| Summary: | The impact of pile driving on marine mammals is one of the major environmental concerns in offshore wind farm (OWF) construction. We assessed the impact of pile driving for the construction of the C-Power OWF (Thorntonbank, Belgian waters) on the spatial and temporal distribution of harbour porpoises during the season in which they occur in a high density in Belgian waters. We combined data collected by passive acoustic monitoring (PAM) devices, moored inside and outside the project area, with standardised aerial surveys covering the Belgian part of the North Sea. The combination and integration of both techniques proved very useful as PAM results in low spatial but high temporal resolution data, while aerial surveys have a complementary low temporal but high spatial resolution. At the end of March 2011, just before construction activities started, aerial surveys yielded an estimate of 2.5 harbour porpoises/km<su>2 . Density estimates in mid April 2011, after the start of the co . Although a decreasing density in Belgian waters towards the end of April should be considered as normal (cf. seasonal migration), changes in the spatial distribution between pre-and post-piling suggested harbour porpoise disturbance. This pattern was further explored with PAM, which indeed showed a clear fine-scale match between acoustic harbour porpoise detections and piling activities. Immediately upon the start of piling activities, harbour porpoise detections at a few km from the piling site fell to virtually zero. After the cessation of piling it took hours to days before new detections were made at this location. Aerial surveys allowed quantifying the distance over which an apparent impact occurred at around 22 km, with a repopulation of part of the area observed after one day with no piling.
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