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With this in mind, NOAA has made significant advances in updating our acoustic thresholds levels by following similar approaches as those made for marine mammals by South- all et al. (2007) and for fishes and sea turtles by Popper et al. (2014). Specifically, the draft Guidance divides sound sources into two groups (impulsive and nonimpulsive) to reflect the higher potential for auditory injury from impul- sive sounds associated with high peak pressures and fast rise times (e.g., explosives, seismic air guns, and impact pile drivers) compared to nonimpulsive sources (e.g., vibratory pile driving, drilling, and most sonars). The draft Guidance provides acoustic thresholds using dual metrics and the ap- plicant uses whichever is most protective (i.e., results in the greatest amount of take). The dual metrics are (1) cumula- tive sound exposure level, acknowledging the importance of exposure duration, and (2) unweighted peak sound level as- sociated with noise-induced hearing loss.
The draft Guidance also divides marine mammals into five functional hearing groups (low-, mid-, and high-frequency cetaceans and otariid and phocid pinnipeds) and employs auditory weighting functions to recognize that all marine mammals do not hear and use sound in the same manner. Because of these differences, it is difficult to make compari- sons between results based on the use of our generic thresh- olds and results associated with the proposed updated acous- tic thresholds. In some situations (e.g., depending on sound source, species, and duration of exposure), updated acoustic thresholds may result in more takes than previously applied thresholds, whereas in others they may result in fewer takes.
Thus the draft Guidance updated acoustic threshold levels are more complex than our previous acoustic thresholds. This added complexity is an important consideration for applicants who have formerly relied on two simple acous- tic thresholds. Once the draft Guidance is finalized, along with any changes based on public comments and further consideration, our two thresholds for acoustic injury (i.e., 180/190 dB) will likely be replaced with 10 sets (5 functional hearing groups with different thresholds for impulsive and nonimpulsive sources) of acoustic thresholds, each with 2 new metrics, including one that incorporates duration of ex- posure (i.e., an applicant needs to incorporate how sources/ animals relate to one another over both space and time to determine exposure). Because of the added complexity, the draft Guidance reflects the best available science on the po- tential for underwater noise to affect marine mammal hear- ing and will contribute to more meaningful analyses.
Importance of Stakeholder and Public Input and Imple- mentation Challenges and Considerations
A critical component of Guidance development involves soliciting stakeholder input and public comment, which fo- cuses on both the technical and application aspects of the document. This process ensures that NOAA transparently address and consider all aspects of implementing the acous- tic thresholds before finalizing the Guidance.
The draft Guidance’s initial public comment period occurred in late 2013/early 2014 (www.Regulations.gov:Docket ID:NOAA-NMFS-2013-0177). We received 129 comments (i.e., individual comments as well as letters containing nu- merous comments) from individuals and groups ranging from members of the public, Congress, and scientists to federal, state, and international government agencies and industry and environmental groups. Many individuals and groups provided substantive comments addressing techni- cal aspects and/or issues relating to the implementation of updated acoustic threshold levels, which we will address via modifications to the Final Guidance as well as in the Federal Register, which will announce the finalization and release of the document. One example of a change based on based on public comment was the addition of a section to the Guid- ance on data gaps and research recommendations. Before the Guidance is finalized, there will be a second public com- ment period (July - September 2015) to address more recent updates (e.g., updated implementation methodology based on public comment as well as more recent scientific meth- odology developed since the draft Guidance was published for public comment).
As mentioned earlier, one of the most difficult challenges NOAA has encountered in finalizing the Guidance is en- suring that applicants can correctly implement the updated acoustic threshold levels given their inherent complexity. The issue of practicality is a fundamental consideration for the regulatory community but is not often encountered or even considered by most scientists in an academic setting. For example, NOAA must recognize that applicants have varying levels of ability (or budgets) to model sound expo- sure, ranging from sophisticated exposure models that can incorporate source and animal movement and propagation models to less sophisticated models that can only make very simplistic approximations. Often these modeling capabili- ties scale with the size of an activity (i.e., small construction project versus large multiday military training exercise), resulting in most applicants for large projects having suf-
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