Passive acoustic methods for fine-scale tracking of harbour porpoises in tidal rapids

Abstract

The growing interest in generating electrical power from tidal currents using tidal turbine generators raises a number of environmental concerns, including the risk that marine mammals might be injured or killed through collision with rotating turbine blades. To understand this risk, information on how marine mammals use tidal rapid habitats and in particular, their underwater movements and dive behaviour is required. Porpoises, which are the most abundant small cetacean at most European tidal sites, are difficult animals to tag, and the limited size of tidal habitats means that any telemetered animal would be likely to spend only a small proportion of time within them. Here, an alternative approach is explored, whereby passive acoustic monitoring (PAM) is used to obtain fine scale geo-referenced tracks of harbour porpoises in tidal rapid areas. Large aperture hydrophone arrays are required to obtain accurate locations of animals from PAM data and automated algorithms are necessary to process the large quantities of acoustic data collected on such systems during a typical survey. Methods to automate localisation, including a method to match porpoise detections on different hydrophones and separate different vocalising animals, and an assessment of the localisation accuracy of the large aperture hydrophone array are presented.

Localizing individual soniferous fish using passive acoustic monitoring

“…Uncertainty increases rapidly when the sound source approaches the plane bisecting at right angles between two hydrophones (the area close to the array) because the position becomes equidistant from each of the hydrophones and the TOAD approaches zero making the choice between two solutions indeterminate (Cato, 1998). Uncertainty in acoustic localization calculations improves as the hydrophone spacing increases, because uncertainty in TOADs are proportionally smaller (Cato, 1998;Macaulay et al, 2017). However, array spacing is ultimately limited by the requirement that the hydrophones must be close enough together to ensure that there is adequate signal to noise ratio for the signal received on the hydrophone farthest from the source (Samaran et al, 2010).…”

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

“…In this section we describe three examples. Hastie et al (2006) A non-static application of PAM to obtain dive data has been used by Macaulay et al (2017) in tidal rapids. In this approach the array drifted with the tides (from a drifting vessel) while collecting the data, which minimises the flow noise from the currents.…”

“…The vertical array (8 hydrophones) and the cluster array (4 hydrophones) used in the tidal stream. From: Macaulay et al (2017).…”

“…Unfortunately, the same characteristics also make this species difficult to track underwater. The high directionality of the narrow-band clicks, and the high attenuation due to the high frequency (among the highest in the animal kingdom), make harbour porpoises especially ill-suited to be localised and tracked through their vocalisations (Macaulay et al, 2017).…”

Passive acoustic methods for tracking the 3D movements of small cetaceans around marine structures

“…64 Quick and Janik, 2008). Arrays of hydrophones can been used to detect and locate cetaceans 65 underwater (e.g., Watkins and Schevill, 1972;Macaulay et al, 2017) and it is possible to track the 66 movements of cetaceans in the vicinity of anthropogenic structures [16].…”