Discussion:
To the best of our knowledge, this study is the first to evaluate the
performance of co-deployed C-POD and F-POD devices in a field setting
for monitoring harbour porpoise. Previous studies have evaluated the
performance of the C-POD with other types of full bandwidth recorders
(incl. Soundtrap, DMON), with the C-POD typically performing well with
an overall high degree of accuracy (Roberts and Read, 2015; Sarnocinska
et al., 2016; Jacobson et al., 2017). Our results suggest that at
appropriate temporal scales, C-PODs provide comparable results to the
newer F-PODs and with certain caveats, F-PODs would be suitable
replacements for C-PODs in existing monitoring programmes as C-PODs
reach the end of their serviceable lifetime.
Data from click detectors such as C-PODs, its predecessor T-PODs, and
now the F-PODs have been used to monitor small cetaceans, as well as
their responses to anthropogenic activities in numerous settings
including pile driving, seismic surveys, and fisheries deterrent devices
(e.g. Philpott et al., 2007; Thompson et al. , 2013; Omeyeret al. , 2020; Todd et al., 2020). Our results show that the F-POD
consistently detects more echolocation clicks and foraging buzzes than
the C-POD across the temporal scales of minutes, hours and days, as well
as all train quality classification groupings. Lower detection rates by
the C-POD is to be expected due to advances in F-POD electronics and
software to capture more information on individual echolocation clicks
and enhance train detection (Chelonia Ltd, 2022). This poses a potential
issue for researchers engaged in long-term monitoring, with questions
about how comparable different POD types may be, potentially affecting
time-series as C-PODs are eventually replaced by F-PODs. This study
shows that both C-PODs and F-PODs detected similar patterns of
occurrence and echolocation activity. As indicated by Garrod et al.
(2018), detection metrics at a minimum of an hourly scale are
representative of relative occurrence, enabling temporal trends to be
determined. Detections at the broader scale of detection positive days
were found to match best between both PODs with little discrepancy
between them. Therefore, for direct comparison between C-POD and F-POD
data, detection positive days, and using combined classifications of Hi
Mod and Lo, is the only detection metric recommended. However, such a
metric would be insufficient for identifying fine-scale temporal
patterns of occurrence of behaviour in response to factors such as
diurnal changes in prey availability or tidal state, both known to
influence harbour porpoise occurrence and feeding behaviour (Schaffeld
et al., 2016; Zein et al., 2019)
Our results highlighted that F-PODs appear to be much more capable at
identifying harbour porpoise click trains with confidence than the C-POD
(i.e. classified as high quality by the KERNO classifier). However, when
considering the combined train quality classification groupings (i.e.
HiMod and HiModLo) the two PODs are substantially more comparable.
Researchers considering using a time series consisting of data from
C-PODs and F-PODs for analysis of temporal trends should consider using
the combined classifications, provided extensive visual validation is
followed, particularly for low quality trains to eliminate possible
false positive detections. The enhanced train detection specified by the
manufacturer has also been demonstrated in our results with F-POD
continually detecting more harbour porpoise detections than the C-POD by
a factor of 1.38 across the deployment period. Comparability between the
PODs was however found to be variable between seasons, with the highest
detection ratio in spring and summer making detection rates on the PODs
less comparable. Detection ratios such as explored here could be
investigated further within monitoring programmes looking to transition
to the use of F-PODs. Understanding how the devices compare in various
deployment sites can help for the interpretation of long-term data
beyond the lifespan of the C-POD and avoid misinterpretation of the data
(for example interpreting a false increase in occurrence due to
differing device sensitivities).
Investigating spatial and temporal patterns in species occurrence is
often the crux of ecological monitoring (e.g., Jones et al., 2014;
Williamson et al., 2017; Zein et al., 2019). Generalised additive models
were used to investigate whether detections from both types of POD have
the capacity to identify the same temporal drivers of porpoise
occurrence and foraging activity. The occurrence models for both PODs
highlighted the same temporal patterns, and same environmental
predictors with very similar effect sizes, suggesting that analyses
using data from both C-PODs and F-PODs will not be affected greatly by
POD type. However, it would be prudent to include POD type as a fixed
factor in any such analysis. Conversely, the models investigating
feeding buzzes did not provide analogous results. No temporal patterns
were found using the C-POD data, possibly due to the much lower
detection rates of feeding buzzes using C-PODs, with the F-POD detecting
three times as many foraging buzzes in comparison, and 10 times as many
echolocation clicks overall. The higher detection rates of feeding
buzzes by the F-POD enabled detection of temporal patterns including an
increase in foraging buzzes from autumn to winter, and an increase in
foraging activity during the day. The specific nature of the
relationships and their ecological context is outside of the scope of
the current study, but the contrasting ability of the PODs to detect
feeding buzzes is particularly relevant in the context of integrating
F-PODS into long-term datasets. The increased click detection capacity
of the F-POD now enables fine-scale analysis of foraging or social
behaviours (demonstrated by high click rates (Clausen et al.,2010)), that has perhaps been missed or underestimated using C-PODs.
Additionally, F-PODs were found to be less effected by environmental
noise levels within the 20-160KHz noise band, as indicated byNall (Clausen et al., 2018). It is plausible that decreased
detections on the C-POD during periods of increasing environmental noise
is a consequence of detector performance, which has been overcome during
the development of the F-POD in conjunction with the increased click
detection ability.
Long-term datasets and consistency of monitoring methods throughout the
duration of monitoring programmes are important to enable long-term
trends to be identified, particularly in areas of high conservation
importance. Changing controllable factors such as monitoring equipment
can skew our understanding of these long-term trends and in turn make it
more difficult to interpret a change in habitat use, or behaviour of a
species, which can be detrimental in the event of a disturbance or
imminent threat. Static acoustic monitoring using PODs has been an
integral part of cetacean monitoring programmes exploring habitat use
and behaviour. Our results show than the C-POD and the F-POD are
consistently comparable at the broad scale of identifying porpoise
presence, and produce similar results when modelling environmental
correlates of occurrence. However, the C-POD failed to detect the more
nuanced patterns detected by the F-POD, particularly when investigating
foraging behaviour versus occurrence. On account of its greater
sensitivity and increased detection rates for harbour porpoise the F-POD
certainly can be a useful tool to integrate into acoustic monitoring
programmes. While the introduction of F-PODs into long-term time series
is unlikely to change our understanding of the environmental drivers of
occurrence, it is advisable that detections from one POD type should not
be directly compared with detections from another as this could give
erroneous results of increased occurrence due to differing detection
rates rather than a true increase in individuals. Furthermore, any
studies transitioning between PODs or combining C-POD and F-POD data
should consider including POD type as a factor when conducting
time-series. While the current study only investigated comparability in
POD performance for detecting acoustic activity of harbour porpoise, it
is likely that analogous differences would be seen for other cetacean
species recorded by PODs. This study has given insights that the F-POD
will be invaluable for future monitoring of harbour porpoise and other
cetacean species, however care and consideration must be taken to make
C-POD data adaptable for the integration into future studies.