Spatiotemporal drivers of harbour porpoise occurrence and foraging
activity
GAMs were run to compare the temporal trends and environmental
predictors of harbour porpoise occurrence between C and F-PODs. The best
models for detection positive minutes per hour (DPM/h) from both PODs
retained all explanatory variables (all variables significant in both
the C-POD and the F-POD models). Effect sizes of variables retained
within the models were remarkably consistent between the C-POD and F-POD
models, with the exception of noise, where this showed a considerably
higher effect within the C-POD model (Table 1).
Similar temporal trends were highlighted by the C-POD and F-POD models
in terms of harbour porpoise DPM/h per month and throughout the diel
cycle (Figure 3). Both tidal range and time to high tide were
significant, reflecting similar trends from both devices with an
increase in detections during the ebb tide and during tidal ranges
associated with spring tides (Figure 3), however, both variables had
relatively small effect sizes within the model (Table 1). The C-POD
model highlighted a much greater effect of noise on harbour porpoise
detections, with a clear decrease in porpoise detections at higher noise
levels, occurring at a much lower noise threshold for the C-POD than the
F-POD (Figure 3). Harbour porpoise detections were found to decrease
with increasing water temperature for both devices (Figure 3).
In contrast to the similar temporal patterns shown via the occurrence,
strong differences occurred between models of foraging buzzes between
C-PODS and F-PODS using buzz clicks per hour (BPH) as the response
variable.
The C-POD foraging model did not retain any temporal variables despite
them remaining the most influential covariates within the F-POD foraging
model, likely due to a much-reduced sample size of identified feeding
buzzes by the C-POD. Neither time difference to high tide or tidal range
were found to influence harbour porpoise foraging behaviour in either
C-POD or F-POD models. The F-POD model suggested a decrease in foraging
buzzes between July and September, and an increase in foraging buzzes
detected during the day (Figure 4). Similar to the detection model,
foraging activity decreased with increasing water temperature, with
highest buzz detections around 10° C. In contrast to the C-POD model,
the F-POD foraging model found no significant effect of noise on
foraging activity (Table 2).