Abstract
In accordance with the monitoring and evaluation plan (MEP) for the ‘Gemini Offshore Wind Farm’ ecological monitoring of harbour porpoises was carried out, concerning the distribution, especially distance and numbers, of harbour porpoises around the wind farm during the first construction phase with pile driving of the fundaments (T-c), without noise mitigation. This study focusses on:
- the effects of the construction of wind turbines at Project Gemini on the presence of harbour porpoises at the construction sites and adjacent areas;
- the assessment of the range and duration of effects during the construction phase;
Furthermore, the questions are addressed if there is an effect on harbour porpoises:
- when using an acoustic porpoise deterrent, the so-called FaunaGuard, before pile driving starts,
- during simultaneous pile driving in both Gemini wind farm areas Buitengaats and ZeeEnergie
For this purpose aerial surveys as well as passive acoustic monitoring (PAM) were performed. In total, five survey flights with 8 out of 10 full flight days were conducted at an average height of 183 m (600 ft) between June and October 2015, covering a total survey area of nearly 7,800 km². For the passive acoustic monitoring, fifteen CPODs were deployed in a design spreading 40 km to the west and 15 km to the south of the centre of the wind farm site. Between June 2015 and February 2016 2,228 days of CPOD recordings were obtained during the construction phase.
Overall 469 harbour porpoises (Phocoena phocoena) were recorded during the aerial surveys. This yields harbour porpoise densities between 0.05 and 3.3 individuals/km² during the five surveys (on average 1.2 individuals/km²). The maximum density was observed on 3 July 2015, two days after the start of pile driving in the Gemini wind farm site. The porpoise density was almost the same on 30 June 2015, one day before the piling started, but the distribution was slightly different. In general, densities of harbour porpoise tended to be lower in the south-western and north-eastern parts of the survey area. Overall, the harbour porpoise density showed a clear decline during the 2015 T-c survey period from a very high level in June to an average level in August/September and a very low level in October. During passive acoustic monitoring harbour porpoises were detected on 96.2% of days sampled. Harbour porpoise acoustic activity showed a strong seasonal pattern. There was a peak in detections in June, followed by lower acoustic activity till the end of the year, and an increase from December onwards.
During T-0 and T-c the same underlying factors explained most variation in acoustic activity. During Tc, however, overall acoustic activity was lower. PAM shows a decrease in acoustic activity before pile driving starts, followed by an increase to pre-piling levels after pile driving stopped. This is interpreted as avoidance of the piling source by harbour porpoises.
PAM shows that the avoidance distance of harbour porpoises lies in the range of 10-20 km. These results are supported by the aerial surveys, which suggest avoidance of a pile driving location during pile driving in a radius from < 15 km up to 25 km. Though the data is scant, the aerial surveys indicate a return to the avoided area a few hours after pile driving has stopped. The avoidance distance might be a restricted by the length of the piling event, that lasted on average too short to allow harbour porpoise to swim further away than 10-20 km during a pile driving event.
PAM did not show a harbour porpoise deterring effect of the FaunaGuard. However, the study design was not primarily aimed at measuring an effect. PAM could not detect an effect of simultaneous pile driving in both Gemini wind farm areas Buitengaats and ZeeEnergie. However, simultaneous pile driving occurred on nine occasions, albeit with short overlap in time (ca 30 min).