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About a half percent of the hammer impact energy goes into waterborne acoustic energy.
In these studies, five species were exposed to a controlled number of pile-driving strikes at known sound levels to pro- duce a predetermined SELcum. The investigators examined the fish for external and internal physiological effects such as hematomas (bleeding), damage to the swim bladder (a bub- ble of air in the abdominal cavity of most), and damage to internal organs using standard necropsy (autopsy) methods.
The five fish species differ in life style and anatomy (chinook salmon, Nile tilapia, hybrid striped bass, hogchoker, and lake sturgeon), and four species showed the same general effects and onset of damage (Halvorsen et al., 2012a,b; Casper et al., 2013a,b). The only species that showed no effect was the hogchoker, a species without a swim bladder (Halvorsen et al., 2012b). The investigators concluded that the causal factor producing internal damage was the repeated motion of the walls of the swim bladder in response to the impulsive pile- driving signals. Because the hogchoker has no swim bladder, nothing would move internally in the body and thus no tis- sues would be damaged. Further supporting this hypothesis is that the major internal damage was to organs most closely positioned to the swim bladder, such as the kidney, gonads, and spleen (Halvorsen et al., 2012a,b).
Based on a quantitative analysis of the effects encountered with different SELcum values, it was found that the onset of physiological effects never occurred until the SELcum was above 203 dB re 1 μPa2∙s and in most species above 207 dB re 1 μPa2∙s. These levels are supported by the results in stud- ies by other groups for both larval (Bolle et al., 2012) and juvenile (Debusschere et al., 2014) fishes. These results led to the conclusion that the SELs proposed earlier need to be changed and that it takes substantially more acoustical en- ergy to damage fish tissues than assumed in the current in- terim regulations.
These data, and current guidelines, focus on the physiologi- cal effects, something only likely to occur if the fish are close to a sound source and stay there long enough to be exposed to a sufficient SELcum. However, because most (although not all) species are likely to move away from a sound source that is too loud, physiological damage is not of greatest concern. What is of far greater concern is, as for marine mammals, the behavioral response that could result in fishes moving from a breeding or feeding site or masking the ability of a fish to hear biologically important sounds ranging from the
overall acoustic scene (or soundscape) (Fay and Popper, 2000) to sounds produced by the same species.
However, almost nothing is known about fish behavioral re- sponses to pile driving or, for that matter, to any man-made sound, and it is thus not possible to provide guidance as to potential behavioral effects. In part, the lack of data results from the difficulties inherent in examining fish behavior in the wild (Hawkins and Popper, 2014; Hawkins et al., 2014a). Unlike marine mammals that come to the surface on a regu- lar basis and are large enough for tags, fishes must be ob- served underwater and even though some tags are available, they only work over short distances and near to underwater receivers. It is possible to use sonars (Hawkins and Popper, 2014; Hawkins et al., 2014b) but only close to the fish where individuals can be identified and their behaviors observed. Another issue is the extraordinary diversity of fishes (more than 32,000 species), in terms of anatomy, physiology, ecol- ogy, and behavior. Thus, the likelihood of “one number fit- ting all species” is probably nil (Hawkins and Popper, 2014; Hawkins et al., 2014a; Popper et al., 2014). Moreover, how fish respond to a particular sound is likely to vary based on the motivational state of the animal at time of sound expo- sure, further complicating any attempts to define criteria levels for potential behavioral effects.
An approach to setting new interim guidelines for fishes (and sea turtles) came in a recent set of guidelines (Popper et al., 2014). This report evaluated all the data on the effects on fish hearing up to 2013 and presented a set of tables for the potential effects from different types of sound sources, in- cluding pile driving. These guidelines are considered inter- im, and there is the expectation that the criteria will change as there are more data. However, the report does suggest the adoption of the criteria proposed in the aforementioned studies by Halvorsen, Casper, and colleagues.
Conclusions
There is little debate concerning the high level of underwater sound associated with impact pile driving. Further details on the mechanism of sound generation remain to be uncov- ered, particularly on quantifying the influence of the seabed on the sound spectrum and level. However, large challenges remain on understanding the potential effects on marine mammals and fishes and thereby improving sound exposure guidelines for all aquatic life. The successful completion of these challenges will be pivotal in future policy decisions.
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