Bubble curtains attenuate noise from offshore wind farm construction and reduce temporary habitat loss for harbour porpoises

Abstract

Effects of constructing the DanTysk offshore wind farm (German Bight, 80 turbines, 6 m diameter foundations) were studied by passive acoustic monitoring of pile-driving noise and harbour porpoise Phocoena phocoena echolocation. An acoustic deterrence device (seal scarer) was used to protect porpoises from hearing loss and bubble curtains were used to attenuate the pile-driving noise. Porpoise occurrence, quantified by echolocation signals, decreased when the seal scarer was engaged, during pile driving and up to 5 h after pile driving stopped. This effect extended out to 12 km, less than the 18−25 km reported from other pile drivings performed without bubble curtains. The bubble curtains thus effectively reduced the temporary habitat loss and risk of hearing loss. The 2 bubble curtains each attenuated the noise by between 7 and 10 dB, when used separately, and 12 dB when used together. Attenuation was most pronounced above 1 kHz, where the pile-driving noise at larger distances was comparable to or lower than ambient noise. This suggests that noise regulation should be based on frequency-weighted sound levels in addition to broadband levels, to assure that mitigation measures are effective in reducing impact on animals and not only in fulfilling legal requirements. The strong reaction to the seal scarer raises concern that it may surpass the reactions to the pile-driving noise itself, when operating with bubble curtains, and calls for a re-evaluation of the specifications of seal scarer sounds.

Using dose–response functions to improve calculations of the impact of anthropogenic noise

“…Some thresholds are applied with respect to the sound source, and others are applied to the sound level as experienced by the animal. As an example of the source-based approach, the German government aims to protect the hearing of marine mammals by limiting noise from impact pile driving to a single-strike sound exposure level L E,p of 160 dB re 1 μPa 2 s and a peak pressure level L p,pk of 184 dB re 1 μPa measured 750 m away from the sound source (Dähne, Tougaard, Carstensen, Rose, & Nabe-Nielsen, 2017). Most approaches to estimating the impacts of noise focus on the sound as received at the animal as opposed to at the source.…”

“…As an example of the source-based approach, the German government aims to protect the hearing of marine mammals by limiting noise from impact pile driving to a single-strike sound exposure level L E,p of 160 dB re 1 μPa 2 s and a peak pressure level L p,pk of 184 dB re 1 μPa measured 750 m away from the sound source (Dähne, Tougaard, Carstensen, Rose, & Nabe-Nielsen, 2017). Some thresholds are applied with respect to the sound source, and others are applied to the sound level as experienced by the animal.…”

Guiding principles for assessing the impact of underwater noise

“…IHC Noise Mitigation System), which are now being routinely deployed in German waters. The application of these technologies reduces the effect zones predicted for auditory injury (Figure c), and has been demonstrated to reduce the distance at which harbour porpoise are displaced from pile driving activities (Dähne, Tougaard, Carstensen, Rose, & Nabe‐Nielsen, ). Nevertheless, in many countries it is rare for such technologies to be required by regulators, and the reduction in effect zones that would be achieved through their use is not typically modelled as part of the assessment process.…”

“…Some developers have also used acoustic deterrent devices (ADDs) to displace animals prior to the activity, with the intent of reducing the risk of auditory injury. Use of ADDs introduces additional acoustic disturbance, and the extent of marine mammal displacement from ADDs may exceed the range of displacement from the activity itself if noise abatement measures are applied (Dähne et al, ). As such, use of ADDs should be considered carefully in the context of the proposed activity.…”

“…However, these in situ measures have been criticised as arbitrary and evidence for their efficacy is lacking (Wright & Cosentino, 2015). noise abatement measures are applied (Dähne et al, 2017) In the case of impulsive noise, many regulators now require licensed activities to be reported to national noise registries, which in turn feed into international registries used in region-scale assessments of impulsive noise activity and its associated risks . There is great potential for these reporting and assessment mechanisms to be integrated into the regulatory process as forward-looking management tools for cumulative effects assessment and marine spatial planning, and to meet the requirements of legislative frameworks such as the EU Marine Strategy Framework Directive (MSFD).…”