Sensitivity mapping of harbour seals, grey seals and harbour porpoises to the construction and operation of offshore wind farms in Danish waters 

Preface

This report contributes to the project “Environmental mapping and screening of the offshore wind potential in Denmark” initiated in 2022 by the Danish Energy Agency. … The project is carried out by NIRAS, Aarhus University (Department of Ecoscience) and DTU Wind. The overall project consists of four tasks …:

1. Sensitivity mapping of nature, environmental, wind and hydrodynamic conditions.
2. Technical fine-screening and assessment of the overall offshore wind potential based on the sensitivity mapping and relevant technical parameters.
3. Assessment of potential cumulative effects from large-scale offshore wind development in Denmark and neighbouring countries.
4. Assessment of barriers and potentials in relation to coexistence.

This report addresses one component of Task 1: sensitivity mapping. Specifically, it provides an overview of areas within Danish offshore regions that are likely to be particularly vulnerable to offshore wind farm development regarding harbour seals, grey seals and harbour porpoises based on available data. …

The project has relied predominantly on historical data, with minimal new data collection. As a result, the sensitivity mapping is largely dependent on the availability and accessibility of pre-existing data across specific subject areas. From the outset, significant effort was made to incorporate all relevant data to comprehensively address the task requirements. However, certain existing datasets could not be accessed. … It is important to recognise that sensitivity mapping serves as a dynamic tool, which can be updated as new data becomes available.

Summary

This report provides an assessment of the sensitivity of harbour seal (Phoca vitulina), grey seal (Halichoerus grypus), and harbour porpoise (Phocoena phocoena) populations to offshore wind farms in Danish waters. We combine species distribution maps and publicly available studies on the effects of construction and operation of offshore wind farms to identify areas with the highest, medium and lowest sensitivity for each species. We define sensitivity as the relative abundance of a species, multiplied by the consequence of constructing a wind farm in a given area. Both are scaled to lie in the range 0–1. Whereas several studies have investigated the short-term effects of individual wind turbines, few studies have addressed the long-term consequences of entire wind farms. Published studies were therefore supplemented with expert knowledge, which enabled pinpointing populations and areas where the consequence of establishing a wind farm would be particularly large (e.g., breeding grounds). The rationale for the judgements is discussed. The authors underline that this assessment is limited due to the scarcity of studies on consequences of offshore wind on the spatial and temporal scales that would be needed to more fully understand animal sensitivities and due to the limited knowledge of the joint impacts of multiple wind farms and the cumulative impacts of different stressors. We highlight specific types of data that should be collected to produce more robust predictions of marine mammal sensitivity to offshore wind farms.

Introduction

The installation of an increasing number of turbines with larger turbine sizes will impact marine species on several levels. The construction of turbines results in high levels of anthropogenic noise, particularly for foundations installed with percussive piling (monopiles and jacket foundations). The noise emitted from percussive piling increases with the increasing hammer energy required to install the larger foundations. Although radiated noise levels can be reduced substantially by appropriate noise abatement techniques such as air bubble curtains, noise levels are expected to continue to affect a wide range of species (in particular marine mammals and fish) representing different trophic levels in the food chain. In addition, the turbines will influence wind speeds and currents in an area extending many kilometers behind the wind farms, yet the consequences to the marine ecosystem are not well understood. As the long-term ecological effects of the establishment of the next generation of offshore wind farms arise through complex interactions between the pre-existing habitats and physical changes induced by the wind farms (e.g. the creation of artificial reefs, changes in currents and turbulence, sand banks and the local sea floor), the impacts of these changes on resident and visiting species are likely site specific and remain largely unknown. Thus, continuous monitoring and development of new modelling frameworks are required.

Impacts of offshore wind farms on marine mammals

Marine mammals depend on sounds for communication and foraging, and concerns regarding the impacts of wind farm construction noise initially sparked investigations of how cetaceans and pinnipeds respond to offshore wind farms. Impacts of high sound levels on marine mammals include acoustic trauma (noise induced hearing loss), communication masking, exclusion from habitats and behavioral changes. One of the major consequences of pile driving is the risk of acoustic trauma. Unmitigated pile driving produces short pulses of very high sound pressure, which affect both porpoises and seals within a range of > 1000 km². In the attempt to reduce the radiated noise, abatement measures such as bubble curtains and noise mitigation screens can be used. Indeed, the use of bubble curtains and other abatement techniques have proven to be effective in minimizing the risk of hearing loss and temporary habitat loss for harbour porpoises and is therefore considered Best Available Technology (BAT). The Danish Guidelines for pile driving (Danish Energy Agency, 2023) specifies when noise abatement is required, which in practice includes all pile driving activities. Such noise abatement includes acoustic deterrent devices (ADD) such as ‘seal scarers’ and pingers. The kind of mitigation measures that are required to avoid undesirable underwater noise levels and impacts on marine mammals varies among projects and is identified as part of the project specific environmental impact assessment. Yet, the use of ADD and other deterrence do not reduce the displacement of cetaceans and pinnipeds caused by the pile driving noise. Only noise abatement or alternative, less noisy installation procedures can alleviate the disturbance impact.

While several studies have investigated the effects of offshore wind farm construction noise on marine mammals, little is known about the potential negative or positive impacts of operating wind farms. These impacts could result from changes in prey densities, e.g. through aggregations of prey around turbine foundations (“artificial reef effects”) or reduced levels of commercial fishing inside the wind farms due to restrictions or difficulties maneuvering around the turbines. Further, operating wind turbines also emit noise, which may affect some marine mammals up to a few hundred meters from the turbines. Very few studies have assessed how marine mammals are influenced by operating wind farms, and it is not well understood whether the overall consequences of wind farms will be positive, negative, or neutral. Further, the consequences of wind farms are likely to depend on the environmental context, including proximity to other wind farms, other sources of disturbance, background noise and prey availability inside and in areas outside the wind farms. Thus, the assessment of how sensitive marine mammals will be to offshore wind farms is inherently associated with a large degree of uncertainty.

Iben Stokholm, Floris M. van Beest, Jonas Teilmann, Signe Sveegaard, Anders Galatius, Rune Dietz, Jakob Tougaard, Jacob Nabe-Nielsen, Institut for Ecoscience, Aarhus Universitet

Scientific Report from DCE – Danish Centre for Environment and Energy, No. 644, 2025

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