In 2002 the Worlds largest offshore wind farm, consisting of 80 2MW wind turbines, was constructed on Horns Reef in the Danish North Sea. Ship based visual surveys and long-term deployment of acoustic dataloggers (PODs) were used to assess short term effects of construction on behaviour and abundance of harbour porpoises (Phocoena phocoena). Most focus was put on mounting of steel mono-pile foundations for the turbines, as they were rammed into the seabed. This type of operation is known to generate very loud underwater sound levels.
Combined evidence from animal densities obtained from visual surveys, behavioural observations during surveys and acoustic activity data in and outside the construction area demonstrated effects on the behaviour and abundance of animals on both short-term (hours) and long term (entire construction period) scales. Acoustic activity by the porpoises decreased dramatically on the entire Horns Reef at the onset of ramming operations and returned to higher levels a few hours after each ramming operation was completed. A reduction in abundance close to ramming operations was anticipated, as deterring devices (pingers and seal scarers) were deployed prior to each ramming operation to deter marine mammals from the area and thus protect them from exposure to the loud sound levels generated by the ramming procedure. The changes in abundance and behaviour over large distances are unlikely to be explained by the deterring sounds, which have comparably lower intensities than the ramming sounds and these effects must be attributed to the ramming.
A general effect on the behaviour of animals was seen during the construction period and at distances of up to 10-15 kilometers from the construction site. Compared to observations before and after construction there was a decrease in non-directional swimming, a behaviour assumed to correlate with feeding activity. Animal density estimates indicates that there were fewer animals present on the entire Horns Reef during the construction period compared to observations before and after the construction phase. Whether these changes are attributable to the construction activities or are related to overall temporal variation cannot be determined without further observations in the post-construction period.