Tracking porpoise underwater movements in tidal rapids using drifting hydrophone arrays

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 cetaceans might be injured or killed through collision with rotating turbine blades. To understand this risk we need better information on how cetaceans use tidal rapid habitats and in particular their underwater movements and dive behavior. Focusing on harbor porpoises, a European protected species, we have developed an approach which uses time of arrival differences of narrow band high frequency (NBHF) clicks detected on large aperture hydrophone arrays drifting in tidal rapids, to determine dive tracks of porpoises underwater. Probabilistic localization algorithms have been developed to filter echoes and provide accurate 2D or geo-referenced 3D locations. Calibration trials have been carried out that show that the system can provide depth and location data with submeter errors. Data collected over three seasons in tidal races around Scotland has provided new insights into how harbor porpoises are using these unique habitats, information vital for assessing the risk tidal turbines may pose.