High resolution three-dimensional beam radiation pattern of harbour porpoise clicks with implications for passive acoustic monitoring

Abstract

The source properties and radiation patterns of animal vocalisations define, along with propagation and noise conditions, the active space in which they can be detected by conspecifics, predators, prey and by passive acoustic monitoring (PAM). Here we report the 4π (360° horizontal and vertical) beam profile of a freeswimming, trained harbour porpoise measured using a 27-element hydrophone array. The forward echolocation beam is highly directional, as predicted by a piston model, and is consistent with previous measurements. However, at off-axis angles greater than ±30°, the beam attenuates more rapidly than the piston model and no side lobes are present. A diffuse back beam is also present with levels about-30 dB relative to the source level. In PAM, up to 50% of detections can be from portions of the beam profile with distorted click spectra, although this drops substantially for higher detection thresholds. Simulations of the probability of acoustically detecting a harbour porpoise show that a traditional piston model can underestimate the probability of detection compared to the actual 3D radiation pattern documented here. This highlights the importance of empirical 4π measurements of beam profiles of toothed whales, both to improve understanding of their biology and to inform PAM.

Vertical sonar beam width and scanning behavior of wild belugas (Delphinapterus leucas) in West Greenland

“…Echolocation is characterized by highly-directional acoustic emissions where signals become increasingly distorted off-axis to the center of the beam [ 38 , 42 , 70 , 71 ]. As a result, accurate sonar parameter estimates are contingent on the ability to isolate on-axis clicks.…”

Boat disturbance affects the acoustic behaviour of dolphins engaged in a rare foraging cooperation with fishers

“…Since the literature states that whistles are key for the flow of information among dolphins (Van Ginkel et al ., 2017) and echolocation clicks are known to be used for prey detection and recognition (Au, 1993), the reduction in whistle and clicks rates in the presence of boats suggest that boat noise has the potential to disrupt the cooperative foraging interaction, or change how dolphins behave during the interaction. For example, the reduction in clicks rates could be due to a change in the position/direction of dolphins in relation to the fishers and the hydrophone, reducing the synchrony of the interaction and also the detection rate by the hydrophone due to the directionality of echolocation clicks (although echolocation signals are detected up to 150° horizontal angle outside the main axis of the transmission beam; see Finneran et al ., 2014; Macaulay et al ., 2020). A reduction in sound emission and interruptions or changes to foraging activity resulting from boat traffic has already been observed in other bottlenose dolphin populations (e.g., Luís, Couchinho & dos Santos, 2014; Pirotta et al ., 2015).…”

An increase in detection rates of the critically endangered Baltic Proper harbor porpoise in Swedish waters in recent years

“…This device contains an omni‐directional hydrophone that records the timing of zero‐crossings (accuracy to 200 ns) and the peak amplitude between zero crossings (Tregenza, Dawson, Rayment, & Verfuss, 2016). These data are then used to identify the narrow‐band high‐frequency (NBHF) clicks of harbor porpoises (Au, Kastelein, Rippe, & Schooneman, 1999; Macaulay, Malinka, Gillespie, & Madsen, 2020; Villadsgaard, Wahlberg, & Tougaard, 2007). Data were collected from 12 stations (average depth 44 m, range 29.0–60.0 m) that were used both as a part of the SAMBAH project (April 2011–July 2013) and the SNMP (April 2017–March 2020) (Figure 1).…”

“…An equation that calculates how the absorption of sound (dB/km) is influenced by various acoustic (frequency) and environmental (temperature, salinity, depth, acidity) factors from a previous study (Ainslie & McColm, 1998) was used to investigate the potential effect of temperature changes over the years on the detection rate. To do this, we assumed a frequency of 130 kHz (based on the likely frequency of harbor porpoise signals (Macaulay et al, 2020; Villadsgaard et al, 2007)), salinity of 8 ppt (given the brackish waters of the Baltic Sea), depth of 44 m (average depth of the C‐PODs), and acidity of 8 pH.…”