to detect cetacean vocalizations from a fixed area, enables the observation of trends in relative abundance and of behaviours of target animals within a focal area (Kimura et al, 2010) and has several advantages over visual techniques.…”

“…The T-POD is a self-contained, submersible hydrophone and digital processor which recognizes and logs the echolocation clicks made by odontocetes. Todd et al (2009) provide a description of the T-POD hardware and of the processes of data collection and classification, while numerous studies have reviewed settings and T-POD functionality (e.g., Thomsen et al, 2005;Philpott et al, 2007;Kyhn et al, 2008;Simon et al, 2010). The onboard data logging nature of the T-POD makes it particularly suited to the study of odontocetes, which use narrow-band high frequency (NBHF) echolocation clicks, such as the Phocoenids (for which the instrument was initially designed; Goodson & Sturtivant, 1996) and members of the Cephalorhynchus genus, as the sampling rate required for real-time recording (> 250 kHz) makes long-term recordings unfeasible.…”

Acoustic monitoring of habitat use by bottlenose dolphins in Doubtful Sound, New Zealand

“…T-POD software allows the user to set each scan independently to match the echolocation characteristics of the target species. T-PODs have become widely used to investigate cetacean habitat use, distribution and behaviour (Jefferson et al 2002;Cox & Read 2004;Leeney et al 2007;Philpott et al 2007;Koschinskietal.2008;Raymentetal.2009;Simon et al 2010;Bailey et al 2010). Doubtful Sound, New Zealand, is home to a resident population of about 56 (CV 01%) bottlenose dolphins (Tursiops truncatus) living approximately at the southern limit of the species' distribution (Curry et al 2007).…”