Evidence Gap FF.06: Underwater noise and vibrations
Review of current knowledge
The majority of fish and invertebrates use sound for vital life functions (communication, detection of prey and predators, mating, orientation and migration, habitat selection, etc) (Spiga et al 2012, Hawkins and Popper 2017, Weilgart, 2018, Popper and Hawkins 2019) and there is growing evidence that the introduction of man-made sound in aquatic environments has the potential to affect (Hawkins and Popper 2018) the ability of fish to detect and use the biologically relevant sounds that are important for their survival. Furthermore, it is well established that intense sounds not only affect fish sound detection and behaviour but also have the potential to have physiological and physical effects that could result in reduced fitness and in some cases death (Hawkins and Popper 2018). Behavioural responses are of particular concern if fish become more exposed to predators, are displaced from key habitats such as feeding and spawning grounds, their migrations are affected or experience disruption of communication between individuals (Hawkins et al 2020).
Concerns on underwater noise related effects on marine fauna associated with MRE development tend to be primarily focused on the potential impact of intense sounds such as those associated with unexploded ordnance (UXO) detonations and impact pile driving during construction works.
Noise associated with wind turbine vibration during operation, and other sources of continuous noise such as engine noise from survey, construction and operation and maintenance vessels, and drilling and dredging activities, however, can also contribute to increased ambient noise levels.
Whilst general effects of noise on fish and invertebrates have been reviewed extensively in recent years (e.g. Popper et al 2014, Radford et al 2014, Williams et al 2015, Kunc et al 2016, Hawkins and Popper 2017, Popper and Hawkins 2019), there are still significant gaps in our current knowledge.
Sound waves have both a sound pressure and a particle motion component, however, most fish and invertebrates primarily sense sound using particle motion rather than sound pressure. Yet, despite its relevance to fish and invertebrates, few studies have measured the particle motion component of sound (Mueller-Blenkle 2010) and the role of particle motion in the biology and ecology of fish and invertebrates is largely unknown (Nedelec et al 2016). In addition, even for species that only sense particle motion, existing noise exposure criteria, based on current best practice guidelines (Popper et al 2014), are fully based on sound pressure.
There is therefore a need to better describe the characteristics of sound propagation in terms of particle motion, and in particular the propagation of sound and vibration through the seabed as this is especially relevant for benthic fish species and invertebrates. The monitoring of particle motion along with sound pressure and the development of instrumentation and software for this purpose has been identified as a high research priority in this field (Hawkins et al 2015). As identified in the current ScotMER fish and fisheries evidence map, best practice guidance on measuring particle motion is currently being developed by Exeter University and partners (publication expected in 2021).
In order to update current exposure criteria and guidelines for fish, taking account of the particle motion component of sound in addition to sound pressure, it is important that data on fish hearing sensitivity to particle motion is also collected (Popper and Howkins 2019). Much of the current data has been obtained either under unsatisfactory acoustic conditions or by means of physiological measurements and do not give an accurate indication of the detection ability of the animals. More detailed knowledge of the hearing abilities of fish and invertebrates is required, including the development of audiograms based on behavioural analysis (Hawkins et al 2015, Popper and Howkins 2019).
Behavioural impacts in fish
Quantitative criteria to address behavioural responses in fish are yet to be developed. Current noise exposure criteria (Popper et al 2014) only provide thresholds for the onset of Temporary Threshold Shifts (TTS), recoverable injury and mortality, in response to various impulsive sound sources (including pile driving), and for shipping and continuous sounds. As behavioural effects are strongly dependent on behavioural context and responses may not scale with sound level, there is considerable uncertainty in assessing risk of behavioural responses (Faulkner et al 2018). The use of experiments using new technologies, such as active acoustics tagging, to gather detailed observations on the behaviour of animals in the natural environment should therefore be encouraged (Hawkins et al 2015).
Impact on invertebrates
In recent years, studies of the effect of underwater noise effects on marine fauna have been increasingly focused on invertebrates, including crustaceans, and molluscs (Tidau and Briffa 2016, Edmonds et al 2016, Solan et al 2016, Jones et al 2020). The existing available data, however, is not considered sufficient to allow the definition of noise exposure criteria for invertebrates (Popper et al 2014). As a result, in the absence of specific criteria, assessments of the impact of noise on invertebrates have to draw on the evidence collected in existing studies (Faulkner et al 2018) and cannot be informed by detailed project specific noise modelling.
In addition to improving the existing knowledge on the effect on noise on fish and invertebrates, suitable approaches to minimise potential impacts also need to be developed and implemented. These require consideration of the use of biological information to minimise impacts as well as of potential changes to the sound sources to minimise noise levels.
Hawkins et al (2015) highlights the need to improve our knowledge on identification of critical habitats, migration routes and reproductive periods so that exposures during these sensitive phases can be avoided. The development of suitable mitigation strategies in relation to underwater noise on fish is therefore closely linked to "Evidence Gap FF.09: Accurate spatio-temporal patterns of spawning activity" and "Evidence Gap FF.10: Essential fish habitat". In this context it is important to note that standard noise mitigation measures implemented in the UK are generally focused on marine mammals (i.e. soft start piling, acoustic deterrent devices (ADDs), Marine Mammal Observers (MMOs)) rather than specifically designed to minimise impacts on fish. To date, where there has been a requirement to mitigate potential noise impacts on specific fish species, this has been generally addressed via consent conditions which impose restrictions over noise generating activities during specific periods of time and/or in specific areas. In some instances, initial restrictions have been lifted or reduced following the provision of additional evidence by developers (i.e. through survey work, additional noise modelling, etc).
With regard to potential mitigation measures in respect the reduction of sound sources to minimise noise levels, Merchant and Robinson (2020) and Verfuss et al (2019) provide comprehensive reviews of the current state of knowledge on the feasibility of different noise abatement options, including detailed information on the following technologies (see Table 4):
- Bubble curtains;
- Resonators; and
- Alternative hammers (i.e. Vibratory Hammer and Blue Hammer).
|Percussive pile driving
|Alternative turbine foundations and piling methods
Merchant and Robinson (2020) noted that no new policy would be needed to implement noise abatement in UK waters for offshore wind farm installation or detonation of UXO and conclude that their deployment would be feasible at locations where offshore wind farms are proposed in UK waters.
The above technologies have been successfully deployed in other parts of the North Sea to reduce the risk of impact on marine life. However, abatement measures are generally implemented to specifically address potential impacts on marine mammals, with fish being of secondary focus in this respect, and tend to be more effective at reducing risks for marine mammals and fish species that are sensitive to high frequencies sounds (>100 Hz) (Verfuss et al 2019).
In many countries, including the UK, it is rare for such technologies to be required by regulators, and the effect zones that would be achieved through their use are generally not modelled as part of the assessment process for MRE projects. The incorporation of noise abatement options in the noise modelling exercise together with the use of standard methods and metrics across EIAs, would help the undertaking of assessments at the project level that can feed into consistent cumulative assessments (Faulkner et al 2018).
Next steps in research
As identified above, there are significant data gaps in our understanding of how underwater noise affects fish and invertebrates and existing tools to help assessments need improving. In order to address the identified knowledge gaps, the following next steps in research are recommended:
- Collection and measurement of noise data, including measurements of particle-motion.
- Development of updated noise exposure criteria for fish to take account of the particle motion component of noise.
- Development of audiograms for key species using behavioural analysis.
- Strategic research to investigate the scale of the effect of noise exposure on fish and invertebrates that may result in population level impact or economic impact to fisheries.
- Development of detailed guidance to help the assessment of behavioural effects on fish.
- Provision of guidance on the approach to be taken for assessment of impacts on invertebrates in the absence of standard noise exposure criteria for this group.
- Collection of improved data and information on the behavioural effect of noise on fish and invertebrates
- Testing of noise abatement methods at wind farms during construction and their effectiveness in mitigating impacts on fish.
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