The monitoring program on harbour porpoises at Horns Rev Offshore Wind Farm in the Danish North Sea, initiated in 1999, has now come to an end with collection of final data in 2005 and spring 2006. Seven years of surveys and five years of acoustic recordings of harbour porpoises on Horns Reef have resulted in a unique set of data documenting effects of the construction and operation of one of the world’s two largest offshore wind farms.
Horns Reef is a shallow reef consisting entirely of sand and with a complex hydrography. The reef and adjacent areas are important habitats for harbour porpoises. The occurrence of porpoises, as documented by visual surveys from ship and airplane as well as with acoustic dataloggers mounted on the seabed, is patchy in both space and time. There is thus a large variation between visual surveys in the number of animals observed and where they are observed. In general the wind farm area seems to be as important to the porpoises as the rest of the reef.
Effects of wind farm
The current dataset, which covers time before, during and after construction of Horns Rev Offshore Wind Farm, indicates a weak negative general effect from the construction and semi-operation on porpoises, with more specific effects linked to pile driving activities. No effects were observed from the operating wind farm.
Acoustic recordings (with T-PODs) did not show any significant change in abundance in the wind farm area as a whole during construction (see figure below). However, there was a significant difference between semi-operation (when intensive maintenance work too place) and operation, measured on the indicator porpoise-positive-minutes (PPM). PPM reached the lowest mean value in the entire monitoring period during semi-operation. Porpoise acoustic activity was higher in the operation phase than during baseline, but this was the case both in the wind farm and in the surrounding reference areas.
Conclusions from the ship surveys point in the same direction as the acoustic data, i.e. a weak negative and local effect of the wind farm during construction but otherwise no significant changes. Also ship survey data indicate more porpoises in the area as a whole during the operational period than for any other of the periods, baseline included.
Specific effects of construction
Although the design of the monitoring program was only aimed at detecting general effects of the construction and operation of the wind farm on porpoise abundance, it was nevertheless possible to document specific effects of a single activity: pile drivings. The T-POD data indicate that porpoises left the entire Horns Reef area in response to the loud impulse sound generated by the pile driving operation. After a period of 6-8 hours, activity returned to levels normal for the construction period as a whole.
Responses of porpoises to the construction and operation of Horns Rev Offshore Wind Farm thus lies within what was anticipated in the Environmental Impact Assessment: a partial displacement during construction and return to baseline activity during normal operation.