Abstract
This report evaluates the cumulative impacts of offshore wind farm (OWF) development and associated vessel traffic on harbour porpoise (Phocoena phocoena) populations in North Sea and inner Danish waters (here defined as southern Kattegat, the Belt Seas, the Sound, and the Western Baltic). Its purpose is to determine whether current and planned OWF construction could result in population-level effects on this protected species, providing evidence to guide strategic environmental planning and marine spatial management.
Harbour porpoises are potentially sensitive to underwater noise from pile-driving and vessel traffic, raising concerns about cumulative impacts as off-shore wind expands. Using the DEPONS agent-based model, we simulated porpoise movements, energetics, and survival under realistic development scenarios for the North Sea and the inner Danish waters. Scenarios incorporated turbine installation noise, construction and maintenance vessel activity, commercial vessels (represented by AIS-recorded traffic), and potential prey enhancement near turbines. Control scenarios included only baseline prey and commercial traffic. Simulations ran for 26 years, comparing population trajectories under impact and control conditions.
Across all scenarios and spatial scales, no measurable population-level impacts were detected. Porpoise abundance and distribution remained stable in both regions, with seasonal fluctuations driven by natural cycles rather than anthropogenic stressors. Even within monitoring blocks surrounding Danish OWFs, no consistent displacement or demographic effects emerged. These results indicate that, under current development trajectories and mitigation practices, OWF construction and associated vessel traffic on their own are unlikely to cause population declines in harbour porpoises.
The findings provide a scientific basis for continued offshore wind develop-ment in Danish waters, provided that cumulative impacts are periodically reassessed. While encouraging, these results do not eliminate all risk. The model assumes effective noise mitigation and does not account for additional stressors such as contaminants, fisheries interactions (e.g. bycatch and food depletion), or climate-driven habitat shifts. Future assessments should refine prey modelling, incorporate a broader range of stressors, and explore multi-species frameworks to capture ecosystem-level effects. Additionally, the Belt Sea population of harbour porpoises inhabiting the inner Danish waters are currently undergoing a large-scale decline. Without understanding and addressing the drivers of these losses, it remains challenging to determine the added contribution of offshore wind development to cumulative impacts.
Under the conditions and assumptions tested, large-scale OWF development in Danish waters appears compatible with harbour porpoise conservation objectives. This supports ongoing planning and licensing decisions, while highlighting the need for adaptive management and updated cumulative impact assessments as offshore wind expands.