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for different effects of noise on marine mammals. Given the evolving requirements, focus shifted from merely documenting hearing to developing science-based noise- exposure criteria. One such endeavor by the United States National Marine Fisheries Service (NMFS) led to an expert panel formed in 2003 to establish quantitative criteria for auditory and behavioral effects. The resulting publication (Southall et al., 2007) derived marine mammal noise- exposure criteria, a first for any nonhuman mammalian group. Innovations included the derivation of (1) species groups based on hearing abilities rather than exclusively taxonomy; (2) group- and frequency-specific auditory- weighting functions; (3) temporary and permanent threshold shift (TTS and PTS, respectively) onset levels; and (4) a descriptive spectrum of behavioral response severity from benign to lethal.
Although an important step, the panel acknowledged limitations to its conclusions, including the omission of some species, simplistic approximation of hearing filters, lack of quantitative behavioral response criteria, and extensive assumptions and extrapolations underlying many predictions. The paper therefore included a sub- stantial section on needed research in specific areas to address data gaps that would improve the criteria.
National and international concerns and calls for action concerning ocean noise have not abated (see Chou et al., 2021) and new research continues to accumulate on both the acute and cumulative effects of noise on marine species (e.g., National Academies of Sciences and Medicine [NAS], 2017; Southall, 2017). A decade after the initial panel, suf- ficient new data were available to reassess exposure criteria. The expert panel was reassembled with added expertise, resulting in three subpanels: (1) auditory impacts, (2) behavioral responses, and (3) impulsive noise categoriza- tion. The remainder of this article summarizes progress and remaining data gaps in each area as well as two recent papers on associated topics (Southall et al., 2019a, 2021).
Hearing and Auditory Effects
The Southall et al. (2007) noise-exposure criteria spurred a number of follow-on assessments either adapting earlier approaches within new quantitative approaches (Finneran, 2016) or proposing alternate methods (e.g., Tougaard et al., 2015). The original panel, augmented with additional experts,agreedtoabroadreassessmentofexposurecriteria, given the substantial increase in scientific data on marine
mammal hearing and the effects of noise research in key areas, thanks in part to earlier calls for targeted research. These include experimental studies of hearing and TTS in
controlled, laboratory settings like the trained harbor seal performing a behavioral hearing test (Figure 1).
Finneran (2016) provided a major step forward by develop- ing a quantitative means of integrating hearing and TTS to derive group-specific audiograms, auditory-weighting shapes, and exposures predicted for different levels of TTS to occur. Finneran’s approach retained the fundamental aspects of Southall et al. (2007), including the use of hearing groups, discrete criteria for impulsive and nonimpulsive exposures, multiple-exposure metrics, methods for estimating PTS onset (where direct measurements remain absent), and precaution- aryapproachesfortaxalackingdirecthearingmeasurements (e.g., baleen whales). After a long progression of efforts to develop regulatory guidance that began during the process leading up to Southall et al. (2007), the United States National Oceanic and Atmospheric Administration (NOAA) largely adopted Finneran’s (2016) recommendations as the basis of their regulatory guidance on hearing effects (NMFS, 2016).
The current iteration of the noise-criteria panel subsequently presented updated marine mammal noise-exposure criteria (Southall et al., 2019a), using the quantitative approach of Finneran (2016) as a foundation. Southall et al. provides group-specific audiograms for most taxa and auditory- weighting functions (filters). Examples of these functions
Figure 1. A harbor seal (Phoca vitulina) positioned at a listening station within a hemianechoic chamber during a behavioral hearing test. When the subject hears a test signal, it leaves the station and presses a response paddle indicating it heard the signal. Photograph courtesy of D. Wharton, taken under National Marine Fisheries Service (NMFS) permit #14535.
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