In July 2022, Aarhus University was contracted by SWECO AB to conduct a one-year baseline study and impact assessment of harbour porpoises as well as noise monitoring within the pre-investigation area for the Arkona Offshore Wind Farm (OWF). Aarhus University recommended passive acoustic monitoring (PAM) within the project site for the Arkona Offshore Wind Farm, for which the deployment of five stations in the area was agreed. The proposal was based on the following:

• There have never been any PAM or other studies for harbour porpoises conducted in the area. The new stations are needed to examine whether this – like the neighbouring Natura 2000 site – is an important area for porpoises and, if so, which seasons are important.
• The OWF site is in the transition zone between the Belt Sea harbour porpoise population and the critically endangered (assessed by International Union for Conservation of Nature (IUCN) and the Helsinki Commission (HELCOM)) Baltic Proper population of harbour porpoises and thus may be used by both populations.
• Five stations have been shown in the Danish National Monitoring Program to provide sufficient data to cover variation in distribution in similar sized areas.

Here, we report the data collected by the five CPOD stations for one year.