Aarhus Universitets segl

No. 432: Geotechnical and geophysical feasibility studies for Energiø Østersø. Assessment of effects on marine mammals

Kyhn, L.A., Sveegaard, S., Galatius, A., Teilmann, J., Tougaard, J. & Mikaelsen, M. 2021. Geotekniske og geofysiske forundersøgelser til Energiø Østersø. Vurdering af påvirkning på havpattedyr. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 44 s. - Videnskabelig rapport nr. 432


The Danish Parliament decided in 2020 to establish two so-called energy islands, one in the North Sea and one in the Baltic Sea, southwest of Bornholm. It has been imposed on Energinet to carry out geotechnical surveys in these areas, prior to the conduction of strategic environmental impact assessments and the tendering of concessions for the offshore windfarms to be built near the energy islands. Energinet has asked University of Aarhus/DCE to assess potential impacts of the geotechnical surveys on marine mammals as well as for Natura 2000 sites appointed for marine mammals. In this report, effects of the geotechnical surveys on the relevant populations of harbour porpoise, harbour seal and grey seal are assessed for the Baltic energy island area. A similar assessment has been conducted for the energy island in the North Sea.


The assessment of potential consequences has been centred on the use of a geophysical sound source (sparker), which was judged to be the most significant source of impact on marine mammals.


The risk of inflicting permanent damage to the hearing of marine mammals (permanent threshold shift, PTS) is limited to the time when the sparker is started prior to survey within a range of up to 840 m. This risk will likely be reduced to insignificant if a suitable soft start mitigation protocol is used.


 It is estimated that porpoises and seals may react to the noise from the survey vessel at distances up to 11 km. The critically endangered population of porpoises in the Baltic Proper is the population of marine mammals in the region most vulnerable to disturbance, whereas porpoises belonging to the Belt Sea population as well as harbour seals and grey seals are assessed as less vulnerable, despite that the conservation status of the grey seal population is unfavourable.


Two Natura 2000 areas, Adler Grund and Rønne Banke on the Danish side and Adlergrund on the German side, are within the impact range and will be affected by the geophysical surveys in the subarea Østersø 1, which borders the Danish Natura 2000 area directly.

The impact on Natura 2000 areas can be reduced by moving the survey area away from the border of the Natura 2000 area Rønne Banke og Adler Grund.