Validating automated click detector dolphin detection rates and investigating factors affecting performance

Abstract

Passive acoustic monitoring (PAM) is a widely used technique for studying the distribution and habitat use of cetaceans. The C-POD, an acoustic sensor with an onboard automated click detector, has been deployed in diverse acoustic environments, but studies verifying its offshore detection rates and factors affecting detection probability are scarce. To empirically evaluate the performance of C-PODs in detecting bottlenose dolphins (), C-PODs were deployed alongside archival acoustic recorders 12-30 km offshore in the Northwest Atlantic Ocean. The C-POD and acoustic recordings, post-processed using PAMGUARD software, were compared for a period of 6852 h. C-POD false positive rates were very low (mean 0.003%), and positive hourly detection accuracy was very high (mean 99.6%). Analysis of the acoustic environment and dolphin click characteristics revealed that true positive detections by C-PODs were significantly more likely to occur when PAMGUARD detected more clicks and there was increased high frequency noise (>20 kHz), likely from distant or unclassified clicks. C-PODs were found to be reliable indicators of dolphin presence at hourly or greater time scales. These results support the application of C-PODs in PAM studies that aim to investigate patterns of dolphin occurrence, such as those related to offshore windfarms.

Empirical evidence that large marine predator foraging behavior is consistent with area‐restricted search theory

“…, Garrod et al. ). These devices continuously monitor the frequency range 20–160 kHz, although they do not record the complete soundstream (details available online ) .…”

“…Data were collected using C-PODs (Chelonia, Cornwall, UK), which detect and log odontocete echolocation clicks. The C-PODs were moored approximately 3 m from the seabed attached to a 45-kg anchor weight and were retrieved and redeployed approximately every 3 months (Wingfield et al 2017, Garrod et al 2018). These devices continuously monitor the frequency range 20-160 kHz, although they do not record the complete soundstream (details available online).…”

Effects of intense storm events on dolphin occurrence and foraging behavior

“…While no dolphins were detected during the 2 loudest days of our study period (average sound levels of up to 126 dB re 1 µPa rms in 2015), on the loudest days in 2016 and 2017, dolphin presence and foraging were detected despite elevated ambient sound levels during the storm. A previous study in this region showed that the C-POD's ability to detect dolphins (based on click trains between 20 and 160 kHz) was not significantly affected by broadband (2 Hz-24 kHz, up to 130 dB re 1 µPa rms) ambient sound levels 53 . Storm noise, which has most of its energy in lower frequencies, did not appear to greatly reduce detections by the C-POD.…”

“…While dolphins were detected throughout our study period, including on days with high ambient sound levels (up to 123 dB re 1 µPa rms), elevated ambient sound levels during the storms likely masked dolphin clicks 50,51 or decreased the detection range of the C-POD 52 . The C-POD's KERNO classifier is also a conservative classifier of dolphin clicks 53 . While no dolphins were detected during the 2 loudest days of our study period (average sound levels of up to 126 dB re 1 µPa rms in 2015), on the loudest days in 2016 and 2017, dolphin presence and foraging were detected despite elevated ambient sound levels during the storm.…”

“…At the four sites in this area (Site 1: nearest inshore-Site 4: farthest offshore, Fig. 1a), water depths ranged from approximately 20-45 m, and acoustic recording instruments were deployed with the hydrophone located approximately 1.5 m above the ocean floor using bottom-anchored moorings (see 53,55 (Fig. 1a).…”