Page 24 - Spring 2015

The Underwater sound field from Impact Pile Driving and Its Potential effects on Marine life
of animals in the area, and on threshold levels above which sound exposure might have a significant impact on protect- ed animals. The initial scientific recommendations for ma- rine mammal noise exposure criteria proposed by Southall et al. (2007) summarized the (limited) state of the art of the knowledge on noise-induced physical injury and behavioral response. Because of the urgent need for this kind of infor- mation for permitting offshore activities, the proposed crite- ria are widely used in impact assessment studies despite the many caveats in this work.
The most abundant cetacean in the European coastal waters is the harbor porpoise (Phocoena phocoena). There is sub- stantial evidence that harbor porpoises are easily disturbed by man-made sounds in coastal waters where offshore wind farms are being developed (Tougaard et al., 2015). During exposure to playback pile-driving sounds in a quiet pool (Kastelein et al., 2013), the respiration rate of a porpoise ap- peared to increase at a threshold SELss of 127 dB re 1 μPa2∙s. When exposed to a SELss of 145 dB re 1 μPa2∙s, the animal tried to avoid the sound by regularly jumping out of the water, whereas it never jumped during the baseline periods without exposure. (It should be noted, however, that one must be very cautious in extrapolating from behavioral re- sults in the laboratory, even in very large enclosures, to how an animal will behave when it is in the wild and able to move around freely [Popper et al., 2014]).
These findings are in line with German field studies, which found their way into legislation. The German government issued a Concept for the Protection of Harbour Porpoises from Sound Exposures during the Construction of Offshore Wind Farms in the German North Sea (BMU, 2014) that states that it is plausible to assume that avoidance and flight behavior are likely to occur at exposure to a received SELss of 140 dB re 1 μPa2∙s. Moreover, the German authorities regard a tem- porary hearing loss (temporary threshold shift [TTS]) in an animal as injury. Therefore, based on the findings of Lucke et al. (2009), the German regulations further established noise- induced injury prevention thresholds that call for a SELss not to exceed 160 dB re 1 μPa2∙s and a peak-to-peak sound pressure level not to exceed 190 dB re 1 μPa at a distance of 750 m from the pile. It is assumed that complying with these criteria will reduce the avoidance distance to ~8 km.
The German regulation has triggered the development of various noise mitigation systems such as bubble curtains, screens, or cofferdams (Bellmann, 2014) that reduce the
sound output of the pile by at least 10 dB. Other North Sea countries apply different regulations. The United Kingdom requires marine mammal observers to visually, and some- times acoustically, monitor an exclusion zone around the pile to ensure the absence of marine mammals. In The Neth- erlands, no piling may occur in the seasons with the highest abundance of sensitive species. And there is an obligation to deter animals from the vicinity of the pile by applying acoustic deterrent devices before the actual start of the pil- ing or by a “soft start” of the pile driving at a lower hammer energy (Robinson et al., 2007) to avoid permanent hearing loss (permanent threshold shift [PTS]) that might occur up to distances of ~1 km from a typical North Sea wind turbine foundation piling without noise mitigation.
Potential effects on fishes and Protective Regulations
The issues discussed for marine mammals parallel those for fishes, although knowledge of the effects and the degree of research currently underway is far less than that for marine mammals. Although fishes are starting to be written into regulations, the only promulgated numbers for the protec- tion of fishes were developed in 2008 by the US National Marine Fisheries Service (NMFS) for migrating salmon on the US west coast (Stadler and Woodbury, 2009).
NMFS interim criteria involve dual criteria for both cumula- tive sound exposure and peak sound pressure (reviewed by Popper and Hastings, 2009). The intent of the dual criteria was that if either value was exceeded, construction could po- tentially stop until a “recovery period” of 12 h was reached (Stadler and Woodbury, 2009). The actual levels were a SELcum of 187 dB re 1 μPa2∙s for fishes above 2 grams and 183 dB re 1 μPa2∙s for smaller animals. The peak sound pressure level was 206 dB re 1 μPa. In each case, these levels were consid- ered to be that for the onset of physiological effects and that lower levels would not cause any effect.
These interim criteria, however, were based on very limited data, most of which were not peer reviewed (Popper and Hastings, 2009). The lack of data generally resulted from an inability to control the pile-driving sounds to which fish were exposed because the studies were done with caged fish near actual driving operations (Popper and Hastings, 2009). To bring pile-driving sounds under control and to bring them into the laboratory, a device was developed that en- abled exposing animals to a far-field signal in a small cham- ber (Halvorsen et al., 2012a,b; Casper et al., 2012, 2013a,b).
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