Acoustic surveys confirm the high-density areas of harbour porpoises found by satellite tracking

The population status of the harbour porpoise (Phocoena phocoena) has long been of concern in European waters. Consequently, the
European Commission (EC) Habitats Directive obligates all EC member states to designate marine protected areas (MPAs) for harbour
porpoises before 2012. These will be designated areas having the greatest density of porpoises. However, little is known about comparability
between the monitoring methods used to examine porpoise distribution and density, and conflicting results may arise,
especially when considering their varying sample size and temporal and spatial scales. Here, vessel-based acoustic surveys are seen
as an independent method of testing the temporal and spatial permanence of previously identified areas of high density of
harbour porpoises found by satellite-tracking them in inner Danish waters. Based on six acoustic surveys, a strong spatial accord
was found between the number of acoustic detections of harbour porpoises and their density distribution obtained from 10 years
of satellite tracking. The results confirm the presence and permanence of areas of high density of porpoises and validate the two
methods for identifying and monitoring future MPAs for the species.