Modelling sound particle motion in shallow watera).

Fish species and aquatic invertebrates are sensitive to underwater sound particle motion. Studies on the impact of sound on marine life would benefit from sound particle motion models. Benchmark cases and solutions are proposed for the selection and verification of appropriate models. These include a range-independent environment, with and without shear in the sediment, and a range-dependent environment, without sediment shear. Analysis of the acoustic impedance illustrates that sound particle velocity can be directly estimated from the sound pressure field in shallow water scenarios, except at distances within one wavelength of the source, or a few water depths at frequencies where the wavelength exceeds the water depth.

Ship noise causes tagged harbour porpoises to change direction or dive deeper.

Shipping is the most pervasive source of marine noise pollution globally, yet its impact on sensitive fauna remains unclear. We tracked 10 harbour porpoises for 5-10 days to determine exposure and behavioural reactions to modelled broadband noise (10 Hz-20 kHz, VHF-weighted) from individual ships monitored by AIS. Porpoises spent a third of their time experiencing ship noise above ambient, to which they regularly reacted by moving away during daytime and diving deeper during night. However, even ships >2 km away (noise levels of 93 ± 14 dB re 1 µPa2) caused animals to react 5-9 % of the time (∼18.6 ships/day). Ships can thus influence the behaviour and habitat use of cetaceans over long distances, with worrying implications for fitness in coastal areas where anthropogenic noise from dense ship traffic repeatedly disrupt their natural behaviour.

Predicting the contribution of climate change on North Atlantic underwater sound propagation.

Since the industrial revolution, oceans have become substantially noisier. The noise increase is mainly caused by increased shipping, resource exploration, and infrastructure development affecting marine life at multiple levels, including behavior and physiology. Together with increasing anthropogenic noise, climate change is altering the thermal structure of the oceans, which in turn might affect noise propagation. During this century, we are witnessing an increase in seawater temperature and a decrease in ocean pH. Ocean acidification will decrease sound absorption at low frequencies (<10 kHz), enhancing long-range sound propagation. At the same time, temperature changes can modify the sound speed profile, leading to the creation or disappearance of sound ducts in which sound can propagate over large distances. The worldwide effect of climate change was explored for the winter and summer seasons using the (2018 to 2022) and (2094 to 2098, projected) atmospheric and seawater temperature, salinity, pH and wind speed as input. Using numerical modelling, we here explore the impact of climate change on underwater sound propagation. The future climate variables were taken from a Community Earth System Model v2 (CESM2) simulations forced under the concentration-driven SSP2-4.5 and SSP5-8.5 scenarios. The sound modeling results show, for future climate change scenarios, a global increase of sound speed at different depths (5, 125, 300, and 640 m) except for the North Atlantic Ocean and the Norwegian Sea, where in the upper 125 m sound speed will decrease by as much as 40 m s-1. This decrease in sound speed results in a new sub-surface duct in the upper 200 m of the water column allowing ship noise to propagate over large distances (>500 km). In the case of the Northeast Atlantic Ocean, this sub-surface duct will only be present during winter, leading to similar total mean square pressure level (SPLtot) values in the summer for both (2018 to 2022) and (2094 to 2098). We observed a strong and similar correlation for the two climate change scenarios, with an increase of the top 200 m SPLtot and a slowdown of Atlantic Meridional Overturning Circulation (AMOC) leading to an increase of SPLtot at the end of the century by 7 dB.

Lack of reproducibility of temporary hearing threshold shifts in a harbor porpoise after exposure to repeated airgun sounds.

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.

Ecological Risk Assessment of Underwater Sounds from Dredging Operations.

There is an increasing international focus to understand and quantify the potential ecological risks of low-frequency underwater sounds produced from anthropogenic activities (e.g., commercial shipping, dredging, construction, and offshore energy production). For dredge operations, a risk-based approach has been proposed for identifying, assessing, and managing risks; however, specific details of the framework and demonstration of the approach are lacking. Thus, the goal of this study was to provide a practical, concise, and reliable framework for assessing the effects of dredging sounds on aquatic life. The specific objectives were to 1) further specify a risk assessment approach for assessing underwater sounds from dredging operations, 2) demonstrate the utility of the approach in practice using a case study, and 3) document the strengths and challenges of the approach. The risk framework was adapted for underwater sounds to include a project formulation step, an analysis step to analyze and assess exposure and biological responses, a risk characterization process in which the preceding steps are integrated and uncertainty is addressed, and a risk management step. A key beneficial component of this framework is the use of a phased approach, whereby a screening step offers a process that utilizes existing or readily available information to evaluate risk. In general, a limitation of evaluating risks due to dredge operations is the degree of uncertainty surrounding effect thresholds for many marine species; however, this approach emphasizes the importance of documenting and communicating uncertainty to regulators, stakeholders, and practitioners in the decision-making process. A case study example is included to illustrate how the framework can be applied in practice. The primary strength of this method is the intrinsic flexibility of the framework to adapt as the scientific understanding improves and new data become available in the rapidly evolving field of underwater acoustics. Integr Environ Assess Manag 2020;16:481-493. © 2020 SETAC.

Temporary hearing threshold shift in a harbor porpoise (Phocoena phocoena) after exposure to multiple airgun sounds.

In seismic surveys, reflected sounds from airguns are used under water to detect gas and oil below the sea floor. The airguns produce broadband high-amplitude impulsive sounds, which may cause temporary or permanent threshold shifts (TTS or PTS) in cetaceans. The magnitude of the threshold shifts and the hearing frequencies at which they occur depend on factors such as the received cumulative sound exposure level (SELcum), the number of exposures, and the frequency content of the sounds. To quantify TTS caused by airgun exposure and the subsequent hearing recovery, the hearing of a harbor porpoise was tested by means of a psychophysical technique. TTS was observed after exposure to 10 and 20 consecutive shots fired from two airguns simultaneously (SELcum: 188 and 191 dB re 1 µPa2s) with mean shot intervals of around 17 s. Although most of the airgun sounds' energy was below 1 kHz, statistically significant initial TTS1-4 (1-4 min after sound exposure stopped) of ∼4.4 dB occurred only at the hearing frequency 4 kHz, and not at lower hearing frequencies tested (0.5, 1, and 2 kHz). Recovery occurred within 12 min post-exposure. The study indicates that frequency-weighted SELcum is a good predictor for the low levels of TTS observed.

Sources of Underwater Sound and Their Characterization.

Because of the history of sonar and sonar engineering, the concept of "source level" is widely used to characterize anthropogenic sound sources, but is it useful for sources other than sonar transmitters? The concept and applicability of source level are reviewed for sonar, air guns, explosions, ships, and pile drivers. International efforts toward the harmonization of the terminology for underwater sound and measurement procedures for underwater sound sources are summarized, with particular attention to the initiatives of the International Organization for Standardization.

Effect of Pile-Driving Sounds on the Survival of Larval Fish.

Concern exists about the potential effects of pile-driving sounds on fish, but evidence is limited, especially for fish larvae. A device was developed to expose larvae to accurately reproduced pile-driving sounds. Controlled exposure experiments were carried out to examine the lethal effects in common sole larvae. No significant effects were observed at zero-to-peak pressure levels up to 210 dB re 1 µPa(2) and cumulative sound exposure levels up to 206 dB re 1 µPa(2)·s, which is well above the US interim criteria for nonauditory tissue damage in fish. Experiments are presently being carried out for European sea bass and herring larvae.

Offshore Dredger Sounds: Source Levels, Sound Maps, and Risk Assessment.

The underwater sound produced during construction of the Port of Rotterdam harbor extension (Maasvlakte 2) was measured, with emphasis on the contribution of the trailing suction hopper dredgers during their various activities: dredging, transport, and discharge of sediment. Measured source levels of the dredgers, estimated source levels of other shipping, and time-dependent position data from a vessel-tracking system were used as input for a propagation model to generate dynamic sound maps. Various scenarios were studied to assess the risk of possible effects of the sound from dredging activities on marine fauna, specifically on porpoises, seals, and fish.

WODA Technical Guidance on Underwater Sound from Dredging.

The World Organization of Dredging Associations (WODA) has identified underwater sound as an environmental issue that needs further consideration. A WODA Expert Group on Underwater Sound (WEGUS) prepared a guidance paper in 2013 on dredging sound, including a summary of potential impacts on aquatic biota and advice on underwater sound monitoring procedures. The paper follows a risk-based approach and provides guidance for standardization of acoustic terminology and methods for data collection and analysis. Furthermore, the literature on dredging-related sounds and the effects of dredging sounds on marine life is surveyed and guidance on the management of dredging-related sound risks is provided.

Summary Report Panel 1: The Need for Protocols and Standards in Research on Underwater Noise Impacts on Marine Life.

As concern about anthropogenic noise and its impacts on marine fauna is increasing around the globe, data are being compared across populations, species, noise sources, geographic regions, and time. However, much of the raw and processed data are not comparable due to differences in measurement methodology, analysis and reporting, and a lack of metadata. Common protocols and more formal, international standards are needed to ensure the effectiveness of research, conservation, regulation and practice, and unambiguous communication of information and ideas. Developing standards takes time and effort, is largely driven by a few expert volunteers, and would benefit from stakeholders' contribution and support.

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.

Validation of finite element computations for the quantitative prediction of underwater noise from impact pile driving.

The acoustic radiation from a pile being driven into the sediment by a sequence of hammer strikes is studied with a linear, axisymmetric, structural acoustic frequency domain finite element model. Each hammer strike results in an impulsive sound that is emitted from the pile and then propagated in the shallow water waveguide. Measurements from accelerometers mounted on the head of a test pile and from hydrophones deployed in the water are used to validate the model results. Transfer functions between the force input at the top of the anvil and field quantities, such as acceleration components in the structure or pressure in the fluid, are computed with the model. These transfer functions are validated using accelerometer or hydrophone measurements to infer the structural forcing. A modeled hammer forcing pulse is used in the successive step to produce quantitative predictions of sound exposure at the hydrophones. The comparison between the model and the measurements shows that, although several simplifying assumptions were made, useful predictions of noise levels based on linear structural acoustic models are possible. In the final part of the paper, the model is used to characterize the pile as an acoustic radiator by analyzing the flow of acoustic energy.

The hearing threshold of a harbor porpoise (Phocoena phocoena) for impulsive sounds (L).

The distance at which harbor porpoises can hear underwater detonation sounds is unknown, but depends, among other factors, on the hearing threshold of the species for impulsive sounds. Therefore, the underwater hearing threshold of a young harbor porpoise for an impulsive sound, designed to mimic a detonation pulse, was quantified by using a psychophysical technique. The synthetic exponential pulse with a 5 ms time constant was produced and transmitted by an underwater projector in a pool. The resulting underwater sound, though modified by the response of the projection system and by the pool, exhibited the characteristic features of detonation sounds: A zero to peak sound pressure level of at least 30 dB (re 1 s(-1)) higher than the sound exposure level, and a short duration (34 ms). The animal's 50% detection threshold for this impulsive sound occurred at a received unweighted broadband sound exposure level of 60 dB re 1 µPa(2)s. It is shown that the porpoise's audiogram for short-duration tonal signals [Kastelein et al., J. Acoust. Soc. Am. 128, 3211-3222 (2010)] can be used to estimate its hearing threshold for impulsive sounds.

Common sole larvae survive high levels of pile-driving sound in controlled exposure experiments.

In view of the rapid extension of offshore wind farms, there is an urgent need to improve our knowledge on possible adverse effects of underwater sound generated by pile-driving. Mortality and injuries have been observed in fish exposed to loud impulse sounds, but knowledge on the sound levels at which (sub-)lethal effects occur is limited for juvenile and adult fish, and virtually non-existent for fish eggs and larvae. A device was developed in which fish larvae can be exposed to underwater sound. It consists of a rigid-walled cylindrical chamber driven by an electro-dynamical sound projector. Samples of up to 100 larvae can be exposed simultaneously to a homogeneously distributed sound pressure and particle velocity field. Recorded pile-driving sounds could be reproduced accurately in the frequency range between 50 and 1000 Hz, at zero to peak pressure levels up to 210 dB re 1µPa(2) (zero to peak pressures up to 32 kPa) and single pulse sound exposure levels up to 186 dB re 1µPa(2)s. The device was used to examine lethal effects of sound exposure in common sole (Solea solea) larvae. Different developmental stages were exposed to various levels and durations of pile-driving sound. The highest cumulative sound exposure level applied was 206 dB re 1µPa(2)s, which corresponds to 100 strikes at a distance of 100 m from a typical North Sea pile-driving site. The results showed no statistically significant differences in mortality between exposure and control groups at sound exposure levels which were well above the US interim criteria for non-auditory tissue damage in fish. Although our findings cannot be extrapolated to fish larvae in general, as interspecific differences in vulnerability to sound exposure may occur, they do indicate that previous assumptions and criteria may need to be revised.

Threshold received sound pressure levels of single 1-2 kHz and 6-7 kHz up-sweeps and down-sweeps causing startle responses in a harbor porpoise (Phocoena phocoena).

Mid-frequency and low-frequency sonar systems produce frequency-modulated sweeps which may affect harbor porpoises. To study the effect of sweeps on behavioral responses (specifically "startle" responses, which we define as sudden changes in swimming speed and/or direction), a harbor porpoise in a large pool was exposed to three pairs of sweeps: a 1-2 kHz up-sweep was compared with a 2-1 kHz down-sweep, both with and without harmonics, and a 6-7 kHz up-sweep was compared with a 7-6 kHz down-sweep without harmonics. Sweeps were presented at five spatially averaged received levels (mRLs; 6 dB steps; identical for the up-sweep and down-sweep of each pair). During sweep presentation, startle responses were recorded. There was no difference in the mRLs causing startle responses for up-sweeps and down-sweeps within frequency pairs. For 1-2 kHz sweeps without harmonics, a 50% startle response rate occurred at mRLs of 133 dB re 1 µPa; for 1-2 kHz sweeps with strong harmonics at 99 dB re 1 µPa; for 6-7 kHz sweeps without harmonics at 101 dB re 1 µPa. Low-frequency (1-2 kHz) active naval sonar systems without harmonics can therefore operate at higher source levels than mid-frequency (6-7 kHz) active sonar systems without harmonics, with similar startle effects on porpoises.

Hearing thresholds of a harbor porpoise (Phocoena phocoena) for helicopter dipping sonar signals (1.43-1.33 kHz) (L).

Helicopter long range active sonar (HELRAS), a "dipping" sonar system used by lowering transducer and receiver arrays into water from helicopters, produces signals within the functional hearing range of many marine animals, including the harbor porpoise. The distance at which the signals can be heard is unknown, and depends, among other factors, on the hearing sensitivity of the species to these particular signals. Therefore, the hearing thresholds of a harbor porpoise for HELRAS signals were quantified by means of a psychophysical technique. Detection thresholds were obtained for five 1.25 s simulated HELRAS signals, varying in their harmonic content and amplitude envelopes. The 50% hearing thresholds for the different signals were similar: 76 dB re 1 µPa (broadband sound pressure level, averaged over the signal duration). The detection thresholds were similar to those found in the same porpoise for tonal signals in the 1-2 kHz range measured in a previous study. Harmonic distortion, which occurred in three of the five signals, had little influence on their audibility. The results of this study, combined with information on the source level of the signal, the propagation conditions and ambient noise levels, allow the calculation of accurate estimates of the distances at which porpoises can detect HELRAS signals.