Passive acoustic monitoring of harbour porpoise behaviour, distribution and density in tidal rapid habitats

Abstract

Toothed whales produce regular echolocation clicks to sense their surroundings and hunt. These clicks can be detected on hydrophones allowing researchers to study and monitor animals, a methodology known as passive acoustic monitoring (PAM). PAM methods can be used to detect behaviours such as foraging, calculate the absolute abundance and determine the 3D positions of soniferous animals. However, this requires robust detection, species classification and localisation algorithms alongside a statistical framework to estimate animal density. Detection, classification and localisation methods were developed to obtain fine scale behavioural information of toothed whales which was then integrated with a distance sampling-based framework to estimate density. These methods were applied to calculate the behaviour, 3D distribution and density of harbour porpoises in a tidal stream habitat.

Toothed whales produce clicks in sequences (clicks trains) which can provide information on behavioural state and aid in species identification. A click train detection algorithm was developed to extract these sequences and performance then tested on PAM data from sperm whales, delphinids and harbour porpoise datasets.

Localisation in tidal rapids is difficult due to fast moving currents and high densities of toothed whales producing a complex soundscape. A drifting free-hanging vertical hydrophone array with movement sensors, click aliasing and track association algorithms was developed to collect geo-referenced 3D tracks of toothed whales in tidal streams.

The full 4π beam profile of a free-swimming captive harbour porpoise was measured. This showed that harbour porpoises have significant acoustic energy at extreme off-axis angles (>30°) which has implications for detectability on PAM devices.

Drifting sound recorders and vertical hydrophones arrays were used to survey a tidal rapid site. Detected click trains, dive data and the measured beam profile were used to inform simulations of detection probability of harbour porpoise clicks and calculate animal density and foraging rates.