What the F-POD? Comparing the F-POD and C-POD for monitoring of harbour
porpoise (Phocoena phococena).
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.