Porpoise Hearing Research Articles

Underwater Noise Level Recordings from a Water Intake Pontoon and Possible Impacts on Yangtze Finless Porpoises in a Natural Reserve.

Simple SummaryRelocation of endangered freshwater cetaceans in natural reserves is an effective conservation strategy to recover their populations. However, due to historical reasons, some natural reserves have facilities that cannot be removed, such as water intake pontoons that are important for residents nearby. These facilities generate underwater noise which may affect cetaceans. To investigate the potential impacts of underwater noise from water intake pontoons on freshwater cetaceans, this study measured such noise within a Yangtze finless porpoise (Neophocaena asiaeorientalis) nature reserve, analyzed its effects on porpoise behavior, vocalization, and hearing, and explored porpoise adaptation to the transient holding pen environment. This study provides a reference for assessing whether human facilities have impacts on freshwater cetaceans.Underwater noise pollution caused by human activities may affect freshwater cetaceans to different degrees. To analyze the impacts of water intake pontoons on Yangtze finless porpoises (Neophocaena asiaeorientalis), this study collected underwater noise data from such a pontoon in a nature reserve, plotted the power spectral density of acoustic signals, and calculated the root mean square sound pressure levels and the magnitude of sound source levels. The 1/3-octave sound pressure level root mean square values at the transient holding pens were <18.0 kHz, 39.5−60.0 kHz, which were slightly higher than the Yangtze finless porpoise hearing threshold curve values and therefore could be perceived. However, the results indicated that the porpoises would not develop a temporary hearing threshold shift. Meanwhile, pontoon noise did not interfere with the porpoises’ high-frequency acoustic signal nor did it affect their echolocation; it significantly interfered with their low-frequency acoustic signal, however, and the mother–child communication of the finless porpoises was affected, but this effect was quickly compensated due to the limited space range of the holding pens. Through this study of Yangtze finless porpoises, this paper provides a reference for assessing whether human facilities have impacts on freshwater cetaceans.

A Result of Accidental Noise Pollution: Acoustic Flowmeters Emit 28 kHz Pulses That May Affect Harbor Porpoise Hearing

Large rivers like the Elbe or the Weser are periodically entered by harbor porpoises of the North Sea. They may even move 97 km upstream to the port of Hamburg, where their presence is highest in spring. This migration is believed to be related to important anadromous prey species travelling upstream for spawning. An acoustic flowmeter in the port of Hamburg emits signals in the hearing range of harbor porpoises. The pulses have a duration of 0.2 ms, a peak frequency of 28 kHz, a source level of 210 dB re 1 µPa and an inter-pulse interval of 4.2 s. The signals are continuously emitted from both sides of the river at a location where the river is 400 m wide. We evaluated the potential of these signals to induce temporary threshold shift (TTS) in harbor porpoise hearing. Hearing tests with a harbor porpoise in human care were conducted to determine TTS onset. We modelled the acoustic field based on underwater noise measurements. The acoustic flowmeters emit pulses in a highly directional beam with a source level high enough for inducing TTS in harbor porpoises by a single exposure up to a distance of approximately 72 m. The received cumulative sound exposure levels for harbor porpoises travelling along the flowmeters are mainly dependent on the timing and distance to the sound source. Accordingly, a close approach to the flowmeter at the time of transmission should be prevented. This could be the case, if vessels force harbor porpoises to displace closer to the flowmeters. We therefore suggest to decrease acoustic flowmeter source levels. This case study emphasizes the need for a mandatory authorization process prior to the use of underwater sound for any purpose with potential effects on aquatic life. Such an authorization process should carefully consider potential effects for target and non-target species.

Effects of multiple exposures to pile driving noise on harbor porpoise hearing during simulated flights—An evaluation tool

Exploitation of renewable energy from offshore wind farms is substantially increasing worldwide. The majority of wind turbines are bottom mounted, causing high levels of impulsive noise during construction. To prevent temporary threshold shifts (TTS) in harbor porpoise hearing, single strike sound exposure levels (SELSS) are restricted in Germany by law to a maximum of 160 dB re 1 μPa2s at a distance of 750 m from the sound source. Underwater recordings of pile driving strikes, recorded during the construction of an offshore wind farm in the German North Sea, were analyzed. Using a simulation approach, it was tested whether a TTS can still be induced under current protective regulations by multiple exposures. The evaluation tool presented here can be easily adjusted for different sound propagation, acoustic signals, or species and enables one to calculate a minimum deterrence distance. Based on this simulation approach, only the combination of SELSS regulation, previous deterrence, and soft start allow harbor porpoises to avoid a TTS from multiple exposures. However, deterrence efficiency has to be monitored.

Temporary Hearing Threshold Shift in Harbor Porpoises (Phocoena phocoena) Due to One-Sixth-Octave Noise Band at 32 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.

Effects of exposure to sonar playback sounds (3.5 – 4.1 kHz) on harbor porpoise (Phocoena phocoena) hearing

Safety criteria for naval sonar sounds are needed to protect harbor porpoise hearing. Two porpoises were exposed to sequences of AN/SQS-53C sonar playback sounds (3.5-4.1 kHz, without significant harmonics), at a mean received sound pressure level of 142 dB re 1 μPa, with a duty cycle of 96% (almost continuous). Behavioral hearing thresholds at 4 and 5.7 kHz were determined before and after exposure to the fatiguing sound, in order to quantify temporary threshold shifts (TTSs) and hearing recovery. Control sessions were also conducted. Significant mean initial TTS1-4 of 5.2 dB at 4 kHz and 3.1 dB at 5.7 kHz occurred after 30 min exposures (mean received cumulative sound exposure level, SELcum: 175 dB re 1 μPa2s). Hearing thresholds returned to pre-exposure levels within 12 min. Significant mean initial TTS1-4 of 5.5 dB at 4 kHz occurred after 60 min exposures (SELcum: 178 dB re 1 μPa2s). Hearing recovered within 60 min. The SELcum for AN/SQS-53C sonar sounds required to induce 6 dB of TTS 4 min after exposure (the definition of TTS onset) is expected to be between 175 and 180 dB re 1 μPa2s.

Assessing auditory evoked potentials of wild harbor porpoises (Phocoena phocoena).

Testing the hearing abilities of marine mammals under water is a challenging task. Sample sizes are usually low, thus limiting the ability to generalize findings of susceptibility towards noise influences. A method to measure harbor porpoise hearing thresholds in situ in outdoor conditions using auditory steady state responses of the brainstem was developed and tested. The method was used on 15 live-stranded animals from the North Sea during rehabilitation, shortly before release into the wild, and on 12 wild animals incidentally caught in pound nets in Denmark (inner Danish waters). Results indicated that although the variability between individuals is wide, the shape of the hearing curve is generally similar to previously published results from behavioral trials. Using 10-kHz frequency intervals between 10 and 160 kHz, best hearing was found between 120 and 130 kHz. Additional testing using one-third octave frequency intervals (from 16 to 160 kHz) allowed for a much faster hearing assessment, but eliminated the fine scale threshold characteristics. For further investigations, the method will be used to better understand the factors influencing sensitivity differences across individuals and to establish population-level parameters describing hearing abilities of harbor porpoises.

Effects of exposure to intermittent and continuous 6-7 kHz sonar sweeps on harbor porpoise (Phocoena phocoena) hearing.

Safety criteria for mid-frequency naval sonar sounds are needed to protect harbor porpoise hearing. A porpoise was exposed to sequences of one-second 6-7 kHz sonar down-sweeps, with 10-200 sweeps in a sequence, at an average received sound pressure level (SPLav.re.) of 166 dB re 1 μPa, with duty cycles of 10% (intermittent sounds) and 100% (continuous). Behavioral hearing thresholds at 9.2 kHz were determined before and after exposure to the fatiguing noise, to quantify temporary hearing threshold shifts (TTS1-4 min) and recovery. Significant TTS1-4 min occurred after 10-25 sweeps when the duty cycle was 10% (cumulative sound exposure level, SELcum: ∼178 dB re 1 μPa(2)s). For the same SELcum, the TTS1-4 min was greater for exposures with 100% duty cycle. The difference in TTS between the two duty cycle exposures increased as the number of sweeps in the exposure sequences increased. Therefore, to predict TTS and permanent threshold shift, not only SELcum needs to be known, but also the duty cycle or equivalent sound pressure level (Leq). It appears that the injury criterion for non-pulses proposed by Southall, Bowles, Ellison, Finneran, Gentry, Greene, Kastak, Ketten, Miller, Nachtigall, Richardson, Thomas, and Tyack [(2007). Aquat. Mamm. 33, 411-521] for cetaceans echolocating at high frequency (SEL 215 dB re 1 μPa(2)s) is too high for the harbor porpoise.

Hearing frequency thresholds of harbor porpoises (Phocoena phocoena) temporarily affected by played back offshore pile driving sounds

Harbor porpoises may suffer hearing loss when exposed to intense sounds. After exposure to playbacks of broadband pile driving sounds for 60 min, the temporary hearing threshold shift (TTS) of a porpoise was quantified at 0.5, 1, 2, 4, 8, 16, 32, 63, and 125 kHz with a psychoacoustic technique. Details of the pile driving sounds were as follows: pulse duration 124 ms, rate 2760 strikes/h, inter-pulse interval 1.3 s, average received single strike unweighted sound exposure level (SEL) 146 dB re 1 μPa(2) s (cumulative SEL: 180 dB re 1 μPa(2) s). Statistically significant TTS only occurred at 4 and 8 kHz; mean TTS (1-4 min. after sound exposure stopped) was 2.3 dB at 4 kHz, and 3.6 dB at 8 kHz; recovery occurred within 48 min. This study shows that exposure to multiple impulsive sounds with most of their energy in the low frequencies can cause reduced hearing at higher frequencies in harbor porpoises. The porpoise's hearing threshold for the frequency in the range of its echolocation signals was not affected by the pile driving playback sounds.

Target‐specific acoustic predator deterrence in the marine environment

AbstractAcoustic deterrent devices (ADDs) have often been considered a benign solution to managing pinniped predation. However, ADDs have also been highlighted as a conservation concern since they can inflict large‐scale habitat exclusion in toothed whales (odontocetes). We tested a new method that selectively inflicted startle responses in harbour seals (Phoca vitulina) at close ranges to the loudspeaker but not in a non‐target species, the harbour porpoise (Phocoena phocoena), by using a frequency range where porpoise hearing was less sensitive than that of phocid seals. The sound exposure consisted of isolated 200 ms long, 2–3 octave‐band noise pulses with a peak frequency of 1 kHz, which were presented at a source level of ∼180 dB re 1 μPa. Field tests were carried out within a 2‐month period on a fish farm on the west coast of Scotland where marine mammal behaviour was observed within three distance categories. Seal numbers dropped sharply during sound exposure compared with control observation periods within 250 m of the sound source but were unaffected at distances further away from the farm. A Poisson regression model revealed that the number of seal tracks within 250 m of the device decreased by ∼91% during sound exposure and was primarily influenced by sound exposure with no evidence for a change in the effect of treatment such as habituation, throughout the experiment. In contrast to seals, there was no shift in the number of porpoise groups in each distance category as a result of sound exposure and porpoises were regularly seen close to the device. We also sighted six common minke whales during sound exposure while only one was seen during control periods. Our data demonstrate that the startle method can be used to selectively deter seals without affecting porpoises.

Hearing thresholds of a harbor porpoise (Phocoena phocoena) for playbacks of multiple pile driving strike sounds

Pile driving is presently the most common method used to attach wind turbines to the sea bed. To assess the impact of pile driving sounds on harbor porpoises, it is important to know at what distance these sounds can be detected. Using a psychophysical technique, a male porpoise's hearing thresholds were obtained for series of five pile driving sounds (inter-pulse interval 1.2-1.3 s) recorded at 100 and 800 m from the pile driving site, and played back in a pool. The 50% detection threshold sound exposure levels (SELs) for the first sound of the series (no masking) were 72 (100 m) and 74 (800 m) dB re 1 μPa(2)s. Multiple sounds in succession (series) caused a ~5 dB decrease in hearing threshold; the mean 50% detection threshold SELs for any sound in the series were 68 (100 m) and 69 (800 m) dB re 1 μPa(2)s. Depending on the actual propagation conditions and background noise levels, the results suggest that pile driving sounds are audible to porpoises at least at tens of kilometers from pile driving sites.

A practical weighting function for harbor porpoise underwater sound level measurements

Harbor porpoise (Phocoena phocoena) are subject to underwater noise disturbance from anthropogenic sources, especially shipping. The underwater audiograms of harbor porpoise were used to create a frequency weighting function, dBht(Phocoena phocoena), to permit estimation of the broadband perceived amplitudes of ambient and shipping noise. An equation was fit to the 0.02-20 kHz range of unmasked detection thresholds and normalizing to 0 dB at 20 kHz; dB = 46.4-35.6 log(kHz). The weighting function de-emphasizes the low frequency components of noise. Harbor porpoise hearing is less sensitive to low frequency shipping noise and, except at high amplitudes, estimating potential noise impacts using linear measurements will be misleading.

Auditory temporal resolution and evoked responses to pulsed sounds for the Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis)

Temporal cues are important for some forms of auditory processing, such as echolocation. Among odontocetes (toothed whales, dolphins, and porpoises), it has been suggested that porpoises may have temporal processing abilities which differ from other odontocetes because of their relatively narrow auditory filters and longer duration echolocation signals. This study examined auditory temporal resolution in two Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis) using auditory evoked potentials (AEPs) to measure: (a) rate following responses and modulation rate transfer function for 100kHz centered pulse sounds and (b) hearing thresholds and response amplitudes generated by individual pulses of different durations. The animals followed pulses well at modulation rates up to 1,250Hz, after which response amplitudes declined until extinguished beyond 2,500Hz. The subjects had significantly better hearing thresholds for longer, narrower-band pulses similar to porpoise echolocation signals compared to brief, broadband sounds resembling dolphin clicks. Results indicate that the Yangtze finless porpoise follows individual acoustic signals at rates similar to other odontocetes tested. Relatively good sensitivity for longer duration, narrow-band signals suggests that finless porpoise hearing is well suited to detect their unique echolocation signals.

The effect of signal duration on the underwater detection thresholds of a harbor porpoise (Phocoena phocoena) for single frequency-modulated tonal signals between 0.25 and 160 kHz

The underwater hearing sensitivity of a young male harbor porpoise for tonal signals of various signal durations was quantified by using a behavioral psychophysical technique. The animal was trained to respond only when it detected an acoustic signal. Fifty percent detection thresholds were obtained for tonal signals (15 frequencies between 0.25-160 kHz, durations 0.5-5000 ms depending on the frequency; 134 frequency-duration combinations in total). Detection thresholds were quantified by varying signal amplitude by the 1-up 1-down staircase method. The hearing thresholds increased when the signal duration fell below the time constant of integration. The time constants, derived from an exponential model of integration [Plomp and Bouman, J. Acoust. Soc. Am. 31, 749-758 (1959)], varied from 629 ms at 2 kHz to 39 ms at 64 kHz. The integration times of the porpoises were similar to those of other mammals including humans, even though the porpoise is a marine mammal and a hearing specialist. The results enable more accurate estimations of the distances at which porpoises can detect short-duration environmental tonal signals. The audiogram thresholds presented by Kastelein et al. [J. Acoust. Soc. Am. 112, 334-344 (2002)], after correction for the frequency bandwidth of the FM signals, are similar to the results of the present study for signals of 1500 ms duration. Harbor porpoise hearing is more sensitive between 2 and 10 kHz, and less sensitive above 10 kHz, than formerly believed.

Testing the acoustic tolerance of harbour porpoise hearing for impulsive sounds

The planned construction of offshore wind turbines in the North and Baltic Seas involves the emission of high numbers of intense impulsive sounds when the foundations of the turbines are being driven into the ground by pile driving. Based on information from other odontocete cetaceans it can be assumed that the source levels which will on average exceed 225 dB re 1 μPa pose a risk at least for temporary threshold shift (TTS) on harbour porpoises which inhabit these waters. In order to base the definition of noise exposure criteria on information on the tolerance of the hearing of this species a TTS study was conducted on one of the harbour porpoises held at the Fjord and Baelt in Kerteminde, Denmark. The hearing data were collected by using the AEP method. An airgun was chosen as sound source for the fatiguing sound stimulus to simulate the impulsive sounds at sufficiently high levels. This study comprises the testing of the animals normal hearing sensitivity and subsequent repetitions of these tests after an exposure to single impulsive sounds from the airgun at increasing levels. The baseline hearing data, thresholds for behavioural reactions and the resulting TTS levels will be presented.