Marine Mammal Hearing Research Articles

Bulk analysis of underwater sound spectra from vibratory pile driving of cylindrical steel piles in coastal waters

Vibratory pile driving is a common means of pile installation in both near-shore and offshore environments. Spectral knowledge of the noise produced by this activity is crucial in modeling and assessing the impacts to aquatic species. With the goal of providing generic representative spectra to better inform propagation modeling and related impacts, approximately 80 near-pile measurements of vibratory pile driving of cylindrical steel piles from more than 19 hydroacoustic reports were analyzed. Included pile diameters range from 24–48 inches and all measurements were of activities in U.S. coastal waters. Results include representative spectra based on the mean and median spectra binned by pile size and the presence of noise mitigation systems, an examination of the variation in the data, marine mammal hearing group weighted spectra, broadband sound pressure levels, and comparisons with general knowledge of vibratory pile driving spectra characteristics in previous literature. Finally, data availability and larger efforts aimed at a comprehensive underwater pile driving acoustic spectral database are discussed.

Applying technical guidance from the USA for management of impacts of anthropogenic noise on wildlife in other countries: the Canadian context

Technical Guidance from the US National Marine Fisheries Service recommends Federal agencies use estimated thresholds for peak sound pressure levels and weighted cumulative sound exposure levels for the onset of permanent (and temporary) hearing threshold shifts in marine mammals. These dual metrics were developed to inform impact assessments within the US legal landscape. Despite its merits, the Technical Guidance contains uncertainties due to limited data on marine mammal hearing and auditory response to noise. The underlying assumptions about how representative existing data are for all marine mammal species also create limitations in the applicability of the Technical Guidance. These limitations warrant consideration before the Technical Guidance can be applied effectively in other jurisdictions with different legal standards. Using the Canadian legal framework as a working example, we found that many Canadian species are underrepresented in the dataset informing the Technical Guidance. The Technical Guidance also does not address all relevant noise impact types. Thus, the Technical Guidance alone cannot address all Canadian legal standards and, if the Technical Guidance is incorporated, some adjustments to the criteria within may be needed to meet the precautionary requirements of many Canadian legal standards.

Temporary threshold shift from continuous 20–40 kHz hyperbolic upsweeps in bottlenose dolphins (Tursiops truncatus)

Naval continuous active sonar (CAS) can operate at high duty cycles and result in sound exposure levels (SELs, in dB re 1 μPa2s) that may induce temporary threshold shift (TTS) in marine mammals. Estimating the impacts of CAS on marine mammal hearing is difficult however, given the scarcity of TTS data for long-duration tonal exposures. For this ongoing study, bottlenose dolphin TTS was measured following exposure to continuous playback of 19-s hyperbolic upsweeps from 20–40 kHz, at multiple sound pressure levels and exposure durations from 2 to 60 min (100% duty cycle). The upsweep was based on characteristics of naval CAS, but frequency-shifted into the dolphin’s range of best hearing to predict how baleen whales might be affected by CAS exposures below 10 kHz. Hearing was measured using both psychophysical and auditory brainstem response measurements. To date, the greatest cumulative exposures of 180 dB SEL (normal-hearing dolphin) and 183 dB SEL (dolphin with elevated hearing thresholds) resulted in less than 6 dB of behavioral TTS. Auditory brainstem response amplitudes for these exposures did not indicate noise-induced fatigue. Future testing will evaluate TTS following exposure to 28-kHz pure tones with comparable SELs. [Work funded by US Navy Living Marine Resources.]

Sam Houston Ridgway 1936–2022

Sam Houston Ridgway 1936–2022

Regulating Impacts of Noise on Marine Mammals in North America: An Overview of the Legal Frameworks in Canada and the United States

Marine mammals are protected under dedicated taxonomic legislation, endangered species legislation, and general environmental stewardship laws in many countries. Governments and agencies within those countries are tasked with assessing and limiting human impacts in accordance with their own laws, including those arising from underwater noise emissions. While the United States (US) has established thresholds for permanent and temporary threshold shifts (PTS and TTS) in the hearing of marine mammals, Canada has not yet established specific numeric onset thresholds for hearing impairment or other noise-related impacts. Given that Canada and the US are jointly responsible for the management of a number of at-risk marine mammals, we provide a brief overview of the main laws and associated standards relevant to management of noise impacts on marine mammals in these two jurisdictions. The US PTS/TTS thresholds are only a small part of a suite of elements collectively applied to assess and mitigate the full range of impacts of noise on marine mammals, and the implementation of these (or any other) thresholds in Canada would not negate the need to conduct case-specific impact assessments to satisfy their own broader requirements. Caution should be taken when applying US thresholds to address Canadian legal standards, as there are substantial differences in the legal definitions to which these thresholds might be applied. Thus, the need for, and application of, similar generalised PTS/TTS thresholds in Canada is still under debate and noise impacts will likely continue to be assessed in different ways in these two bordering nations.

Theoretical study of vibro-acoustics of fluid-pile-soil coupled system and experimental research of noise reduction of small-scale pile driving

Excited by a hydraulic hammer, the steel pipe pile makes a quite strong noise, which would cause enormous damage to the hearing of marine mammals. Therefore, a fluid-pile-soil coupled theoretical model is established to predict the environmental impact and underwater noise in pile driving and obtain the near-field acoustic information. Based on the Rayleigh-Ritz theorem, a differential equation of pile vibration that considers soil as a distributed damped spring is derived, and the analytical solution of vibro-acoustic response of pipe pile is given. Then the influence of fluid and soil on natural frequency and sound pressure radiated from pipe pile is discussed. A new type of double-shell pipe pile is also proposed to explore the noise reduction. Several model tests are carried out and compared double-shell with single pipe piles to validate the hypothesis. Numerical results show that the existence of fluid loading causes a decrease in the natural frequency, and when the spring stiffness, especially the radial stiffness, becomes stronger, the sound pressure peaks shift toward higher frequencies. Double-shell pipe piles have good performance in noise reduction as the noise transmission between the inner shell and the outer shell is eliminated.

In-air hearing in Hawaiian monk seals: implications for understanding the auditory biology of Monachinae seals

The auditory biology of Monachinae seals is poorly understood. Limited audiometric data and certain anatomical features suggest that these seals may have reduced sensitivity to airborne sounds compared to related species. Here, we describe the in-air hearing abilities of a Hawaiian monk seal (Neomonachus schauinslandi) trained to participate in a psychophysical paradigm. We report absolute (unmasked) thresholds for narrowband signals measured in quiet conditions across the range of hearing and masked thresholds measured in the presence of octave-band noise at two frequencies. The behavioral audiogram indicates a functional hearing range from 0.1 to 33 kHz and poor sensitivity, with detection thresholds above 40 dB re 20 µPa. Critical ratio measurements are elevated compared to those of other seals. The apparently reduced terrestrial hearing ability of this individual—considered with available auditory data for a northern elephant seal (Mirounga angustirostris)—suggests that hearing in Monachinae seals differs from that of the highly sensitive Phocinae seals. Exploration of phylogenetic relationships and anatomical traits support this claim. This work advances understanding of the evolution of hearing in amphibious marine mammals and provides updated information that can be used for management and conservation of endangered Hawaiian monk seals.

Using western science and Inuit knowledge to model ship-source noise exposure for cetaceans (marine mammals) in Tallurutiup Imanga (Lancaster Sound), Nunavut, Canada

Tallurutiup Imanga (TI) is a National Marine Conservation Area (NMCA) established in 2019 at the eastern entrance of the Northwest Passage in Lancaster Sound, Nunavut, Canada, to protect 110,000 square kilometres of core habitat for cetaceans. This study examines the potential impacts of underwater noise from increased ship traffic in TI NMCA on three cetaceans: narwhal, beluga whale, and bowhead whale. Automatic Identification System data from 2015 to 2018 were used to spatially analyse ship traffic in the area. Sound propagation loss was modelled using vessel positions along major routes and then used to model vessel acoustic noise outputs. Areas populated by narwhal, beluga and bowhead whales were identified using western science and Inuit knowledge and then overlapped with the vessel noise outputs. Results indicate that an increasing number of ships are transiting important habitat areas for cetaceans and that this has resulted in some areas where the National Oceanic and Atmospheric Association 120 dB behavioural threshold for marine mammals has been exceeded. This suggests that in some areas of TI NMCA ship noise may negatively impact marine mammal hearing and behaviour, with the highest potential exposures in Eclipse Sound and Milne Inlet.

Audiogram of a Cook Inlet beluga whale (Delphinapterus leucas).

Noise is a stressor to wildlife, yet the precise sound sensitivity of individuals and populations is often unknown or unmeasured. Cook Inlet, Alaska belugas (CIBs) are a critically endangered and declining marine mammal population. Anthropogenic noise is a primary threat to these animals. Auditory evoked potentials were used to measure the hearing of a wild, stranded CIB as part of its rehabilitation assessment. The beluga showed broadband (4-128 kHz) and sensitive hearing (<80 dB) for a wide-range of frequencies (16-80 kHz), reflective of a healthy odontocete auditory system. Data were similar to healthy, adult belugas from the comparative Bristol Bay population (the only other published data set of healthy, wild marine mammal hearing). Repeated October and December 2017 measurements were similar, showing continued auditory health of the animal throughout the rehabilitation period. Hearing data were compared to pile-driving and container-ship noise measurements made in Cook Inlet, two sources of concern, suggesting masking is likely at ecologically relevant distances. These data provide the first empirical hearing data for a CIB allowing for estimations of sound-sensitivity in this population. The beluga's sensitive hearing and likelihood of masking show noise is a clear concern for this population struggling to recover.

Techniques for distinguishing between impulsive and non-impulsive sound in the context of regulating sound exposure for marine mammals.

Regulations designed to mitigate the effects of man-made sounds on marine mammal hearing specify maximum daily sound exposure levels. The limits are lower for impulsive than non-impulsive sounds. The regulations do not indicate how to quantify impulsiveness; instead sounds are grouped by properties at the source. To address this gap, three metrics of impulsiveness (kurtosis, crest factor, and the Harris impulse factor) were compared using values from random noise and real-world ocean sounds. Kurtosis is recommended for quantifying impulsiveness. Kurtosis greater than 40 indicates a sound is fully impulsive. Only sounds above the effective quiet threshold (EQT) are considered intense enough to accumulate over time and cause hearing injury. A functional definition for EQT is proposed: the auditory frequency-weighted sound pressure level (SPL) that could accumulate to cause temporary threshold shift from non-impulsive sound as described in Southall, Finneran, Reichmuth, Nachtigall, Ketten, Bowles, Ellison, Nowacek, and Tyack [(2019). Aquat. Mamm. 45, 125-232]. It is known that impulsive sounds change to non-impulsive as these sounds propagate. This paper shows that this is not relevant for assessing hearing injury because sounds retain impulsive character when SPLs are above EQT. Sounds from vessels are normally considered non-impulsive; however, 66% of vessels analyzed were impulsive when weighted for very-high frequency mammal hearing.

Measuring responses of harbour seals to potential aversive acoustic mitigation signals using controlled exposure behavioural response studies

Abstract Some anthropogenic activities pose acute risks for marine species. For example, pile driving could damage the hearing of marine mammals while underwater explosions can also result in physical damage or death. Effective mitigation is required to reduce these risks, but the exclusion zones specified in regulations can extend over hundreds or thousands of metres and seals pose particular problems because they are difficult to detect at sea. Aversive sound mitigation aims to exclude animals from high‐risk areas before dangerous activities take place by broadcasting specific acoustic signals. Field research is needed to identify signals that might be effective in eliciting short‐term avoidance by marine species such as harbour seals (Phoca vitulina). A series of controlled‐exposure experiments (CEEs) were undertaken to measure seal movements in response to acoustic deterrent devices (ADD) and predator calls, and to assess the effectiveness of candidate signals for aversive sound mitigation. Seals were fitted with UHF/GPS transmitters providing continuous high‐resolution tracks and real‐time transmissions of their locations. A tracking/playback vessel located seals at sea and transmitted either ADD signals or orca (Orcinus orca) calls over a range of distances while seals were foraging or moving between sites. Behaviour before, during and after exposure was analysed to assess responses. One‐hundred and ten CEEs were assessed as being of at least ‘adequate’ quality. Of the 71 adequate trials with the Lofitech ADD, all 38 at ranges of &lt;1 km (predicted received level 134.6 dB RMS re 1 μPa) elicited a response. The maximum response range was 3123 m (predicted RL: 111 dB RMS re 1 μPa). However, the responses observed did not always result in substantial movements away from the source, especially for seals that were travelling at the time of the exposures. More work is needed to better understand how exposure risks would be reduced in different scenarios. The mean net speed of horizontal movements for seals responding to aversive sounds (1.15 m s−1) was only 7% higher than their mean travel speed. Responses to broadcasts of orca calls were highly variable. The results suggest that signals similar to those generated by a Lofitech ADD could be used to reduce risks to harbour seals from pile driving and underwater explosions in coastal waters. More work will be needed to develop systems that match the requirements of industry and regulators and to explore whether these results can be generalized to offshore waters and to other phocids.

Acoustic characterization of sensors used for marine environmental monitoring

Active acoustic sensors are widely used in oceanographic and environmental studies. Although many have nominal operating frequencies above the range of marine mammal hearing, they can produce out-of-band sound that may be audible to marine mammals. Acoustic emissions from four active acoustic transducers were characterized and compared to marine mammal hearing thresholds. All four transducers had nominal operating frequencies above the reported upper limit of marine mammal hearing, but produced measurable sound below 160 kHz. A spatial map of the acoustic emissions of each sonar is used to evaluate potential effects on marine mammal hearing when the transducer is continuously operated from a stationary platform. Based on the cumulative sound exposure level metric, the acoustic emissions from the transducers are unlikely to cause temporary threshold shifts in marine mammals, but could affect animal behavior. The extent of audibility is estimated to be, at most, on the order of 100 m.

Measured sound levels in ice-covered shallow water caused by seismic shooting on top of and below floating ice, reviewed for possible impacts on true seals

Seismic surveying of the Arctic is important for several reasons, but also introduces some challenges. One is the concern that seismic data may affect the hearing of marine mammals living there, including true seals. We performed two seismic experiments on floating ice on Svalbard in the Norwegian Arctic in early March 2016 and late May 2017, just before and right after the ringed seal breeding period. We used a single airgun below ice and detonating cord on ice, measured sound levels in the water column, compared these with hearing capabilities of true seals found from previous studies, and observed the animals’ reactions when exposed to seismic waves in the field. We found that these actual seismic experiments have little potential to cause physical hearing damage, but temporary behavioural change may occur. We also observed a difference in measured sound levels, frequency content, and animal reactions, depending on the type of source used.

The history of marine mammal bioacoustics at the Navy Marine Mammal Program

The United States Navy Marine Mammal Program (MMP) has been in existence for over 50 years. Following its inception, the program quickly became involved in the study of marine mammal sensory systems and bioacoustics. Early studies included the pioneering work of C. Scott Johnson in obtaining the first behavioral audiogram in a dolphin and Sam Ridgway’s electrophysiological studies of dolphin hearing and sound production. Marine mammal bioacoustic studies grew substantially in the decades following the MMP’s inception, and included numerous investigations into odontocete biosonar, pinniped and odontocete hearing (using both behavioral and physiological methods), and the impact of human-made sound on the hearing, behavior, and physiology of marine mammals. The MMP’s bioacoustic research has significantly contributed to the Navy’s environmental stewardship mandate (i.e. to predict and mitigate the impact of Navy activities on marine mammals), the development of bio-inspired sonar systems, and the ability to assess the hearing capabilities of marine mammals in the wild, under human care, and in stranded or rehabilitation scenarios. The MMP continues its bioacoustic studies today with investments focused on bottlenose dolphin and sea lion bioacoustics, but with expansion to the passive acoustic monitoring of wild marine mammals.

Development of an S3/SC1 standard for auditory evoked potential hearing tests in toothed whales

The National Marine Fisheries service regulates the impact of anthropogenic ocean noise to marine mammals. Other government and commercial groups that produce ocean noise are subject to regulation. Both groups desire greater knowledge of hearing in marine mammals to better predict, mitigate, and regulate noise exposure. Auditory evoked potential (AEP) hearing tests have become a primary method by which hearing tests are performed in toothed whales (e.g., dolphins, porpoises). The synthesis of AEP data to address noise impacts to marine mammals is limited, in part, because of the variability between AEP thresholds measured by different researchers and laboratories using different methodologies. Methodological differences exist for AEP detection, threshold estimation, calibration, and stimulus delivery. Threshold variability could be reduced through standardization of AEP hearing test methods, which is desired by regulatory agencies and sound producers. An S3/SC1 standard for AEP hearing test methods in toothed whales has been initiated to address this need. The draft standard will be submitted to ANSI for consideration by the end of 2017. Adoption of the standard internationally would further improve threshold comparability by ensuring researchers globally utilize the same methods. The process should increase AEP data reliability for purposes of addressing ocean noise issues.

Implementing NOAA Fisheries’ 2016 Marine Mammal Acoustic Guidance: Challenges and lessons learned

The National Oceanic and Atmospheric Administration’s (NOAA) first comprehensive Guidance addressing the effects of noise on marine mammal hearing is intended for use by NOAA managers and applicants to better predict acoustic exposures that have the potential to trigger certain requirements under various statutes (e.g., U.S. Marine Mammal Protection Act; Endangered Species Act). The Guidance was developed by compiling, interpreting, and synthesizing scientific information on the effects of anthropogenic sound on marine mammal hearing. The Guidance’s updated acoustic thresholds are more sophisticated than our previous thresholds. This added complexity is an important consideration for applicants who have formerly relied on more simple acoustic thresholds to evaluate potential impacts. Thus, the development of user-friendly tools is a fundamental issue for the regulatory community that is not often considered by most outside this group. As NOAA implements the Guidance, we have entered a new phase that consists of its own inherent issues and challenges associated with the practicality of employing more complex science to real-world applications. Throughout this process, NOAA has learned several valuable lessons, which will help improve the process of updating this document as well as drafting future guidance (e.g., marine mammal behavioral guidance; guidance for other protected species).

Marine mammal hearing and the effects of noise

Marine mammals use sound for critical life functions through passive listening and, in some species, active sonar. Major recent advances have been made in understanding what sounds they make, how they hear, and how noise affects their hearing, behavior, and physiology. While data are lacking for whole groups, notably baleen whales, direct hearing measurements are available for most taxa. Some species, such as odontocete cetaceans, hear extremely broad frequency ranges, well into ultrasonic biosonar frequencies. Pinnipeds have more limited frequency ranges but are adapted for aerial and underwater hearing. Indirect data suggest whales use and are more sensitive to low frequencies. For most species, quite high received sound levels are required to temporarily or permanently affect hearing. Direct experimental measurements of behavioral responses have revealed strong species and context dependencies, with factors such as proximity, novelty, and spatial relationships influencing response probability as well as relative loudness. For active surveillance sonar systems operating in the 30-200 kHz range, whales and otariid pinnipeds are of much less concern in terms of audibility and potential behavioral disturbance than phocid pinniped and odontocete cetaceans, notably porpoises. Hearing loss or other injuries from such systems are quite unlikely in almost all practical scenarios.

Cumulative Effects of Exposure to Continuous and Intermittent Sounds on Temporary Hearing Threshold Shifts Induced in a Harbor Porpoise (Phocoena phocoena).

The effects of exposure to continuous and intermittent anthropogenic sounds on temporary hearing threshold shifts (TTSs) in a harbor porpoise were investigated by testing hearing before and after exposure to 1- to 2-kHz downsweeps of 1 s, without harmonics, presented as paired-intermittent and continuous-exposure combinations with identical cumulative sound exposure levels (SEL(cum)). Exposure to intermittent sounds resulted in lower TTSs than exposure to continuous sounds with the same SEL(cum). Therefore, the hearing of marine mammals is at less risk from intermittent anthropogenic noises than from continuous ones at the same received sound pressure level and duration.

Hearing in Whales and Dolphins: Relevance and Limitations.

Understanding the hearing of marine mammals has been a priority to quantify and mitigate the impact of anthropogenic sound on these apex predators. Yet our knowledge of cetacean hearing is still limited to a few dozen species, therefore compromising any attempt to design adaptive management strategies. The use of auditory evoked potentials allows scientists to rapidly and noninvasively obtain the hearing data of species rarely available in captivity. Unfortunately, many practical and ethical reasons still limit the availability of large whales, thus restricting the possibility to effectively ensure that anthropogenic sounds have minimum effects on these species. The example of a recent Blainville's beaked whale (Mesoplodon densirostris) audiogram collected after a stranding indicated, for instance, very specialized hearing between 40 and 50 kHz, which corresponded to the frequency-modulated upsweep signals used by this species during echolocation. The methods used during a stranding event are presented along with the major difficulties that have slowed down the scientific community in measuring the audition of large whales and the potential value in obtaining such results when successful.

Airgun inter-pulse noise field during a seismic survey in an Arctic ultra shallow marine environment.

Offshore oil and gas exploration using seismic airguns generates intense underwater pulses that could cause marine mammal hearing impairment and/or behavioral disturbances. However, few studies have investigated the resulting multipath propagation and reverberation from airgun pulses. This research uses continuous acoustic recordings collected in the Arctic during a low-level open-water shallow marine seismic survey, to measure noise levels between airgun pulses. Two methods were used to quantify noise levels during these inter-pulse intervals. The first, based on calculating the root-mean-square sound pressure level in various sub-intervals, is referred to as the increment computation method, and the second, which employs the Hilbert transform to calculate instantaneous acoustic amplitudes, is referred to as the Hilbert transform method. Analyses using both methods yield similar results, showing that the inter-pulse sound field exceeds ambient noise levels by as much as 9 dB during relatively quiet conditions. Inter-pulse noise levels are also related to the source distance, probably due to the higher reverberant conditions of the very shallow water environment. These methods can be used to quantify acoustic environment impacts from anthropogenic transient noises (e.g., seismic pulses, impact pile driving, and sonar pings) and to address potential acoustic masking affecting marine mammals.