Visual monitoring of harbour porpoises (Phocoena phocoena) is challenging, and restrained by weather conditions and light levels. Their small size and quiet surface behaviour makes them difficult to spot in even calm conditions. Harbour porpoises are vocal animals that emit narrowband high frequency clicks almost continuously, which makes them ideal candidates for acoustic research. Click detectors, such as the T-POD (www.t-podporpoiseclickdetector.co.uk), are used commonly. Once moored, they can remain in situ, monitoring 24 hours a day for months at a time.
The number of offshore wind farms in European waters is increasing every day. When considering the impact this may have on marine mammals, all stages from pre-construction to normal operation need to be considered. Pile driving, which occurs during construction, is thought to have the highest impact on porpoises, and operational wind farm noise is thought to have less effect. Deploying T-PODs in wind farm and control areas prior to, during, and following construction enables long-term changes in marine mammal presence to be identified.
WIND FARM PILE DRIVING
Pile driving refers to the hammering of piles into the seabed, and is a source of high-amplitude, low frequency, impulsive sound.
Carstensen et al. (2006) (www.int-res.com) assessed harbour porpoise activity prior to, and during construction of the Nysted wind farm in the Western Baltic Sea (http://en.wikipedia.org). Three T-PODs were deployed within the wind farm area and three in a reference area 10 km east. Prior to construction, harbour porpoises were detected more commonly in the wind farm area, but during construction, more were encountered in the reference area. The time between detections (waiting time) increased at all six sites during the construction phase, but the increase was larger on the T-PODs located in the wind farm area. The first waiting time after piling ceased increased significantly at all sites, with an increase of 41 hours noted at the T-POD closest to the sound source. Observed increases in subsequent waiting times were not significant; thus, impacts were considered short term.
Tougaard et al. (2009) examined the response of harbour porpoises to construction of the Horns Rev wind farm in the North Sea (http://en.wikipedia.org/wiki/Horns_Rev). Back-calculated source levels of the pile driving were estimated to be 235 dB re 1 µPa @ 1 m p-p (peak-to-peak). Peak frequency of the noise was 0.16 kHz, but energy up to at least 100 kHz was recorded. Three T-PODs were used to measure porpoise activity. One was located within the wind farm area, the second 7.5 km to the east, and the third 21 km to the west. Harbour porpoise presence was reduced at all three stations, indicating that the noise was detectable up to at least 21 km from the source.
Thompson et al. (2010) (http://bit.ly) used T-PODs to assess the impact of pile driving on harbour porpoises and dolphins during the installation of two wind turbines in NE Scotland in July/August 2006. Few comparisons could be made, but a clear distinction was evident when porpoise detections within the wind farm area were compared between the piling activity in 2006, and when the wind farm was operational in 2007. Detection rates in July/August 2006 were significantly lower during the construction phase in than in the July/August 2007 operational phase. To test to see if there was any difference in detection rates when the 2006 piling was inactive to when the farm was operational in 2007, specific months were compared. Detections during inactive piling in the months June, September, and October 2006, compared with the same months in 2007, found no significant difference in porpoise detections, indicating that operational sound may have less effect. It should be noted however, that effects of other biological factors, such as prey availability, were not explored.
Brandt et al. (2011) (www.miriambrandt.de) recorded noise measurements 720 m and 2,300 m from pile driving during the construction of the Horns Rev II offshore wind farm in the Danish North Sea (http://en.wikipedia.org). At the maximum strike energy of 850 kJ, peak levels reached 196 dB re 1 µPa and 184 dB re 1 µPa at 720 m and 2,300 m respectively. Peak frequency was 80–200 Hz. Porpoise activity was assessed using T-PODs deployed at six positions between 2.5 km and 21.2 km from pile driving. Porpoise presence decreased significantly at the closest three positions (2.5, 3.2, and 4.8 km), but effects were insignificant at the two furthest away (17.8 and 21.2 km). Data were incomplete at the fourth location. Porpoise numbers were reduced for up to 72 hours 2.5 km from pile driving, and for nine and 10 hours at distances of 10.1 km and 17.8 km respectively. Conversely, the number of porpoises increased at the T-POD furthest from the pile driving, but numbers returned to normal 35 hours after pile driving ceased. At the three sites closest to the pile driving, porpoise activity was lower for the entire construction period (5 months) than during the baseline period.