Assessing the potential acoustic impact of floating offshore wind farms in the Central Mediterranean Sea

The Strait of Sicily, a vital marine passage with diverse fauna, is seeing a steep rise in the planning of offshore wind farm projects. This study assesses the acoustic impact of these wind farms on local marine species. Underwater propagation was modeled for three proposed floating wind farms using JASCO's Marine Operations Noise Model (MONM), which integrates a parabolic equation method for frequencies from 10 to 800 Hz and a beam-tracing model for 1 to 25 kHz. Propagation losses were calculated in one-third octave bands for ten source locations selected to represent the variability in bathymetry, and considering sound speed profiles for February and August. Sound levels from floating turbines were used to estimate exceedance ranges to known acoustic thresholds for marine species. Modeling indicated that sound levels could exceed temporary threshold shift and, for some species, permanent threshold shift criteria within a few tens of meters, but only if animals were to remain for 24 h at such small distances from a turbine. Behavioral disturbance thresholds for marine mammals were exceeded up to 68 km from the wind farms' boundaries. The study emphasizes considering species-specific sensitivities and ecological contexts in environmental impact assessments, recommending mitigation measures, such as the strategic placement of the turbines and continuous monitoring, to minimize adverse effects on local marine fauna, including marine mammals and turtles.

Bottlenose dolphin temporary threshold shift following exposure to 10-ms impulses centered at 8 kHza)

Studies of marine mammal temporary threshold shift (TTS) from impulsive sources have typically produced small TTS magnitudes, likely due to much of the energy in tested sources lying below the subjects' range of best hearing. In this study of dolphin TTS, 10-ms impulses centered at 8 kHz were used with the goal of inducing larger magnitudes of TTS and assessing the time course of hearing recovery. Most impulses had sound pressure levels of 175-180 dB re 1 μPa, while inter-pulse interval (IPI) and total number of impulses were varied. Dolphin TTS increased with increasing cumulative sound exposure level (SEL) and there was no apparent effect of IPI for exposures with equal SEL. The lowest TTS onset was 184 dB re 1 μPa2s, although early exposures with 20-s IPI and cumulative SEL of 182-183 dB re 1 μPa2s produced respective TTS of 35 and 16 dB in two dolphins. Continued testing with higher SELs up to 191 dB re 1 μPa2s in one of those dolphins, however, failed to result in TTS greater than 14 dB. Recovery rates were similar to those from other studies with non-impulsive sources and depended on the magnitude of the initial TTS.

Conditioned attenuation of dolphin monaural and binaural auditory evoked potentials after preferential stimulation of one ear

Previous studies have demonstrated that some species of odontocetes can be conditioned to reduce hearing sensitivity when warned of an impending intense sound; however, the underlying mechanisms remain poorly understood. In the present study, conditioned hearing attenuation was elicited in two bottlenose dolphins by pairing a 10-kHz tone (the conditioned stimulus) with a more intense tone (the unconditioned stimulus) at 28 kHz. Testing was performed in air, with sounds presented via contact transducers. Hearing was assessed via noninvasive measurement of monaural auditory nerve responses (ANR) and binaural auditory brainstem responses (ABR). ABRs/ANRs were measured in response to 40-kHz tone bursts, over 2 to 3-s time intervals before and after the conditioned and unconditioned stimuli. Results showed reductions in ABR/ANR amplitude and increases in latency after pairing the warning and more intense tones. Monaural ANRs from the left and right ears were attenuated by similar amounts when the warning and more intense sounds were preferentially applied to the right ear. The data support a neural mechanism operating at the level of the cochlea and/or auditory nerve and suggest the involvement of neural projections that can affect the contralateral ear.

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 (SEL_SS) 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 SEL_SS regulation, previous deterrence, and soft start allow harbor porpoises to avoid a TTS from multiple exposures. However, deterrence efficiency has to be monitored.

The use of seal scarers as a protective mitigation measure can induce hearing impairment in harbour porpoises

Acoustic deterrent devices (ADDs) are used to deter seals from aquacultures but exposure of harbour porpoises (Phocoena phocoena) occurs as a side-effect. At construction sites, by contrast, ADDs are used to deter harbour porpoises from the zone in which pile driving noise can induce temporary threshold shifts (TTSs). ADDs emit such high pressure levels that there is concern that ADDs themselves may induce a TTS. A harbour porpoise in human care was exposed to an artificial ADD signal with a peak frequency of 14 kHz. A significant TTS was found, measured by auditory evoked potentials, with an onset of 142 dB re 1 μPa²s at 20 kHz and 147 dB re 1 μPa²s at 28 kHz. The authors therefore strongly recommend to gradually increase and down regulate source levels of ADDs to the desired deterrence range. However, further research is needed to develop a reliable relationship between received levels and deterrence.

Long-term evidence of noise-induced permanent threshold shift in a harbor seal (Phoca vitulina)

In psychophysical studies of noise-induced hearing loss with marine mammals, exposure conditions are often titrated from levels of no effect to those that induce significant but recoverable loss of auditory sensitivity [temporary threshold shift (TTS)]. To examine TTS from mid-frequency noise, a harbor seal was exposed to a 4.1-kHz underwater tone that was incrementally increased in sound pressure level (SPL) and duration. The seal's hearing was evaluated at the exposure frequency and one-half octave higher (5.8 kHz) to identify the noise parameters associated with TTS onset. No reliable TTS was measured with increasing sound exposure level until the second exposure to a 60-s fatiguing tone of 181 dB re 1 μPa SPL (sound exposure level 199 dB re 1 μPa²s), after which an unexpectedly large threshold shift (>47 dB) was observed. While hearing at 4.1 kHz recovered within 48 h, there was a permanent threshold shift of at least 8 dB at 5.8 kHz. This hearing loss was evident for more than ten years. Furthermore, a residual threshold shift of 11 dB was detected one octave above the tonal exposure, at 8.2 kHz. This hearing loss persisted for more than two years prior to full recovery.

Influence of background noise on auditory evoked responses to rippled-spectrum signals

The resolution of spectral patterns in adaptation background noise was investigated in a beluga whale, Delphinapterus leucas, using the evoked-potential technique. The resolution of spectral patterns was investigated using rippled-spectrum test stimuli of various levels and ripple densities and recording the rhythmic evoked responses (the rate following response, RFR) to ripple phase reversals. In baseline (no adaptation background noise) experiments, the highest RFR magnitude was observed at signal sound pressure levels (SPLs) of 100-110 dB re 1 μPa; at SPLs both below the optimum (down to 80 dB re 1 μPa) and above the optimum (up to 140 dB re 1 μPa), the RFR magnitude decreased. For high signal levels (above 110 dB re 1 μPa), low-level adaptation background noise (from -10 to -20 dB re signal level) increased RFR magnitude compared to baseline. This effect is considered to be a result of the optimization of the sensation level of the high-SPL signals due to decreasing hearing sensitivity caused by the adaptation background noise.

A review of the history, development and application of auditory weighting functions in humans and marine mammals

This document reviews the history, development, and use of auditory weighting functions for noise impact assessment in humans and marine mammals. Advances from the modern era of electroacoustics, psychophysical studies of loudness, and other related hearing studies are reviewed with respect to the development and application of human auditory weighting functions, particularly A-weighting. The use of auditory weighting functions to assess the effects of environmental noise on humans-such as hearing damage-risk criteria-are presented, as well as lower-level effects such as annoyance and masking. The article also reviews marine mammal auditory weighting functions, the development of which has been fundamentally directed by the objective of predicting and preventing noise-induced hearing loss. Compared to the development of human auditory weighting functions, the development of marine mammal auditory weighting functions have faced additional challenges, including a large number of species that must be considered, a lack of audiometric information on most species, and small sample sizes for nearly all species for which auditory data are available. The review concludes with research recommendations to address data gaps and assumptions underlying marine mammal auditory weighting function design and application.

Auditory sensitivity to local stimulation of the head surface in a beluga whale (Delphinapterus leucas)

Using the auditory evoked response technique, sensitivity to local acoustic stimulation of the ventro-lateral head surface was investigated in a beluga whale (Delphinapterus leucas). The stimuli were tone pip trains of carrier frequencies ranging from 16 to 128 kHz with a pip rate of 1 kHz. For higher frequencies (90-128 kHz), the low-threshold point was located next to the medial side of the middle portion of the lower jaw. For middle (32-64 kHz) and lower (16-22.5 kHz) frequencies, the low-threshold point was located at the lateral side of the middle portion of the lower jaw. For lower frequencies, there was an additional low-threshold point next to the bulla-meatus complex. Based on these data, several frequency-specific paths of sound conduction to the auditory bulla are suggested: (i) through an area on the lateral surface of the lower jaw and further through the intra-jaw fat-body channel (for a wide frequency range); (ii) through an area on the ventro-lateral head surface and further through the medial opening of the lower jaw and intra-jaw fat-body channel (for a high-frequency range); and (iii) through an area on the lateral (near meatus) head surface and further through the lateral fat-body channel (for a low-frequency range).

Auditory evoked potentials in the auditory system of a beluga whale Delphinapterus leucas to prolonged sound stimuli

The effects of prolonged (up to 1500 s) sound stimuli (tone pip trains) on evoked potentials (the rate following response, RFR) were investigated in a beluga whale. The stimuli (rhythmic tone pips) were of frequencies of 45, 64, and 90 kHz at levels from 20 to 60 dB above threshold. Two experimental protocols were used: short- and long-duration. For the short-duration protocol, the stimuli were 500-ms-long pip trains that repeated at a rate of 0.4 trains/s. For the long-duration protocol, the stimuli were continuous pip successions lasting up to 1500 s. The RFR amplitude gradually decreased by three to seven times from 10 ms to 1500 s of stimulation. Decrease of response amplitude during stimulation was approximately proportional to initial (at the start of stimulation) response amplitude. Therefore, even for low stimulus level (down to 20 dB above the baseline threshold) the response was never suppressed completely. The RFR amplitude decay that occurred during stimulation could be satisfactorily approximated by a combination of two exponents with time constants of 30-80 ms and 3.1-17.6 s. The role of adaptation in the described effects and the impact of noise on the acoustic orientation of odontocetes are discussed.

Noise-induced hearing loss in marine mammals: A review of temporary threshold shift studies from 1996 to 2015

One of the most widely recognized effects of intense noise exposure is a noise-induced threshold shift—an elevation of hearing thresholds following cessation of the noise. Over the past twenty years, as concerns over the potential effects of human-generated noise on marine mammals have increased, a number of studies have been conducted to investigate noise-induced threshold shift phenomena in marine mammals. The experiments have focused on measuring temporary threshold shift (TTS)—a noise-induced threshold shift that fully recovers over time—in marine mammals exposed to intense tones, band-limited noise, and underwater impulses with various sound pressure levels, frequencies, durations, and temporal patterns. In this review, the methods employed by the groups conducting marine mammal TTS experiments are described and the relationships between the experimental conditions, the noise exposure parameters, and the observed TTS are summarized. An attempt has been made to synthesize the major findings across experiments to provide the current state of knowledge for the effects of noise on marine mammal hearing.

Mapping probability of shipping sound exposure level

Mapping vessel noise is emerging as one method of identifying areas where sound exposure due to shipping noise could have negative impacts on aquatic ecosystems. The probability distribution function (pdf) of sound exposure levels (SEL) is an important metric for identifying areas of concern. In this paper a probabilistic shipping SEL modeling method is described to obtain the pdf of SEL using the sonar equation and statistical relations linking the pdfs of ship traffic density, source levels, and transmission losses to their products and sums.

Effects of multiple impulses from a seismic air gun on bottlenose dolphin hearing and behavior

To investigate the auditory effects of multiple underwater impulses, hearing thresholds were measured in three bottlenose dolphins before and after exposure to 10 impulses produced by a seismic air gun. Thresholds were measured at multiple frequencies using both psychophysical and electrophysiological (auditory evoked potential) methods. Exposures began at relatively low levels and gradually increased over a period of several months. The highest exposures featured peak sound pressure levels from 196 to 210 dB re 1 μPa, peak-peak sound pressure levels of 200-212 dB re 1 μPa, and cumulative (unweighted) sound exposure levels from 193 to 195 dB re 1 μPa(2)s. At the cessation of the study, no significant increases were observed in psychophysical thresholds; however, a small (9 dB) shift in mean auditory evoked potential thresholds, accompanied by a suppression of the evoked potential amplitude function, was seen in one subject at 8 kHz. At the highest exposure condition, two of the dolphins also exhibited behavioral reactions indicating that they were capable of anticipating and potentially mitigating the effects of impulsive sounds presented at fixed time intervals.

Conditioned frequency-dependent hearing sensitivity reduction in the bottlenose dolphin (Tursiops truncatus)

The frequency specificity of conditioned dampening of hearing, when a loud sound is preceded by a warning sound, was investigated in a bottlenose dolphin. The loud sounds were 5 s tones of 16, 22.5 or 32 kHz, sound pressure level of 165 dB root mean square (RMS) re. 1 µPa. Hearing sensitivity was tested at the same three frequencies. Hearing sensitivity was measured using pip-train test stimuli and auditory evoked potential recording. The test sound stimuli served also as warning sounds. The durations of the warning sounds were varied randomly to avoid locking a conditioning effect to the timing immediately before the loud sound. Hearing thresholds before the loud sound increased, relative to the baseline, at test frequencies equal to or higher than the loud sound frequency. The highest threshold increase appeared at test frequencies of 0.5 octaves above the loud sound frequencies.

Cetacean noise criteria revisited in the light of proposed exposure limits for harbour porpoises

The impact of underwater noise on marine life calls for identification of exposure criteria to inform mitigation. Here we review recent experimental evidence with focus on the high-frequency cetaceans and discuss scientifically-based initial exposure criteria. A range of new TTS experiments suggest that harbour and finless porpoises are more sensitive to sound than expected from extrapolations based on results from bottlenose dolphins. Furthermore, the results from TTS experiments and field studies of behavioural reactions to noise, suggest that response thresholds and TTS critically depend on stimulus frequency. Sound exposure levels for pure tones that induce TTS are reasonably consistent at about 100 dB above the hearing threshold for pure tones and sound pressure thresholds for avoidance reactions are in the range of 40-50 dB above the hearing threshold. We propose that frequency weighting with a filter function approximating the inversed audiogram might be appropriate when assessing impact.