The best available science? Are NOAA Fisheries marine mammal exposure noise guidelines up to date?
Michael Stocker – mstocker@OCR.org
Ocean Conservation Research
P.O. Box 559
Lagunitas, California 94938
Popular version of paper 4aAB1
Presented Thursday morning, May 16, 2019
177th ASA Meeting, Louisville, KY
NOAA Fisheries employs a set of in-water noise exposure guidelines that establish regulatory thresholds for ocean actions that impact marine mammals. These are established based on two impact criteria: Level A – a physiological impact, and Level B – a behavioral impact or disruption. Since the introduction of these exposure definitions, much more work has been published on behavioral impacts of various noise exposures, and consideration of other variables such as frequency, sound quality, and multiple sound-source exposures. But these variables have not yet been incorporated into the NOAA Fisheries exposure guidelines.
Determining regulatory thresholds
In the Marine Mammal Protection Act (MMPA) sound exposure levels are categorized in two levels, Level A” and “Level B.” “Level A Take” defined by the National Marine Fisheries Service (NMFS) as a “do not exceed” threshold below which physical injury would not occur. In whales and whales, dolphins, and porpoises this was 180dB (re: 1μPa).
A “Level B Take” is defined as “any act that disturbs or is likely to disturb a marine mammal or marine mammal stock in the wild by causing disruption of natural behavioral patterns, including, but not limited to, migration, surfacing, nursing, breeding, feeding, or sheltering, to a point where such behavioral patterns are abandoned or significantly altered.” But defining what constitutes “disruption” is fraught with threshold vagaries – given that behavior is always contextual, and the weight of the “biological significance” of the disruption hinges on a human value scale. How biologically significant is it when Bowhead whales change their vocalization rates in response to barely audible airgun exposure, well below the Level B threshold? How biologically significant is it when a sea lion risks exposure to loud, intentionally (above Level A) Acoustic Harassment Devices intended to scare sea lions away from fish farms actually attracts them by letting them know that “dinner” is available.
Regulations work best when they are unambiguous. Regulators are not fond of nuance. Dichotomous decisions of Yes/No, Go/No-Go are their stock and trade. It was for this reason that until just recently the marine mammal exposure guidelines were really simple:
Noise exposure above 180dB = Level A exposure.
Noise exposure above 160dB = Level B exposure (for impulsive sounds)
Noise exposure above 120dB = Level B exposure (for continuous sounds)
But it was clear that these original regulatory thresholds were actually too simple. When dolphins ride the bow waves of seismic survey vessels – frolicking in a Level A noise field, it was apparent that the regulatory thresholds did not reflect common field conditions. This was recently addressed in guidelines that more accurately reflected the noise exposure criteria relative to the hearing ranges of a range of the various marine mammal species – from large “Low Frequency” baleen whales, to small “High Frequency” dolphins and porpoises. While this new standard more accurately reflects the frequency-defined hearing ranges of the exposed animals, it does not accurately address the complexity of the noise exposures in terms of sound qualities, nor in terms of the complexity of the sound environments in which the exposures would typically occur.
Actual sound exposures
Increasingly complex signals are being used in the sea for underwater communication and equipment control. These communication signals can be rough or “screechy” sounding and more disturbing and more damaging than the simple signals used for auditory testing.
Additionally, when sounds presented in a typical Environmental Impact Statements, they are presented as single sources of sound. And while there is some consideration for accumulated noise impacts, the accumulation period “resets” after 24 hours, so the metric only reflects accumulated noise exposure and does not address the impacts of a habitat completely transformed by continuous, or ongoing noise. Given that typical seismic airgun surveys run around the clock for weeks to months at a time, and have an acoustical reach of hundreds to thousands of kilometers, the activity is likely to have much greater behavioral impact than is reflected in accumulating and dumping of a noise exposure index every 24 hours.
Furthermore, operations such as seismic survey, or underwater extraction industry operations typically use a lot of different, but simultaneous sound sources. Seismic surveys may include seafloor profiling with multi-beam or side-scan sonars. Underwater extraction industries such as seafloor processing for oil and gas extraction, or seafloor mining operations will necessarily have multiple sound sources – with noisy equipment, along with acoustical communications for status monitoring, and acoustical remote control of the equipment. These concurrently operating compliments of equipment can create a very complex soundscape. And even if the specific pieces of equipment don’t in-and-of-themselves exceed regulatory thresholds, they may nonetheless create acoustically-hostile soundscapes likely to have behavioral and metabolic impacts on marine animals. So far there is no qualitative metrics for compromised soundscapes, but modeling for concurrent sound exposures is possible, and in this context, many concurrent sounds would constitute “continuous sound,” thereby qualifying the soundscape as a whole under the Level B continuous sound criteria of 120dB.
This is particularly the case for a proposed set of seismic surveys in the Mid-Atlantic, wherein three separate geophysical surveys would be occurring simultaneously in close proximity. “Incidental Harassment Authorizations” have been released by NOAA Fisheries for these surveys which have not taken the ‘concurrent noise exposures’ into account.
Additionally, while sound sources in the near-field may be considered “impulsive sounds.” And thus regulated under “Level B” criteria for impulse sounds, due to reverberation, louder sounds which have a long reach should be considered as “continuous sound sources” and thus be regulated under the Level B ‘continuous sound’ criteria of 120dB.
1. NOAA sound exposure metric should be updated to reflect sound quality (accommodating for signal characteristics) as well as amplitude.
2. “Soundscapes” need qualitative and quantitative definitions, and then incorporated into the regulatory framework.
3. Exposure metrics needs to accommodate for concurrent sound source exposures.
4. The threshold for what constitutes “continuous sound” needs to be more clearly defined, particularly in terms of loud sound sources in the far field subject to reverberation and “multi-path” echoes.