Diurnal variation in harbour porpoise detection—potential implications for management

Abstract

Robust information on animal distributions and foraging behaviour is required to target management and conservation measures for protected species and populations. Visual survey data are commonly used to model these distributions. However, because visual data can only be collected in daylight, modelled distributions and consequent management actions may fail to identify or protect important nocturnal habitats. We explored this issue using data from the Moray Firth, Scotland, where visual survey data have previously been used to characterise habitat use and distribution patterns of harbour porpoises Phocoena phocoena. Marine predators such as harbour porpoises have a widespread distribution, are highly mobile and are known to exhibit behavioural variation in relation to diel cycles. Here, we used long-term passive acoustic data which revealed habitat-specific differences in diel patterns of detection. Harbour porpoises were detected consistently during night and day in sandy areas, with peaks in detection around sunrise and sunset, and at night in muddy areas. Detections also varied with depth, with the greatest proportion of daytime detections recorded in shallower sandy areas, and the most nighttime detections recorded in deeper muddy areas. The proportion of detections with foraging buzzes increased slightly during the night and in muddy habitats. These findings suggest that the importance of muddy habitats could be underestimated when using visual survey data alone. This highlights the value of using a combination of visual and acoustic methods both to characterise species distribution and to support efforts to develop appropriate spatio-temporal management of key habitats.

Fine scale spatial variability in the influence of environmental cycles on the occurrence of dolphins at coastal sites

“…This study highlights the importance of understanding temporal variation in behaviour to underpin more dynamic management of Marine Protected Areas. Although nocturnal activity has been detected in many studies of these animals 23,38,63 , visual methods are still widely used to study their distribution 64,65 . Visual surveys only provide data during daylight hours and may be underestimating the importance of certain locations during the night 23 .…”

“…For instance, without including temporal variables in the models, it was found that a sandy bank was an important site for harbour porpoises in the Moray Firth 22 . However, once the diel cycle was included, it was found that adjacent muddy areas were also important habitats for them during the night 23 . The management of potential stressors relies on accurate information about the distribution of focal species; consequently, the inclusion of both spatial and temporal variables becomes necessary to implement efficient protection measures for highly mobile species.…”

Feasibility of a Passive Acoustic Monitoring (PAM) network for Harbour Porpoises on the Dutch Continental Shelf

“…Apart from seasonal patterns harbour porpoises exhibit circadian patterns in their behaviour. PAM studies showed diurnal patterns in harbour porpoise acoustic activity in a wide range of places, from Danish and German North Sea waters (Teilmann et al, 2013;Schaffeld et al, 2016), via the Moray Firth in northern Scotland (Williamson et al, 2017), to the Wadden Sea (Zein et al, 2019). Porpoise click activity in most locations seems to be highest at night or in the morning.…”

Presentation of Binning-Based Inter-Click Interval Data from Passive Acoustic Monitoring of Free-Ranging Harbour Porpoises (Phocoena Phocoena)

“…Previous studies investigating characteristics of harbour porpoise echolocation, both in captivity and in the wild, have reported that variations in click rates and minICI values appear to be correlated strongly with behaviour and habitat usage. 16,35 Observed differences in click rates and minICI values showed diel changes in harbour porpoise habitat usage within the survey area, and were likely correlated with habitat complexity.…”

“…14 C-PODs (and their analogue predecessor, T-PODs), have been used widely to study many odontocete species, including inter alia, harbour porpoises. 15,16 C-PODs are described in the available literature. 11 C-POD click-train recognition algorithms filter out non-cetacean clicks and give reliable data on cetacean presence and echolocation behaviour by using digital waveform characterisation to select and log time of cetacean tonal ultrasonic sounds from 120-160 kHz.…”