Hot Topic – Guidance on Determining Potential Effects to Marine Mammal Hearing

The National Marine Fisheries Service (NMFS) is the U.S. federal agency that is responsible for the stewardship of the nation’s ocean resources, including the management of the majority of marine mammals, such as whales, dolphins, porpoises, seals, and sea lions. The U.S. Fish and Wildlife Service (USFWS) has jurisdiction over walruses, polar bears, manatees, dugongs, and sea otters. The NMFS is a branch of the National Oceanic and Atmospheric Administration (NOAA), and they are also known simply as NOAA Fisheries.

In 2016, NMFS published a technical memorandum that provides guidance on determining the effects of anthropogenic underwater sound on the hearing of the marine mammals under their jurisdiction. The guidance divides species into hearing groups defined by the frequencies that each group can hear. Each hearing group has an auditory weighting function that defines frequency weighting, which accounts for their varying sensitivities to sounds of different frequencies. The net effect of frequency weighting is that sounds at frequencies outside of those to which an animal is most sensitive must be louder to have the same level of potential impact as a sound at a frequency to which an animal is more sensitive. Finally, the guidance defines the received levels, or acoustic thresholds, at which temporary threshold shift (TTS) or permanent threshold shift (PTS) is predicted to occur from exposure to impulsive sounds or non-impulsive sounds.

To account for the fact that different species use and hear sound differently, five hearing groups were defined, which includes low-frequency (LF) cetaceans, mid-frequency (MF) cetaceans, high-frequency (HF) cetaceans, phocidpinnipeds underwater (PW), and otariid pinnipeds underwater (OW). A composite audiogram was determined for each group from direct data (behavioral and electrophysiological measurements) and predictions based on morphology, modeling, behavior, vocalizations, or taxonomy. The generalized hearing range for each group was defined based on its composite audiogram. Outside of the generalized hearing range, the potential for effects to hearing is considered highly unlikely or very low.

 

Not only do species hear different frequencies, but their ability to hear sounds varies across their hearing range. An audiogram displays the hearing sensitivity of a species at different frequencies. The typical U-shape of an audiogram means that at the bottom of the “U”, the animal is most sensitive to those frequencies. When determining the potential effects of sound exposure, it is important to compensate for the fact that animals do not hear equally well at all frequencies and therefore do not have the same potential for effects to their hearing at all frequencies. Auditory weighting functions define mathematical equations that represent the susceptibility of the ear to noise-induced hearing threshold shifts (e.g., PTS or TTS). The frequency weighting functions were created using data on hearing ability, effects of noise on hearing, and data on equal latency. Separate functions were defined for each marine mammal hearing group. The auditory weighting functions are to be used with the cumulative sound exposure level (SEL_cum) acoustic thresholds, where the acoustic energy received over a 24-hour period is accumulated.

Acoustic thresholds for the onset of PTS were extrapolated from measurements of TTS in studies with cetaceans and pinnipeds. A threshold shift of 6 dB is what NMFS has defined as the onset of TTS, with a 40 dB threshold shift approximating PTS onset. There have been no studies of PTS onset with marine mammals, so this value was extrapolated from human hearing studies. There are two criteria for PTS onset for impulsive sources, such as airguns, pile driving, or explosive sound sources. The first metric is not weighted (“flat”) and is based on the peak (“pk”) received sound pressure level (L_pk,flat). The second metric is weighted (“WF” represents the auditory weighting function defined for each hearing group) and is based on the accumulated acoustic energy (“E”) over a 24-hour period (L_E,WF,24h). If either metric is reached or surpassed, an animal is considered to have experience PTS onset. For non-impulsive sources, such as projectors, echosounders, and side scan sonars, there is only the weighted cumulative sound exposure metric (L_E,WF,24h). As the cumulative sound exposure level, it is abbreviated as SEL_cum and takes into account both received levels and duration of exposure (ANSI 2013), both factors that contribute to noise-induced hearing loss.

Additional Links on DOSITS

• Airgun
• Echosounder
• Explosive Sound Sources
• Frequency Weighting of Signal Levels
• Hearing Loss
• Hearing Sensitivity Studies
• Pile driving
• Projector (sound source)
• Side Scan Sonar
• Sound Pressure Levels and Sound Exposure Levels

Additional Resources

• NOAA’s Marine Mammal Acoustic Technical Guidance