activity was observed, diminished with distance away from the foundation, with reduced click activity observed out to 17.8km. At the farthest T-POD position of 22km from the pile, no reduction in click activity was observed, rather there was an increase in detections following the onset of piling. The recorded recovery periods observed for ranges up to 4.7km were longer than the period between foundation installations. The implication may be that the harbour porpoise abundance within this range remained reduced over the entire construction period of the Horns Rev II windfarm (Brandt et al. 2011). Assuming a homogenous propagation environment in all directions from the pile, the noise levels from impact piling at which reduced harbour porpoise acoustic click detections were observed by Brandt et al. (2011) indicate that the received level thresholds stated by the US Marine Mammal Criteria Group (Southall et al. 2007) for behavioural disturbance of high-frequency cetaceans, of 224dB re 1 μPa peak pressure level and 183dB re 1 μPa2·s SEL, are probably not conservative enough.
111. As discussed above, this is also supported by Lucke et al. (2009) who suggested that averse behavioural reactions to a representative impulsive sound sources may be expected to occur at received level of 168dB re 1 μPa peak pressure level and 145dB re 1 μPa2·s SEL. In general, observational studies indicate that harbour porpoise will potentially avoid an area around marine impact piling which can extend out to ranges of several km, with the actual extent depending on the specifics of the sound source and the propagation environment. It is likely that ranges of about 20km reported from near-shore waters generally 10m or less in water depth will correspond to larger ranges in deeper water environment further offshore.
112. A more recent publication describing results from the first German offshore windfarm also reported an avoidance response of harbour porpoise to impact piling (Dähne et al. 2013). The study utilised both visual and acoustic detection methodology in the form of aerial surveys and monitoring with data loggers, respectively. The authors also provide estimates of expected sound levels at various ranges from the pile, although these were obtained by extrapolation and from a single measurement range and were derived using a simple transmission loss formula, thus these should be used with caution until more robust acoustical data are obtained. Further published work of the behavioural response of harbour porpoise to impact piling in Germany may be expected in due course.
113. Although no published studies of pinniped response directly to pile driving during windfarm construction are currently available in the literature, a study in the Beaufort Sea (Blackwell et al. 2004) during pile driving activities showed no aversive response at any distance for resident ringed seals Phoca hispida in air or in water.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 49
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