Abstract
With the ongoing development of offshore wind energy in the North Sea, a substantial part of the distribution of harbour porpoises (Phocoena phocoena) will overlap with offshore wind farms (OWFs) in the near future. Effects of the construction of OWFs on the distribution and ecology of harbour porpoises has received a lot of attention in research and impact assessments for many years, however, data on the presence of harbour porpoises in and around operational OWFs are relatively scarce. Gaining more knowledge on the spatial and temporal occurrence of harbour porpoises in OWFs is a prerequisite for adequate impact management. As part of ornithological monitoring programmes, 174 visits to multiple OWFs in the North Sea were carried out between 2007 and 2023. Following standardized observation protocols for birds, marine mammal observations were also recorded during these visits. Additionally, harbour porpoises were recorded during Digital Aerial Surveys (DAS) in and around the operational wind farm Borssele in 2021. We combined all harbour porpoise observations from these studies with the aim to study temporal and spatial use of operational OWFs. Harbour porpoises were seen year-round inside OWFs with highest abundances in winter. We found additional (smaller) peaks in autumn (September and October), which, to our knowledge, has not yet been reported before. A comparison of observations in two nearby OWFs separated by a decade suggests an increase in the abundance of harbour porpoises in the area over time. Behaviour was not systematically recorded but foraging behaviour inside the wind farm area was observed on a small number of occasions. Harbour porpoises were occassionally observed at close distances to operational wind turbines, even foraging. However, data of DAS shows that the probability of observing a harbour porpoise significantly decreases closer to wind turbines, which strongly suggests that harbour porpoises avoid close distances to operational wind turbines. Their numbers stabilise at distances of around 500m to wind turbines. Furthermore, we found that harbour porpoise densities do not significantly differ inside a wind farm corridor compared to the wind farm border or inside the wind farm area.