High-amplitude anthropogenic sounds may result in hearing damage in marine mammals that are under water and close to the sound source. Most energy in anthropogenic sound is < 4 kHz. In the wild, harbor seals (Phoca vitulina) often rest while “bottling” with their head and ears above the water surface; this behavior may also serve as a self-mitigation method to reduce sound pressure levels (SPLs) received by their ears. We quantified the hearing sensitivity of two harbor seals for underwater sounds when they were bottling, and we simulated sound propagation near a bottling seal. We also assessed the physiological mode of hearing during hearing tests and the pathways through which sound reaches the inner ears. The head-above-water (HAW) hearing thresholds of the two seals for underwater sounds were very similar. For hearing test signals between 0.031 and 6.3 kHz, the HAW hearing thresholds were 14 to 62 dB higher than the underwater hearing thresholds of the same seals for the same sound frequencies, showing that they were able to reduce their exposure to these underwater sounds by bottling. For signals between 8 and 80 kHz, the HAW and underwater hearing thresholds were much more similar, differing by only 0 to 8 dB. Numerical simulations (< 20 kHz) were consistent with exposure level differences at the ears between HAW and submerged positions, providing theoretical background for the observations. Between 0.1 and 4 kHz, the mean corresponding aerial SPLs radiated by the underwater signals at threshold levels were similar to the theoretical masked thresholds for harbor seals (i.e., background noise spectral density level + critical ratio). The aerial hearing thresholds at 0.031 kHz (84 dB re 20 µPa) and 0.063 kHz (80 dB re 20 µPa) can be added as data points to the existing unmasked aerial audiogram of harbor seals (0.1 to 32.5 kHz). Hearing test signals < 4 kHz were probably heard via the “air-outer ear-middle ear-inner ear” pathway (aerial hearing; unmasked between 0.031 and 0.063 kHz, and masked between 0.1 and 4 kHz), and signals > 8 kHz were heard via a “water-body tissue-cochlea” pathway (underwater hearing). When estimating the effect on harbor seal hearing of high-amplitude, long-duration, continuous underwater sound (e.g., from continuous active naval sonar, offshore vibratory pile driving, or marine vibroseis) or high-amplitude, repetitive impulsive underwater sound (e.g., from offshore percussion pile driving), the seals’ ability to self-mitigate their exposure to sounds < 8 kHz, at times when they are not diving or foraging, should be taken into account.

Susceptibility to temporary hearing threshold shift (TTS) in harbor seals (Phoca vitulina) depends, in part, on the frequency of the fatiguing sound (the sound causing the shift). The TTS induced and the pattern of hearing recovery were documented in two female harbor seals after exposure for one hour to a continuous, constant-amplitude one-sixth-octave noise band (NB) at 8 kHz. This fatiguing sound was emitted at average received sound pressure levels (SPLs) estimated at between 138 and 156 dB re 1 µPa, resulting in sound exposure levels (SELs) of 174 to 192 dB re 1 µPa2s. Hearing thresholds for narrow-band sweeps were determined at 8, 11.3, and 16 kHz. The hearing frequency most affected was 11.3 kHz, half an octave above the fatiguing sound’s center frequency. Higher SELs were more likely to result in TTS than lower SELs. At hearing frequencies 8 and 16 kHz, initial TTS (1 to 4 min after the sound stopped) only occurred after exposure to the highest SEL (192 dB re 1 µPa2s). Recovery of hearing took longer after large TTSs than after small TTSs. The equal-energy hypothesis was tested by exposing the seals to the same continuous fatiguing sound with SPLs between 149 and 165 dB re 1 µPa, and exposure durations between two and 80 min; all seven combinations had the same SEL of 186 dB re 1 µPa2s. The equal-energy hypothesis was supported in both seals for the frequency, SPL, and duration ranges that were tested; thus, SEL can be used to predict the TTS elicited in harbor seals by continuous, constant-amplitude sound around 8 kHz. The TTS-onset SEL for the NB at 8 kHz, taken together with the TTS-onset SELs for fatiguing sound frequencies tested in previous studies, can form the basis for a revised TTS-onset function for harbor seals.

Two experiments were conducted to quantify the behavioral response of harbor seals (Phoca vitulina) to impulsive underwater sounds as produced during impact pile driving for offshore wind turbines. In Experiment 1 (dose-response relationship), two female harbor seals in a quiet pool were exposed to playbacks of minimally filtered pile-driving sounds (46 strikes/min) recorded at one location in the North Sea, at seven unweighted broadband single-strike sound exposure levels (SELss) at 6 dB steps between 125 and 161 dB re 1 µPa2s. Considering the dose-response relationship as expressed by the harbor seals’ position and their jumps out of the water, the onset of behavioral response occurred at 131 dB re 1 µPa2s in seal F01 and at 137 dB re 1 µPa2s in seal F02. The response was very clear in both harbor seals ≥ 143 dB re 1 µPa2s. Experiment 2 (effect of weighting) assessed whether sound levels used in predictions of harbor seal behavioral responses to pile-driving sounds should be frequency-weighted to reflect hearing sensitivity. The seals were exposed for 15 min to the minimally filtered pile-driving playback sound (PS), a low-pass filtered version (LP, filtered at 0.5 kHz), and a high-pass filtered version (HP, filtered at 4 kHz), with the same mean received unweighted broadband SELss (161 dB re 1 µPa2s). With the auditory weighting function for Phocidae in water applied, SELss were 156 (PS), 151 (LP), and 161 (HP) dB re 1 µPa2s. Both seals responded to all three pile-driving sounds but were most affected by the PS and HP sounds, showing that the high-frequency components of pile-driving playback sounds caused most of the behavioral effects. The second experiment showed that weighting of SELss is useful when setting underwater sound criteria for behavioral responses in Phocidae, as weighted SELss was a better predictor of behavioral response than unweighted SELss. The results indicate that the design of noise mitigation measures for harbor seals should focus on the reduction of the high-frequency components of impact pile-driving sounds.

California sea lions (Zalophus californianus) exposed to anthropogenic noise may experience temporary hearing threshold shift (TTS). The function used in regulations to protect their hearing from such damage in the Pacific Ocean is based on only one datapoint, so more data are needed. To determine their frequency-dependent susceptibility to noise-induced TTS, two California sea lions were exposed for 60 minutes to a continuous one-sixth-octave noise band (NB) centered at 40 kHz as the fatiguing sound, at sound pressure levels of 119 to 143 dB re 1 µPa, resulting in sound exposure levels (SELs) of 155 to 179 dB re 1 µPa2s. TTSs were quantified at the center frequency of the fatiguing sound and up to one octave above that frequency (at 40, 50, 56.5, 63, and 80 kHz). Statistically significant TTS occurred at all hearing test frequencies; higher SELs caused greater TTSs. Significant onset of TTS(1-4 min) occurred after exposure to a minimum SEL of 167 dB re 1 µPa2s—a shift of 5.2 dB at hearing frequency 56.5 kHz. At other hearing frequencies, onset of TTS1-4 occurred at SEL 173 dB re 1 µPa2s. TTSs1-4 ≤ 8 dB recovered within 12 min, and TTSs1-4 of > 8 dB recovered within 60 min. TTSs and hearing recovery patterns were similar in both subjects. Comparison with TTS data for the species’ hearing frequency range (0.6 to 40 kHz) shows that after exposure to fatiguing sound frequencies of 0.6, 1, 4, 8, and 16 kHz, the largest TTS1-4 occurred half an octave above the frequency of each of the fatiguing sounds. After exposure to fatiguing sound frequencies 2, 32, and 40 kHz, the largest TTS occurred at the frequency of the fatiguing sounds. Recovery patterns after exposure to the NB at 40 kHz were similar to those after exposure to NBs at 0.6, 1, 2, 4, 8, 16, and 32 kHz. Over almost the entire hearing range, the shape of the audiogram is a poor predictor of the shape of the TTS-onset function. The low TTS-onset SELs show that the hearing of California sea lions is more vulnerable to injury by anthropogenic sound in the oceans than was previously thought.

Susceptibility to temporary hearing threshold shifts (TTS) in harbor porpoises (Phocoena phocoena) depends in part on the frequency of the fatiguing sound (the sound causing the shift). The TTS induced and the pattern of recovery were documented in a female porpoise after exposure for one hour to a continuous one-sixth-octave noise band centered at 8 kHz. This fatiguing sound was emitted at average received sound pressure levels (SPLs) between 126 and 144 dB re 1 µPa, resulting in average sound exposure levels (SELs) of 162 to 180 dB re 1 µPa2s. Hearing thresholds for narrow-band sweeps centered at 8, 11.3, and 16 kHz were determined before and after exposure. Control sessions were used to determine which SELs resulted in statistically significant TTS in the first four minutes after the fatiguing sound stopped (TTS1-4). At 8 kHz, the lowest SEL that resulted in significant TTS1-4 (4.4 dB) was 174 dB re 1 µPa2s; at 11.3 kHz, the lowest SEL that resulted in significant TTS1-4 (4.9 dB) was 168 dB re 1 µPa2s; and at 16 kHz, the lowest SEL that resulted in significant TTS1-4 (1.3 dB) was 174 dB re 1 µPa2s. The hearing frequency that was most affected was 11.3 kHz, half an octave above the fatiguing sound’s center frequency. The equal-energy hypothesis was tested by exposing the porpoise to the same noise band with SPLs of 137 to 153 dB re 1 µPa and exposure durations between two and 80 minutes; all seven combinations resulted in the same fatiguing SEL of 174 dB re 1 µPa2s; and for these combinations, the equal-energy hypothesis was upheld. The results add to the body of data on TTS-onset SELs that were used to generate a revised auditory weighting function and, thus, enhance regulatory protection of wild harbor porpoises that are exposed to anthropogenic noise at sea.

To determine their frequency-dependent susceptibility to noise-induced temporary hearing threshold shift (TTS), two California sea lions (Zalophus californianus) were exposed for 60 min to a continuous one-sixth-octave noise band (NB) centered at 32 kHz as the fatiguing sound, at sound pressure levels of 132 to 156 dB re 1 µPa (sound exposure levels [SELs] of 168 to 192 dB re 1 µPa2s). Using a psychoacoustic technique, TTSs were quantified at the center frequency of the fatiguing sound and at half an octave and one octave above the center frequency (at 32, 44.8, and 63 kHz). When significant TTS occurred, higher SELs resulted in greater TTSs. TTSs and hearing recovery patterns were similar in both sea lions. The mean onset of TTS1-4 min (defined as 6 dB TTS) in sea lion F01 is estimated to occur after exposure to an SEL of 179 dB re 1 µPa2s (at hearing test frequency 44.8 kHz). After exposure to an SEL of 180 dB re 1 µPa2s, a mean TTS1-4 min of 6.7 dB was measured at hearing test frequency 44.8 kHz. In California sea lions, TTS onset levels are not as closely related (especially at the lower and higher frequencies) to the unmasked hearing thresholds (audiograms) as was previously assumed.

Masking can reduce the efficiency of communication and prey and predator detection. Most underwater sounds fluctuate in amplitude, which may influence the amount of masking experienced by marine mammals. The hearing thresholds of two harbor seals for tonal sweeps (centered at 4 and 32 kHz) masked by sinusoidal amplitude modulated (SAM) Gaussian one-third octave noise bands centered around the narrow-band test sweep frequencies, were studied with a psychoacoustic technique. Masking was assessed in relation to signal duration, (500, 1000, and 2000 ms) and masker level, at eight amplitude modulation rates (1-90 Hz). Masking release (MR) due to SAM compared thresholds in modulated and unmodulated maskers. Unmodulated maskers resulted in critical ratios of 21 dB at 4 kHz and 31 dB at 32 kHz. Masked thresholds were similarly affected by SAM rate with the lowest thresholds and the largest MR being observed for SAM rates of 1 and 2 Hz at higher masker levels. MR was higher for 32-kHz maskers than for 4-kHz maskers. Increasing signal duration from 500 ms to 2000 ms had minimal effect on MR. The results are discussed with respect to MR resulting from envelope variation and the impact of noise in the environment on target signal detection.

Unmasked behavioral audiograms of two California sea lions (Zalophus californianus), an adult female (F01) and a subadult male (M02), were recorded using narrow-band frequency-modulated hearing test signals. Signals had a duration of 1 s and center frequencies ranging from 0.031 to 80 kHz. Hearing thresholds were measured by varying test signal amplitude according to the up-down staircase method. The resulting underwater audiograms (50% detection thresholds) of the two sea lions were similar and showed the typical mammalian U-shape. Maximum hearing sensitivity (58 and 57 dB re 1 mPa) occurred at 11.3 kHz for F01 and at 8 kHz for M02, respectively. The range of best hearing (defined as < 10 dB from the maximum sensitivity) was from 1 to 16 kHz (four octaves). The detection thresholds for hearing test signal frequencies 0.031, 0.040, and 0.050 kHz were lower than expected, possibly caused by a shift in perceptional modality from auditory to vibrotactile, or due to the difficulty in measuring accurate SPLs of such low frequencies in a pool. Measurements of particle motion deemed detection of these very low frequencies via the vibrissae unlikely. The present study extends the frequency range for which the hearing of California sea lions has been tested. Based on the two audiograms of the present study and audiograms reported by Reichmuth et al. (2013) and Cunningham & Reichmuth (2016), a revised generic audiogram for California sea lions is proposed.

The loud, impulsive, broadband underwater sounds produced during offshore pile driving are known to have auditory and behavioral effects on harbor porpoises (Phocoena phocoena) in the areas around piling sites. Thresholds to prevent behavioral effects have not yet been set, and it is unclear whether or not auditory frequency weighting of piling sounds, as used for criteria to protect hearing (Southall et al., 2019), is also useful for predicting behavioral responses and therefore required to set safety criteria and develop mitigation measures. A harbor porpoise in a pool was exposed to playbacks of piling sounds, and her behavioral responses were quantified in comparison to baseline periods without piling sounds. The full-spectrum playback piling sound was recorded at 100 m from a piling site for an offshore wind turbine. For comparison, five low-pass filtered (6.3, 3.2, 1.5, 1.0, and 0.5 kHz) versions of the sound in which the bandwidth decreased were played back at the same duty cycle (46 strikes/min) and similar single-strike sound exposure levels (power average in the pool: 135 dB re 1 µPa²s; t90: 90 to 100 ms). As the bandwidth of the piling sounds decreased, the porpoise’s behavioral response became weaker. Although these results are based on only one porpoise, they indicate that harbor porpoises respond most strongly to the higher frequencies in piling sounds. Therefore, frequency weighting of the sound exposure level (SEL) will improve prediction of behavioral responses, and behavioral response threshold levels for criteria should also be expressed as weighted SELs. However, it is unclear whether the weighting for predicting auditory effects is also the best weighting to predict behavioral effects. Mitigation of the effects of piling sounds on harbor porpoise behavior should be focused on reducing the high-frequency part of the spectrum.

To determine the frequency-dependent susceptibility of California sea lions (Zalophus californianus) to noise-induced temporary hearing threshold shift (TTS), one of two subjects were exposed for 60 minutes to two continuous one-sixth-octave noise bands (NBs) as fatiguing sounds: one centered at 0.6 kHz, at sound pressure levels (SPLs) of 168 to 174 dB re 1 µPa (sound exposure levels [SELs] of 204 to 210 dB re 1 µPa2s), or one centered at 1 kHz, at SPLs of 144 to 159 dB re 1 µPa (SELs of 180 to 195 dB re 1 µPa2s). Using a psychoacoustic technique, TTSs were quantified at 0.6, 0.85, 1, 1.2, 1.4, and 2 kHz (at the center frequency of each NB, half an octave higher, and one octave higher). When significant TTS occurred, higher SELs resulted in greater TTSs. In the sea lion that was tested 1 to 4 minutes after exposure to the fatiguing sounds, the largest TTSs occurred when the hearing test frequency was half an octave higher than the center frequency of the two fatiguing sounds. The highest TTS levels elicited were 8.7 dB at 0.85 kHz and 9.6 dB at 1.4 kHz. When their hearing was tested at the same time after the fatiguing sounds stopped, initial TTSs and hearing recovery patterns were similar in both sea lions. These findings will contribute to the protection of hearing of species in the Otariidae family from anthropogenic noise by facilitating the development of an evidence-based underwater sound weighting function.

To determine the frequency-dependent susceptibility of California sea lions (Zalophus californianus) to noise-induced temporary hearing threshold shift (TTS), two subjects were exposed for 60 min to two fatiguing sounds: continuous one-sixth-octave noise bands (NBs) centered at 8 kHz (at sound exposure levels [SELs] of 166 to 190 dB re 1 µPa2s) and at 16 kHz (at SELs of 183 to 207 dB re 1 µPa2s). Using a psychoacoustic technique, TTSs were quantified at 8, 11.3, 16, 22.4, and 32 kHz (at the center frequency of each NB, half an octave higher, and one octave higher). For both NBs, higher SELs resulted in greater TTSs. In the SEL ranges that were tested, the largest TTSs occurred when the hearing test frequency was half an octave higher than the frequency of the fatiguing sound. When their hearing was tested at the same time after the fatiguing sounds stopped, initial TTSs and hearing recovery patterns were similar in both sea lions. The effect of fatiguing sound duty cycle on TTS was investigated with the 8 kHz NB, using 1,600 ms signals at a mean sound pressure level (SPL) of 154 dB re 1 µPa. Duty cycle reduction from 100 to 90% resulted in a large decrease in TTS; no TTS was observed at duty cycles ≤ 30%. The equal-energy hypothesis was tested with the 8 kHz NB and found to hold true: five combinations of SPL and exposure duration all resulting in a 182 dB SEL produced similar initial TTSs in both sea lions. These findings will contribute to the protection of otariid hearing from anthropogenic noise by facilitating the development of evidence-based underwater sound weighting functions. Our results also show that the introduction of short inter-pulse intervals to underwater sounds aids in the protection of otariid hearing by allowing recovery to take place.

In one of a series of studies of noise-induced hearing loss to determine the frequency-dependent susceptibility of California sea lions (Zalophus califor-nianus) to temporary hearing threshold shift (TTS), two subjects were exposed for 60 min to two different fatiguing sounds. These were continuous one-sixth-octave noise bands (NBs), centered at 2 kHz, at sound pressure levels (SPLs) of 138 to 167 dB re 1 µPa (resulting in sound exposure levels [SELs] of 174 to 203 dB re 1 µPas), and at 4 kHz, at SPLs of 133 to 169 dB re 1 µPa (resulting in SELs of 169 to 205 dB re 1 µPas). Using a psychoacoustic technique, TTSs were quantified at 2,2.8,4.2,5.6, and 8 kHz (at the center frequency of each NB, at half an octave higher, and at one octave higher). After exposure to both NBs, higher SELs resulted in greater TTS at all healing frequencies that were tested. TTSs and healing recovery patterns were similar in both sea lions. The effect of fatiguing sound duty cycle on TTS was investigated with the NB at 4 kHz and with 1.6-s signal duration, at a mean SPL of 169 dB re 1 µPa. Duty cycle reduction from 100 to 90% resulted in the largest decrease in TTS, and no TTS occurred at duty cycles ≤ 60%. The equal-energy hypothesis was investigated with the NB at 4 kHz: five combinations of SPL and exposure duration that resulted in the same SEL (197 dB re 1 µPas) produced similar initial TTSs in both sea lions. Susceptibility of California sea lions to TTS is higher than previously believed; for sounds around 2 and 4 kHz, it is similar to the susceptibility of harbor seals (Phoca vitulina). These data will contribute towards the development of an evidence-based underwater sound weighting function for the protection of Otariidae.

Susceptibility to temporary hearing threshold shift (TTS) in harbor porpoises (Phocoena phocoena) depends on the frequency of the fatiguing sound causing the shift. This study is part of a larger project in which TTS in harbor porpoises was tested alter exposure to fatiguing sounds within the 0.5 to 88.4 kHz frequency range. Herein, we report on the ITS induced and hearing recovery in a female harbor porpoise after exposure to 1, 2, and 4 h of continuous one-sixth-octave noise band centered at 0.5 kHz, which is within the frequency range of many high-amplitude anthropogenic sounds. This fatiguing sound was emitted at an average received sound pressure level of 163 dB re 1 mu Pa, resulting in sound exposure levels (SELs) of 199 to 206 dB re 1 mu Pa(2)s. hearing thresholds for 0.5, 0.71, and 1 kHz tonal signals were determined before and after exposure. Control sessions were used as a baseline and to determine which SELs resulted in statistically significant TTS in the first 4 min after the fatiguing sound stopped (TTS1-4). At 0.5 kHz, the lowest SEL that resulted in significant TTS1-4 (4.3 dB) was 202 dB re 1 mu Pa(2)s. At 0.71 and 1 kHz, the lowest SEL that resulted in significant TTS1-4 (5.8 dB and 3.8 dB, respectively) was 205 dB re 1 mu Pa(2)s. Hearing always recovered within 60 min after the fatiguing sound stopped. Within the SEL range that was tested, the greatest mean TTS1-4 (7.6 dB) occurred at 0.5 kHz, the center frequency of the fatiguing sound, after exposure to an SEL of 205 dB re 1 mu Pa(2)s (4-h exposure); 0.5 kHz is close to the lower bound of porpoise hearing, and ecological impacts of reduced hearing at this frequency are unclear. Results will be used in a future study of this series to generate an auditory weighting curve and to enhance regulatory protection of the harbor porpoise.

Acoustic masking reduces the efficiency of communication, prey detection, and predator avoidance in marine mammals. Most underwater sounds fluctuate in amplitude. The ability of harbor porpoises (Phocoena phocoena) to detect sounds in amplitude-varying masking noise was examined. A psychophysical technique evaluated hearing thresholds of three harbor porpoises for 500-2000 ms tonal sweeps (3.9-4.1 kHz), presented concurrently with sinusoidal amplitude-modulated (SAM) or unmodulated Gaussian noise bands centered at 4 kHz. Masking was assessed in relation to signal duration and masker level, amplitude modulation rate (1, 2, 5, 10, 20, 40, 80, and 90 Hz), modulation depth (50%, 75%, and 100%) and bandwidth (1/3 or 1 octave). Masking release (MR) due to SAM was assessed by comparing thresholds in modulated and unmodulated maskers. Masked thresholds were affected by SAM rate with the lowest thresholds (i.e., largest MR was 14.5 dB) being observed for SAM rates between 1 and 5 Hz at higher masker levels. Increasing the signal duration from 500-2000 ms increased MR by 3.3 dB. Masker bandwidth and depth of modulation had no substantial effect on MR. The results are discussed with respect to MR resulting from envelope variation and the impact of noise in the environment.

Noise-induced temporary hearing threshold shift (TTS) was studied in a harbor porpoise exposed to impulsive sounds of scaled-down airguns while both stationary and free-swimming for up to 90 min. In a previous study, ∼4 dB TTS was elicited in this porpoise, but despite 8 dB higher single-shot and cumulative exposure levels (up to 199 dB re 1 μPa2s) in the present study, the porpoise showed no significant TTS at hearing frequencies 2, 4, or 8 kHz. There were no changes in the study animal's audiogram between the studies or significant differences in the fatiguing sound that could explain the difference, but audible and visual cues in the present study may have allowed the porpoise to predict when the fatiguing sounds would be produced. The discrepancy between the studies may have resulted from self-mitigation by the porpoise. Self-mitigation, resulting in reduced hearing sensitivity, can be achieved via changes in the orientation of the head, or via alteration of the hearing threshold by processes in the ear or central nervous system.

As part of a series of studies to determine frequency-dependent susceptibility to temporary hearing threshold shifts (TTS), two female harbor seals (F01 and F02) were exposed for 60 min to a one-sixth-octave noise band centered at 40 kHz at mean sound pressure levels ranging from 126 to 153 dB re 1 μPa [mean received sound exposure level (SEL) range: 162-189 dB re 1 μPa2s]. TTSs were quantified at 40, 50, and 63 kHz within 1-4 min of the exposure for F02 and within 12-16 min of the exposure for F01. In F02, significant TTS1-4 (1-4 min post exposure) occurred at 40 kHz with SELs of ≥183 dB re 1 μPa2s and at 50 kHz with SELs of ≥174 dB re 1 μPa2s. At 63 kHz, TTS1-4 occurred with SELs ≥186 dB re 1 μPa2s. In F01, significant TTS12-16 (12-16 min post exposure) occurred only at 50 kHz with SELs of ≥177 dB re 1 μPa2s. The highest TTSs (27.5 dB in F02, 29.8 dB in F01) occurred at 50 kHz, one-third of an octave above the fatiguing sound's center frequency (SEL = 189 dB re 1 μPa2s); recovery took 2 days in F02 and 4 days in F01. In most other cases, recovery was within 1 h. The seals have a similar susceptibility to TTS from 4 to 40 kHz.

Temporary hearing threshold shift (TTS) caused by fatiguing sounds in the 1.5 to 16 kHz range has been documented in harbor porpoises (Phocoena phocoena). To assess impacts of anthropogenic noise on porpoise hearing, TTS needs to be investigated for other frequencies, as susceptibility appears to depend on the frequency of the fatiguing sound. TTS was quantified after two porpoises (Porpoises F05 and M06) were exposed for 1 hour to a continuous one-sixth-octave noise band centered at 32 kHz, at average received sound pressure levels of 118 to 148 dB re 1 µPa, and at a sound exposure level (SEL) range of 154 to 184 dB re 1 µPa2s. Hearing thresholds for 32, 44.8, and 63 kHz tonal signals were determined before and after exposure to quantify initial TTS and recovery. Porpoise M06’s hearing was tested 1 to 4 min after exposure. At 32 kHz, the lowest SEL that resulted in significant TTS1-4 (3.4 dB) was 166 dB re 1 µPa2s. At 44.8 kHz, the lowest SEL that resulted in significant TTS1-4 (5.2 dB) was 178 dB re 1 µPa2s. The highest TTS1-4 (18.3 dB) occurred at 44.8 kHz after exposure to 184 dB SEL. Porpoise F05’s hearing was tested 12 to 16 min after exposure. At 32 kHz, the lowest SEL that resulted in significant TTS12-16 (3.5 dB) was 184 dB re 1 µPa2s. At 44.8 kHz, the lowest SEL that resulted in significant TTS12-16 (1.2 dB) was 178 dB re 1 µPa2s. The highest TTS12-16 (8.2 dB) occurred in Porpoise F05 at 44.8 kHz after exposure to 184 dB SEL. At 63 kHz, no TTS could be elicited in either animal. Considering that Porpoise F05 had more time than Porpoise M06 for recovery, the susceptibility of the two porpoises to TTS after exposure to sounds of 32 kHz was similar. In the range investigated so far (1.5 to 32 kHz), susceptibility to TTS appears to increase with increasing frequency below ~6.5 kHz, and to decrease with increasing frequency above ~6.5 kHz.

Susceptibility to temporary hearing threshold shift (TTS) in harbor porpoises (Phocoena phocoena) depends on the frequency of the fatiguing sound causing the shift. TIS in harbor porpoises has been tested for sounds within the 1 to 63 kHz frequency range. Susceptibility to TTS caused by sounds of similar to 88 kHz is ecologically relevant since these sounds are expected to affect hearing in the frequency range of harbor porpoise echolocation signals. US was quantified in a female porpoise after exposure for 1 h to a continuous one-sixth-octave noise band centered at 88.4 kHz, at average received sound pressure levels of 137 to 161 dB re 1 mu Pa (resulting sound exposure levels [SEWs]: 173 to 197 dB re 1 mu Pa(2)s). To quantify TTS and recovery, hearing thresholds for 88.4, 100, and 125 kHz tonal signals were determined before and after exposure. Control trials were used as a baseline and to determine which exposure levels resulted in statistically significant TTS in the 4 min after the fatiguing sound stopped (TTS1-4). At 88.4 kHz, the lowest SEL, that resulted in significant TTS1-4 (3.6 dB) was 185 dB re 1 mu Pa(2)s; at 100 kHz, the lowest SEL that resulted in significant TTS1-4 (5.2 dB) was 191 dB re 1 mu Pa(2)s; and at 125 kHz, the lowest SEL that resulted in significant TTS1-4 (5.4 dB) was 191 dB re 1 mu Pa(2)s. At higher SELs, the TTS at this frequency remained similar. The highest TTS1-4 (13.1 dB) occurred at 100 kHz after exposure to an SEL of 197 dB re 1 mu Pa(2)s. In most cases, hearing recovered within 12 min after the fatiguing sound stopped; in the remaining cases, recovery took less than 1 h. TTS onset (defined as 6 dB TTS; Southall et al., 2019) occurred after exposures to SELs of similar to 191 dB re 1 mu Pa(2)s (when hearing was measured at 100 kHz, one third of an octave above the center frequency of the fatiguing sound).

This study concludes a larger project on the frequency-dependent susceptibility to noise-induced temporary hearing threshold shift (TTS) in harbor seals (Phoca vitulina). Here, two seals were exposed to one-sixth-octave noise bands (NBs) centered at 0.5, 1, and 2 kHz at several sound exposure levels (SELs, in dB re 1 μPa2s). TTSs were quantified at the center frequency of each NB, half an octave above, and one octave above, at the earliest within 1-4 min after exposure. Generally, elicited TTSs were low, and the highest TTS1-4 occurred at half an octave above the center frequency of the fatiguing sound: after exposure to the 0.5-kHz NB at 210 dB SEL, the TTS1-4 at 0.71 kHz was 2.3 dB; after exposure to the 1-kHz NB at 207 dB SEL, the TTS1-4 at 1.4 kHz was 6.1 dB; and after exposure to the 2-kHz NB at 215 dB SEL, TTS1-4 at 2.8 kHz was 7.9 dB. Hearing always recovered within 60 min, and susceptibility to TTS was similar in both seals. The results show that, for the studied frequency range, the lower the center frequency of the fatiguing sound, the higher the SEL required to cause the same TTS.

Two female harbor seals were exposed for 60 min to a continuous one-sixth-octave noise band centered at 32 kHz at sound pressure levels of 92 to 152 dB re 1 μPa, resulting in sound exposure levels (SELs) of 128 to 188 dB re 1 μPa2s. This was part of a larger project determining frequency-dependent susceptibility to temporary threshold shift (TTS) in harbor seals over their entire hearing range. After exposure, TTSs were quantified at 32, 45, and 63 kHz with a psychoacoustic technique. At 32 kHz, only small TTSs (up to 5.9 dB) were measured 1-4 min (TTS1-4) after exposure, and recovery was within 1 h. The higher the SEL, the higher the TTS induced at 45 kHz. Below ∼176 dB re 1 μPa2s, the maximum TTS1-4 was at 32 kHz; above ∼176 dB re 1 μPa2s, the maximum TTS1-4 (up to 33.8 dB) was at 45 kHz. During one particular session, a seal was inadvertently exposed to an SEL of ∼191 dB re 1 μPa2s and at 45 kHz, her TTS1-4 was >45 dB; her hearing recovered over 4 days. Harbor seals appear to be equally susceptible to TTS caused by sounds in the 2.5-32 kHz range.