The National Oceanic and Atmospheric Association has released a document outlining at what point man-made sounds become harmful to marine mammals. Called “Draft Guidance for Assessing the Effects of Anthropogenic Sound on Marine Mammals,” the 83-page, peer-reviewed document is currently open for public comment.
The document divides mammals into five different “functional hearing groups” based on the frequencies they can hear, a change from previous guidelines.
“Given the specific nature of the guidance updates (e.g., different metrics, auditory weighting functions, etc.), it is not possible to directly compare the updated acoustic threshold levels with the thresholds previously used by NOAA,” according to a NOAA web page.
NOAA Field Office Supervisor with the Protected Resources Division Brad Smith said marine mammals in Alaska have “almost certainly” been negatively impacted by sound.
“We issue authorizations for incidental harassment under MMPA (the Marine Mammal Protection Act) for this purpose, and those animals ‘taken’ are by definition negatively affected,” he said.
According to the document, “take” means harassing, hunting, capturing or killing an animal, or attempting to do any of those things.
Marine mammals use sound for navigation, communication and finding food. Some strandings have been attributed to auditory damage. This document doesn’t address behavioral changes associated with sound; that’s something NOAA is planning to study over the next year.
The draft outlines a decibel level at which point different mammals’ hearing will be permanently injured, which is classified as “Level A Harassment” under the MMPA and “harm” under the Endangered Species Act.
The National Marine Fisheries Service defines “harm” as “an act which actually kills or injures fish or wildlife.”
The document also outlines at which point different mammals’ hearing will be temporarily injured, which is “Level B harassment” under the MMPA and “harassment” under the ESA. Smith said the guidelines “will not normally result in the ‘take’ bar being lowered. Rather, the guidelines are an additional tool to characterize the result of acoustic exposures.”
Data about permanent injuries is limited, as it’s gathered accidentally (it’s unethical to cause permanent hearing loss on purpose).
The lowest threshold for temporary hearing loss is estimated at 146 decibels, cumulatively, for “high-frequency cetaceans (like true porpoises) exposed to underwater impulsive (quickly rising, quickly falling) sound sources.”
The document notes that cumulative limits “could be exceeded in a multitude of ways (i.e., varying exposure levels and durations).”
Sound is measured differently underwater than above the water. According to the Federation of American Scientists, sounds underwater measure around 63 decibels higher than sounds in air (though it notes this is a generalization, as frequency can affect sound level.) Ambient ocean noise, it says, is around 74 to 100 decibels. A supertanker 340 meters long and traveling at 20 knots, for comparison, would reach a maximum of 190 decibels underwater.
According to NOAA, the activities most likely to impact marine mammals are seismic airguns, explosions, construction activities like impact pile driving and high-energy sonar used by the military.
“Many activities in Alaska generate these levels; pile driving, seismic research and even ships,” said Smith. “Because these (guidelines) introduce time as a factor, the longer an exposure, the lower the acoustic (decibel) threshold becomes.”
After the draft was released, Senator Lisa Murkowski requested NOAA’s acting administrator extend the deadline.
“This is a very complex issue that will have a significant effect on Alaskans and NOAA decided to unveil its own holiday surprise of sophisticated research when most Americans were understandably focused on family matters,” Murkowski stated in a press release. “I believe more time is needed to ensure that stakeholders, including coastal communities, not only are aware of this document (but also) have a chance to review this document and provide their input.”
NOAA responded to “requests from members of the public” by extending the comment deadline for 45 days, until March 13. NOAA is planning on releasing a user guide with the finalized document this summer.
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