Harbour porpoise (Phocoena phocoena) in the Wadden Sea World Heritage Site and requirements for trilateral monitoring

The harbour porpoise (Phocoena phocoena) is considered part of the ‘Outstanding Universal Value’ characterising
the Wadden Sea World Heritage Site (WS WHS). The Trilateral Wadden Sea Plan aims to preserve the conservation
status of the Trilateral Wadden Sea Cooperation Area, encompassing the WS WHS. The plan has specified two
conservation targets for the harbour porpoise: (1) viable stocks and a natural reproduction capacity and (2) conservation
of habitat quality for its conservation. To assess the current occurrence of the harbour porpoise in the Wadden
Sea area, we collated and analysed data from regional and national research projects using telemetry, aerial surveys,
strandings and passive acoustic monitoring, obtained over the years 1990–2020. The results illustrate that porpoises
occur in both offshore and intertidal waters, showing seasonal movements and changes in local occurrence over time.
Some porpoises displayed limited home ranges throughout the year, suggesting a possible residency for some of the
animals using the Wadden Sea area. We also showed that methods, frequency and spatial coverage of monitoring
activities vary among the countries Denmark, Germany and the Netherlands. We discuss the suitability of the different
methods both regarding the challenges of monitoring in the complex Wadden Sea habitat as well as their ability
to target the conservation aims of the WHS. We give several recommendations to assess the status of the species to
meet the identified conservation aims.