Page 54 - Summer 2021

NOISE EXPOSURE CRITERIA
Figure 2. Estimated underwater group audiograms. Top; group estimates (black lines) from individual measurements (gray lines). Auditory-weighting functions (bottom) for phocid (true seal) marine carnivores in water (PCW; solid line) and other marine carnivores in water (OCW; dashed line). From Southall et al., 2019a, with permission.
for the true (phocid) seals and other marine carnivores are shown in Figure 2.
The resulting group- and frequency-specific hearing, frequency weighting, and TTS onset functions are quan- titatively identical to those for species groups considered in Finneran (2016) and adopted by the NMFS (2016). However, Southall et al. (2019a) offered several substan- tive additions and is in a peer-reviewed publication rather than an unpublished guidance document specific to regu- lations and subject to the politics of one nation.
For instance, only underwater noise was addressed by the NMFS (2016), whereas Southall et al. (2019a) con- sidered both airborne and underwater hearing impacts for amphibious marine mammals. Furthermore, South- all et al. comprehensively reviewed the available data on hearing and the auditory effects of noise, with additional insight from auditory anatomy and sound commu- nication. The extensive, integrated data syntheses are included as detailed appendices, summarizing relevant information for each marine mammal species, clarifying
how what is known about the functional anatomy relative to proposed hearing groupings, underscoring remaining data gaps, and suggesting potential new directions.
For example, Southall et al. (2019a) proposed the addi- tion of two cetacean groups, segregating baleen whales into “very low” and “low-frequency” categories and denoting the largest of the toothed odontocetes (killer whales [Orcinus orca]; see bit.ly/31mpBsX), sperm whales (Physeter macrocephalus; see bit.ly/3lXzkiZ), and beaked whales (family Ziphiidae; see bit.ly/3d9Tu5m) as a new “mid-frequency” group. The remaining odontoce- tes then comprise “high” and “very high frequency” groups. These developments were regarded as a justified deviation from the groups adopted by NMFS (2016), but they were not made lightly. Substantial data gaps were identified, but the new groupings were nevertheless proposed based on the comprehensive integrated assessment of all forms of avail- able data. Furthermore, the overall process led Southall et al. (2019a) to further research recommendations in emerg- ing fields, including TTS growth rates, anatomical and functional modeling approaches (e.g., Tubelli et al., 2018), and self-mitigation from intense exposure events whereby animals reduce hearing sensitivity before anticipated noise events (Nachtigall and Supin, 2015).
As intended by Southall et al. (2007), these recent develop- ments represent subsequent steps in what will continue to be an evolving, challenging, and extensive journey. Future noise-exposure criteria for marine mammals should con- tinue to evolve, incorporating new minds and new science in this fast-moving field and considering effects not yet considered, such as auditory masking and nonauditory physiological effects (e.g., increases in stress hormones and associated impacts on the immune or other systems).
Behavioral Response Severity
Another subgroup of the new panel addressed behavioral responses to noise on marine mammals, building from the earlier severity assessments in Southall et al. (2007). Consid- erations for this group were fundamental differences in the nature and severity of the behavioral effects of noise in captive and field settings, how effects can be studied for individuals and populations, and how best to evaluate the rapidly expand- ing literature in this area (Southall et al., 2021).
Regulators have long sought and, in some cases, applied a simple, overarching step-function threshold (i.e., 160 dB)
54 Acoustics Today • Summer 2021