Noise mitigation reduces negative effects of pile driving on harbour porpoises
1, Ansgar Diederichs1, Armin Rose1, Michael Bellmann2, Martin Scott3, Georg Nehls1
1BioConsult SH GmbH & Co. KG, 25183 Husum, Germany, 2itap GmbH - Institute for technical and applied Physics, 26129 Oldenburg, Germany, 3HiDef Aerial Surveying Ltd, Cleator Moor / Cumbria, UK
Construction of offshore wind farms goes along with considerable noise emissions during pile driving. Several studies demonstrated clear avoidance behaviour of harbour porpoises (Phocoena phocoena) in quite extended areas around such construction sites due to underwater noise. In order to protect marine mammals, especially harbour porpoises, noise mitigation techniques have to be implemented during pile driving in German waters. The aim is to reduce noise to a threshold of 160 dB SEL at a distance of 750 m to the sound source to avoid an auditory trauma in highly protected species like the porpoises. The construction process of three German offshore windfarms (Borkum West II, Global Tech 1, and DanTysk) was accompanied by research projects in order to investigate the effectiveness of noise mitigation tools (e.g. the Big Bubble Curtain). The results show as well positive effects of reduced sound exposure levels as the improvement of the bubble curtain with growing handling experience.
The behaviour of harbour porpoises was investigated by use of up to 26 passive acoustic data loggers (C-PODs) per project, placed at different distances around the respective construction area. The devices record porpoise echolocation clicks and thus give information on the presence of these animals on a high temporal resolution. Selected Stations were also equipped with underwater hydrophones to measure perceived sound levels (SEL and Lpeak) in distances between 300m and up to 20km. Data were analysed with respect to whether the spatial and temporal scale of porpoise avoidance behaviour differed with different noise levels and other factors, e.g., duration of piling, number of blow or total blow energy.
Results show that the application of noise mitigation techniques like the bubble curtain (often in combinations with other mitigation and deterrence techniques) reduced the impact area of porpoise avoidance behaviour of up to 90 %. Noise levels below 160dB also led to reduced activity delay after piling and thus helped to shorten the duration of the negative impact on animal behavior.
Minimising the impact zones of sound emission during pile driving may be the most successful way to mitigate negative effects of offshore construction on marine mammals. This is particularly relevant with respect to working schedules containing simultaneous piling in different wind parks or simultaneous piling in larger windfarms. In combination with deterrence from the nearfield of a construction site, long lasting negative effects on individual harbour porpoises or even on population levels can be eliminated.
Noise mitigating system like bubble curtains or noise mitigation screens are effective tools to avoid injuries in marine mammals and can be efficiently and savely handled. These tools, as well as marine mammal deterrence, can be included in the installation process without causing a delay of the installation works.