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Federal Register / Vol. 86, No. 23 / Friday, February 5, 2021 / Notices
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or inferred from evidence of masking compensation. There are few studies addressing real-world masking sounds likely to be experienced by marine mammals in the wild e.g., Branstetter et al., 2013.
Masking affects both senders and receivers of acoustic signals and can potentially have long-term chronic effects on marine mammals at the population level as well as at the individual level. Low-frequency ambient sound levels have increased by as much as 20 dB more than three times in terms of SPL in the worlds ocean from pre-industrial periods, with most of the increase from distant commercial shipping Hildebrand, 2009. All anthropogenic sound sources, but especially chronic and lower-frequency signals e.g., from vessel traffic, contribute to elevated ambient sound levels, thus intensifying masking.
Potential Acoustic Effects of Proposed Activities Acoustic effects on marine mammals during the specified activity can occur from impact pile driving, vibratory pile driving/removal, and HRG surveys. The effects of underwater noise from construction of the SFWF and SFEC
have the potential to result in PTS
Level A harassment or disruption of behavioral patterns Level B
harassment of marine mammals in the action area.
The effects of pile driving on marine mammals are dependent on several factors, including the size, type, and depth of the animal; the type impact or vibratory, depth, intensity, and duration of the pile driving sound; the depth of the water column; the substrate of the habitat; the distance between the pile and the animal; and the sound propagation properties of the environment.
When piles are driven with impact hammers, they deform, sending a bulge travelling down the pile that radiates sound into the surrounding air, water, and seabed. This sound may be received by biological receivers such as marine mammals through the water, as the result of reflected paths from the surface, or re-radiated into the water from the seabed See Figure 3 Appendix J1 of the SFWF COP for a schematic diagram illustrating sound propagation paths associated with pile driving.
Noise generated by impact pile driving consists of regular, impulsive sounds of short duration. These impulsive sounds are typically high energy with fast rise times. Exposure to these sounds may result in harassment depending on proximity to the sound source and a variety of environmental
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and biological conditions Dahl et al.
2015; Nedwell et al., 2007. Illingworth & Rodkin 2007 measured an unattenuated sound pressure within 10
m 33 ft at a peak of 220 dB re 1 mPa for a 2.4 m 96 in steel pile driven by an impact hammer, and Brandt et al.
2011 found that for a pile driven in a Danish wind farm in the North Sea, the peak pressure at 720 m 0.4 nm from the source was 196 dB re 1 mPa. Studies of underwater sound from pile driving finds that most of the acoustic energy is below one to two kHz, with broadband sound energy near the source 40 Hz to >40 kHz and only low-frequency energy <400 Hz at longer ranges Bailey et al., 2010; Erbe, 2009;
Illingworth & Rodkin, 2007. There is typically a decrease in sound pressure and an increase in pulse duration the greater the distance from the noise source Bailey et al., 2010. Maximum noise levels from pile driving usually occur during the last stage of driving each pile where the highest hammer energy levels are used Betke, 2008.
Available information on impacts to marine mammals from pile driving associated with offshore wind is limited to information on harbor porpoises and seals, as the vast majority of this research has occurred at European offshore wind projects where large whales are uncommon. Harbor porpoises, one of the most behaviorally sensitive cetaceans, have received particular attention in European waters due to their protection under the European Union Habitats Directive EU
1992, Annex IV and the threats they face as a result of fisheries bycatch.
Brandt et al. 2016 summarized the effects of the construction of eight offshore wind projects within the German North Sea between 2009 and 2013 on harbor porpoises, combining PAM data from 20102013 and aerial surveys from 20092013 with data on noise levels associated with pile driving. Baseline analyses were conducted initially to identify the seasonal distribution of porpoises in different geographic subareas. Results of the analysis revealed significant declines in porpoise detections during pile driving when compared to 2548
hours before pile driving began, with the magnitude of decline during pile driving clearly decreasing with increasing distances to the construction site. During the majority of projects, significant declines in detections by at least 20 percent were found within at least 510 km of the pile driving site, with declines at up to 2030 km of the pile driving site documented in some cases. However, there were no
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indications for a population decline of harbor porpoises over the five year study period based on analyses of daily PAM data and aerial survey data at a larger scale Brandt et al., 2016. Despite extensive construction activities over the study period and an increase in these activities over time, there was no long-term negative trend in acoustic porpoise detections or densities within any of the subareas studied. In some areas, PAM data even detected a positive trend from 2010 to 2013. Even though clear negative short-term effects 12 days in duration of offshore wind farm construction were found based on acoustic porpoise detections, the authors found no indication that harbor porpoises within the German Bight were negatively affected by wind farm construction at the population level Brandt et al., 2016.
Monitoring of harbor porpoises before and after construction at the Egmond aan Zee offshore wind project in the Dutch North Sea showed that more porpoises were found in the wind project area compared to two reference areas post-construction, leading the authors to conclude that this effect was linked to the presence of the wind project, likely due to increased food availability as well as the exclusion of fisheries and reduced vessel traffic in the wind project Lindeboom et al., 2011. The available literature indicates harbor porpoise avoidance of pile driving at offshore wind projects has occurred during the construction phase.
Where long term monitoring has been conducted, harbor porpoises have repopulated the wind farm areas after construction ceased, with the time it takes to re-populate the area varying somewhat, suggesting that while there are short-term impacts to porpoises during construction, population-level or long-term impacts are unlikely.
Harbor seals are also a particularly behaviorally sensitive species. A harbor seal telemetry study off the East coast of England found that seal abundance was significantly reduced up to 25 km from WTG pile driving during construction, but found no significant displacement resulted from construction overall as the seals distribution was consistent with the non-piling scenario within 2 hours of cessation of pile driving Russell et al., 2016. Based on 2 years of monitoring at the Egmond aan Zee offshore wind project in the Dutch North Sea, satellite telemetry, while inconclusive, seemed to show that harbor seals avoided an area up to 40
km from the construction site during pile driving, though the seals were documented inside the wind farm after construction ended, indicating any
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