During summer 2008, 92 monopile foundations of 3.9 m diameter were rammed into the seabed of the Danish North Sea west of Esbjerg to construct the offshore wind farm Horns Rev II. Effects of pile driving on harbour porpoise behaviour were studied by Brandt et al. (2009a) and a strong effect with reduced porpoise recordings that lasted up to 23 hours in the vicinity was found. The length of this effect rapidly decreased with distance. At more than 10 km distance it only lasted about 1-2 hours and was thus more or less limited to the time that pile driving took place.
In this study, we tested the effects of the construction of the research platform FINO III in the German North Sea and the wind farm Horns Rev II in the Danish North Sea on harbour porpoises on a large scale by passive acoustic monitoring. As the previous study at Horns Rev II only covered a maximum distance of about 20 km, the aim of this study was to investigate potentially further reaching effects of pile driving. For this purpose five C-PODs were deployed along a 50 km transect reaching from the Horns Rev II construction site southwest to the research platform FINO III.
Unlike during the previous study the parameter PP10M/day and PPM/H did not show a clear effect of pile driving on harbour porpoises at any location. However, the duration of waiting time indicated an effect which was statistically detectable at close distance (7 km) to the construction site. There may be some methodological reasons, why an effect was detectable by analysing waitingtimes but not the other parameters, which are discussed.. Compared to random waiting times first waiting times after pile driving increased 4.5fold by 7.6 hours (from 2.2 hours to 9.8 hours) at a distance of 7 km. A somewhat weaker effect was found at greater distances (15 - 37 km) where the increase of first waiting times was between 1.3 and 3.6 hours compared to random waiting times. However, here the difference was not statistically significant. A significant effect at 46 km was likely to have been an artefact.
The results are discussed in relation to expected noise levels at large distances and possibly diverging responses of harbour porpoises in habitats of differing function or quality.
From available data it cannot be finally concluded until what range piling noise from Horns Rev 2 was audible to harbour porpoises, but they are likely to be masked by background noise at 50 km. It is concluded, that harbour porpoise responses at distances beyond 15 km, if at all present, are certainly weak and of short duration.