Marine mammal auditory systems: a summary of audiometric and anatomical data and its implications for underwater acoustic impacts

TitleMarine mammal auditory systems: a summary of audiometric and anatomical data and its implications for underwater acoustic impacts
Publication TypeReport
Year of Publication1998
AuthorsKetten, D. R.
Series TitleNOAA Technical Memorandum NMFS
Date Published2008
InstitutionNational Oceanic and Atmospheric Administration (NOAA) / National Marine Fisheries Service (NMFS) / Southwest Fisheries Science Center
CityLa Jolla
TypeTechnical Memorandum
Report NumberNOAA-TM-N M FS-SW FSC-256
Keywordsanthropogenic noise, audiometric data, auditory system, by catch, commercial fisheries, fishery assessment, marine mammal hearing, sound impacts, underwater acoustic impacts, underwater noise impacts

This report summarizes and critiques existing auditory data for marine mammals. It was compiled primarily as a background or reference document for assessing probable impacts of long-range detection devices that may be employed in tuna fisheries. To that end, it has the following emphases: a description of currently available data on marine mammal hearing and ear anatomy, a discussion and critique of the methods used to obtain these data, a summary and critique of data based on hearing models for untested marine species, and a discussion of data available on acoustic parameters that induce auditory trauma in both marine and land mammals. In order to place these data in an appropriate context, summaries are incorporated also of basic concepts involved in underwater vs. air-borne sound propagation, fundamental hearing mechanisms, and mechanisms of auditory trauma in land mammals.

Although the primary purpose of this report is to provide a reference document on the state of knowledge of marine mammal hearing, it is expected that the material will be used as a resource for assisting with the design and assessment of the safety and efficacy of acoustic detection and censusing devices used in fisheries, particularly for the Eastern Tropical Pacific region. Consequently, to maximize the utility of this document, a brief discussion has been included on the potential for impact on hearing from several recently proposed devices and an outline of research areas that need to be addressed if we are to fill the relatively large gaps in the existing data base.

The data show that marine mammals have a fundamentally mammalian ear that through adaptation to the marine environment has developed broader hearing ranges than those common to land mammals. Audiograms are available for 11 species of odontocetes and pinnipeds. For most marine mammal species, we do not have direct behavioral or physiologic audiometric data. For those species for which audiograms are not available, hearing ranges can be estimated with mathematical models based on ear anatomy or inferred from emitted sounds and play back experiments. The combined data show there is considerable variation among marine mammals in both absolute hearing range and sensitivity, and the composite range is from ultra to infrasonic. Odontocetes, like bats, are excellent echolocators, capable of producing, perceiving, and analyzing ultrasonics fiequencies (defined as >20 kHz). Odontocetes commonly have good functional hearing between 200 Hz and 100 kHz, although individual species may have functional ultrasonic hearing to nearly 200 kHz. The majority of odontocetes have peak sensitivities in the ultrasonic ranges although most have moderate sensitivity from 1 to 20 kHz. No odontocete has been shown audiometrically to have acute hearing (>80 dB re 1 μPa) below 500 Hz.

Good lower frequency hearing is confined to larger species in both the cetaceans and pinnipeds. No mysticete has been directly tested for any hearing ability, but functional models indicate that their functional hearing range commonly extends to 20 Hz, with several species expected to hear well into infrasonic frequencies. The upper functional range for most mysticetes has been predicted to extend to 20-30 kHz.

Most pinniped species have peak sensitivities from 1-20 kHz. Some species, like the harbour seal, have best sensitivities over 10 kHz; only the elephant seal has been shown to have good to moderate hearing below 1 kHz. Some pinniped species are considered to be effectively double-eared in that they hear moderately well in two domains, air and water, but are not particularly acute in either. Others however are clearly best adapted for underwater hearing alone.

To summarize, marine mammals as a group have functional hearing ranges of 10 Hz to 200 kHz with best thresholds near 40 dB re 1 µPa. They can be divided into infrasonic balaenids (probable functional ranges of 15 Hz to 20 kHz; good sensitivity from 20 Hz to 2 kHz; threshold minima unknown, speculated to be 80 dB re 1 μPa); sonic to high frequency species (100 Hz to 100 kHz; widely variable peak spectra; minimal threshold commonly 50 dB re 1 μPa), and ultrasonic dominant species (500 Hz to 200 kHz general sensitivity; peak spectra 16 kHz to 120 kHz; minimal threshold commonly 40 dB re 1 μPa).

The consensus of the data is that virtually all marine mammal species are potentially impacted by sound sources with a frequency of 500 HZ or higher. Relatively few species are likely to receive significant impact for lower frequency sources. Those that are likely candidates for LFS impact are all mysticetes and the elephant seal. By contrast, most pinnipeds have relatively good sensitivity in the 1 - 15 kHz range while odontocetes have peak sensitivities above 20 kHz. These "typical" ranges are generalities based on the mode of the data available for each group. It must be remembered that received levels that induce acoustic trauma, at any one frequency, are highly species dependent and are a complex interaction of exposure time, signal onset and spectral characteristics, and received vs. threshold intensity for that species at that frequency. Pilot studies show that marine mammals are susceptible to hearing damage but are not necessarily as fragile as land mammals. The available data suggest that a received level of 80 to 140 dB over species-specific threshold for a narrow band source will induce temporary to permanent loss for hearing in and near that band in pinnipeds and delphinids (Ridgway, pers. comm.; Schustennan, pers. comm.). Estimates of levels that induce temporary threshold shift in marine mammals can be made, at this time, only by extrapolation from trauma studies in land mammals. By comparison, because of mechanistic differences, blasts or rapid onset sources are capable of inducing broad hearing losses in virtually all species. Incidence of damage from blasts that results from middle ear air volume effects is speculated to be, to some extent, animal mass dependent rather than auditorially dependent.

For all devices, given that impulsive noise can be avoided, the question of impact devolves largely to the coincidence of device signal characteristics with the species audiogram. Because the majority of devices proposed use frequencies below ultra or high sonic ranges, odontocetes, with relatively poor sensitivity below 1 kHz as a group, may be the least likely animals to be impacted. Mysticetes and pinnipeds have substantially greater potential than odontocetes for direct acoustic impact because of better low to mid-sonic range hearing. Behavioral perturbations are not assessed in the report, but a concern is noted that they may be equally or more important as acoustic impacts. Mitigation, like estimation of impact, requires a case by case assessment, and therefore suffers from the same lack of data. To provide adequate estimates for both, substantially better audiometric data are required from more species. To obtain these data requires an initial three-pronged effort of behavioural audiograms, evoked potentials recordings, and post-mortem examination of ears across a broad spectrum of species. Crosscomparisons of the results of these efforts will provide a substantially enhanced audiometric data base and should provide sufficient data to predict all levels of impact for most marine mammals.