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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.

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.

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.

Seasonal changes in food consumption, respiration rate, and body condition in a healthy captive male harbor porpoise (Phocoena phocoena) from the North Sea were recorded over 8 y. He was kept at water and air temperatures similar to those experienced by wild conspecifics. At the age of 3 y and 10 mo, the porpoise's body length stabilized at 148 cm. Body mass, an indicator of body condition, increased to 40 kg between the ages of 2 and 5.5 y, after which it fluctuated seasonally by 5 to 10 kg. The porpoise's food consumption was 1,200 to 4,400 g/d but was generally 2,400 g/d (nearly 7% of body mass). Based on the caloric content of the fish diet, his energy intake was 9,000 to 26,000 kJ/d; the average was 18,000 kJ/d. Once his body length had stabilized, the porpoise's daily mean respiration rate was 17 to 26 breaths per 5 min (3 to 5 breaths/min). Correlation analysis revealed that respiration rate and body mass declined with increasing water temperature and that respiration rate increased with increasing food consumption. When the porpoise's body length was stable, his food consumption also decreased as the water temperature increased. If the data from the present study are representative of other male harbor porpoises, these results indicate that male harbor porpoises may need different amounts of food depending on the season and on whether they are growing or adult. Food consumption peaks in winter; thus, seasonality should be taken into account in energetics studies. Depending on food availability at sea, harbor porpoises may be more or less vulnerable to disturbances that decrease their foraging efficiency. With information from this longitudinal study, experts will be better informed on typical body condition patterns when considering the Interim Population Consequences of Disturbance (iPCoD) model. In addition, hypotheses about the effects of climate change on cetaceans' susceptibility to disturbance, in relation to seasons and life history, can be generated.

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.

When wild harbor porpoises (Phocoena phocoena) are disturbed by (and perhaps flee from) anthropogenic sound, they probably do not forage, and thus they fast for a period of time. The rate of body mass loss during fasting is probably related to internal parameters such as initial body mass and blubber thickness, hormonal and reproductive state, and anxiety and activity levels, as well as to environmental parameters such as water and air temperature. Reduced blubber thickness causes reduced insulation which, in turn, increases heat loss to the environment. If blubber cannot be replenished by eating extra food, porpoises' fitness may decline, which may eventually result in hypothermia and pneumonia. To increase understanding of the effects of fasting, the body condition of two captive porpoises was quantified while they were kept under ambient temperature conditions similar to those experienced by wild conspecifics in the North Sea, and while they were near-fasting (i.e., almost fasting) for 24 hours (consuming 3 to 10% of the average daily food intake of their normal ration in each period). Replicated near-fasting periods took place during each of the four seasons of the year, and body mass (an indicator of body condition) declined in all 30 near-fasting periods (15 for each animal). For both porpoises in all seasons, body mass loss represented approximately 4% of initial body mass (of which 0.7% was due to loss of food in the alimentary canal). Blubber thickness was difficult to quantify due to low measurement accuracy in relation to loss, but small decreases in blubber thickness (0 to 3 mm) occurred. A linear mixed-effects model showed that mass loss was greatest overall in autumn, lowest in summer, and intermediate in winter and spring. Harbor porpoises, therefore, appear to be most vulnerable to the effects of fasting due to disturbance in autumn, perhaps because their blubber layer has to increase in autumn to cope with the decreasing water temperature.

Susceptibility to temporary threshold shift (TTS) depends on the frequency of the fatiguing sound. So far, TTS in harbor porpoises (Phocoena phocoena) has been tested for sounds in the 1 to 7 kHz range. To assess the impact of anthropogenic noise, TTS needs to be investigated for other frequencies within the porpoise hearing range. TTSs were quantified in two porpoises that were exposed for one hour to a continuous one-sixth octave noise band centered at 16 kHz, at average received sound pressure levels (SPLs) of 117 to 145 dB re 1 [micro]Pa, and a sound exposure level (SEL) range of 153 to 181 dB re 1 [micro][Pa.sup.2]s. Hearing thresholds for 16, 22.4, and 32 kHz signals were determined before and after exposure, to quantify TTS and recovery. The highest TTS, measured 1 to 4 minutes after exposure, occurred at 22.4 kHz. Statistically significant TTS occurred at 16 kHz after exposure to 159 dB SEL, at 22.4 kHz after exposure to 165 dB SEL, and at 32 kHz after exposure to 181 dB SEL. The susceptibility of the two porpoises to TTS induced by the exposures (16 kHz; 1 h) was similar. Below 6.5 kHz, it appears that susceptibility to TTS increases with increasing frequency; whereas above 6.5 kHz, it appears that susceptibility to TTS decreases with increasing frequency (for the frequency range tested so far).