Abstract
A vertical hydrophone array has been developed to provide new insights into harbour porpoise dive depths, underwater movements and echolocation behaviour in tidal habitats. This information is of direct relevance to understanding and reducing environmental impacts of tidal current driven turbines. Analysis of field data is on-going however we have demonstrated that it is possible to investigate the behaviour of porpoises in tidal areas using passive acoustics and further work is likely to yield reliable depth distributions.