Abstract:
Passive acoustic monitoring (PAM) is a cost-effective method for
monitoring cetacean populations compared to techniques such as aerial
and ship-based surveys. The C-POD (Cetacean POrpoise Detector) has
become an integral tool in monitoring programmes globally for over a
decade, providing standardised metrics of occurrence that can be
compared across time and space. However, the phasing out of C-PODs
following development of the new F-POD (Full waveform capture Pod) with
increased sensitivity, improved train detection, and reduced false
positive rates, represents an important methodological change in data
collection, particularly when being introduced into existing monitoring
programmes. Here, we compare the performance of the C-POD with that of
its successor, the F-POD, co-deployed in a field setting for 15 months,
to monitor harbour porpoise (Phocoena phocoena) . While similar
temporal trends in detections were found for both devices, the C-POD
detected only 58% of the detection positive minutes (DPM), recorded by
the F-POD. Differences in detection rates were not consistent through
time making it difficult to apply a correction factor or directly
compare results obtained from the two PODs. To test whether these
differences in detection rates would have an effect on analyses of
temporal patterns and environmental drivers of occurrence, generalised
additive models (GAMs) were applied. No differences were found in
seasonal patterns or the environmental correlates of porpoise occurrence
(month, diel period, temperature, environmental noise, and tide).
However, the C-POD failed to detect sufficient foraging buzzes to
identify temporal patterns in foraging behaviour that were clearly shown
by the F-POD. Our results suggest that the switch to F-PODs will have
little effect on determining broad-scale seasonal patterns of
occurrence, but may improve our understanding of fine-scale behaviours
such as foraging. We highlight how care must be taken interpreting F-POD
results as indicative of increased occurrence when used in time-series
analysis.