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Figure 4. Sound pressure-time series for the radiated underwater noise from a pile undergoing impact pile driving measured at a 114-m (top) and a 495-m (bot- tom) range with the same signal. Red segments represent the time segment within which 90% of the signal energy resides. Circles identify the maximum absolute value of pressure.
Potential effects on Marine Mammals and fishes
It is necessary to understand the potential ef- fects of the sound produced during pile driving on marine mammals and fishes to determine whether mitigation is developed to protect the animals from exposure to high-energy sounds. The best way to determine the degree of miti- gation required is to understand how various sound levels affect animals. Mitigation is not necessary when there is sufficient evidence that the animals are not affected. This also ap- plies to how much mitigation is required for a particular project. If it is known that a certain level of sound is being produced and that this signal level is only 5 dB above the level where no effects are seen, then there is no need to have 10 or 15 dB of attenuation of the signal. The following sections provide a global over- view of the current knowledge of the potential effects on marine mammal and fish species and of the status of current protective regulations.
Potential effects on Marine Mammals and Protective Regulations
The intense sound impulses of the impact pil- ing are likely to disrupt the behavior of marine mammals at ranges of many kilometers (Tou-
of a single strike (SELss) as well as cumulated over multiple strikes (SELcum). This is a useful metric to assess cumulative noise exposure and allows for the comparison of sound with a varying duration, giving an indication of the total energy received by an organism over the course of an event or over a day (Popper and Hastings, 2009).
Dividing the signal energy by signal duration (τ) gives the mean square pressure from which arises the root-mean- square pressure on taking the square root, with the decibel equivalent being the root-mean-square sound pressure level (Lp,rms) metric in decibels re 1 μPa. A meaningful definition of the signal duration is the time segment of the signal with- in which 90% of the signal energy resides (Madsen, 2005). This segment begins when the cumulative signal energy ex- ceeds 0.05 of the total signal energy and ends when it reach- es 0.95 of this value; hence the term “90% of signal energy.” It is important to observe that SEL is immune to changing τ; in contrast, computing the Lp,rms metric using a different τ changes its value, usually in a nonmeaningful way.
gaard et al., 2009; Brandt et al., 2011) and have the potential to induce hearing impairment at close range (Madsen et al., 2006). In environmental impact assessment studies, it is be- coming increasingly clear that the most significant popula- tion consequences are likely to occur as a result of a behav- ioral response rather than direct physical injury or mortality due to the sound (Ellison et al., 2012). The implies that the focus has turned toward assessing a longer term impact of the behavioral responses through changes in energy costs related to food-foraging costs, survival, or fecundity (Bailey et al., 2014). However, because of a lack of data, the current impact assessment methods are still largely based on expert judgment (Thompson et al., 2013; Harwood et al., 2014).
An assessment of the effects of piling sound on marine mammals starts with determining the number of animals that may be exposed to sound levels that are likely to cause injury or behavioral disturbance. This requires information on the sound distribution around the pile, on the abundance
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