Abstract
In accordance with the monitoring and evaluation plan (MEP) for the ‘Gemini Offshore Wind Farm’ the ecological monitoring of harbour porpoises was carried out by IMARES and IBL Umweltplanung, concerning the distribution and numbers of harbour porpoises in and around the wind farm prior to construction (T-0). For this purpose aerial surveys as well as passive acoustic monitoring were performed.
In total, seven of eight planned survey flights were conducted at a height of 183 m (600 ft) between August 2013 and June 2014, covering a survey area of nearly 7,800 km². A survey scheduled for December 2013 had to be postponed to December 2014 due to adverse weather conditions, but could not be conducted in 2014 as well due to unfavorable weather conditions. For the passive acoustic monitoring, 15 CPODs were deployed in a design spreading 40 km to the west and 15 km to the south of the intended wind farm site. Between September 2011 and July 2014, 6,881 days of CPOD recordings were obtained during two distinct one-year sampling periods.
Overall 691 harbour porpoises, and 54 harbour and grey seals were recorded during the aerial surveys. This yields harbour porpoise densities between 0.3 and 1.5 individuals/km² (on average 1.0 /km²). The maximum was observed in April 2014, following lowest numbers in February and early March. Densities of harbour porpoise tended to be higher in the central part of the survey area where the intended Gemini offshore wind farm is located. In June and August 19 calves were seen during the flight surveys.
By passive acoustic monitoring harbour porpoises were detected on 93.2% of days sampled. Harbour porpoise acoustic activity showed a strong seasonal pattern. There was a peak in detections in March and lowest acoustic activity occurred in May-June and December-January. Detection peaks were also associated with high-tide cycles and morning periods.
Although the results of aerial surveys and passive acoustic monitoring do not show exactly the same pattern for the flight dates, they reveal a quite similar porpoise phenology over the year and show that the Gemini offshore wind farm is very frequently used by harbour porpoises, especially in spring.