9.4. Effects of Noise and Particle Motion

  1. Potential effects of noise and particle motion on fishes and invertebrates can be summarised as follows (Popper et al., 2014; Popper and Hawkins, 2018; Nedelec et al., 2016):
  • Death and injury
  • Exposure to very high amplitude noises can cause injury and death in fish and other marine species. In addition, the effect of sudden pressure changes (barotrauma) must be considered.

–           Barotrauma is the tissue injury that is caused by a sudden change in pressure resulting in a shock wave effect (e.g. primarily caused by explosions, as opposed to non-shock wave propagation as is typically caused by impulsive piling). Rapid pressure changes can cause the gases in blood to come out of solution and can cause rapid movement in the swim bladder. This can damage other organs and even rupture the swim bladder.

–           Sudden changes in pressure (such as that from impulsive noises) are more likely to cause damage than gradual ones.

–           Extreme levels of particle motion may have the potential to cause tissue damage, but this has not been proven yet (Popper et al., 2014).

–           Hearing loss can be permanent or temporary (PTS and TTS) with PTS being caused by damage to the tissue in the auditory pathway (including the swim bladder)

–           TTS results from temporary damage to the hairs in the inner ear or to the auditory nerves. In fish (unlike in mammals) the hairs of the inner ear are constantly added and replaced if damaged. Therefore, loss of hearing due to damage to these hairs may be mitigated over time in fish.

–           While experiencing TTS, fish may have a decrease in fitness in terms of communication, detecting predators or prey, and/or assessing their environment.

–           Masking is an impairment with respect to the relevant noise sources normally detected within the noisescape. The consequences of masking are not fully understood for fish. It is likely that higher levels of masking occur with a higher noise level from the masker.

–           It is possible that anthropogenic noise will have a detrimental effect on the communication of species between conspecifics, it may also hinder their identification of predator and prey.

–           There have been a variety of behavioural reactions observed from fish, including changes in swimming patterns and startle reactions. These reactions may habituate over repeated exposure to the noise.

–           There has been very limited research carried out to date in relation to the effects of particle motion on marine invertebrates (Popper and Hawkins, 2018). However, they are expected to have the same types of effect even if the severity is unclear.

  1. Popper et al. (2014) categorised fish species into the following identifiable groups:
  • Fishes with no swim bladder or other gas chamber. These fish are less susceptible to barotrauma and only detect particle motion, however, some barotrauma may occur from exposure to sound pressure.
  • Fish with swim bladders in which hearing does not involve the swim bladder or some other gas volume. These species again only detect particle motion; however, they are susceptible to barotrauma due to the presence of the swim bladder.
  • Fish in which the swim bladder (or other gas volume) is involved in hearing. These species detect sound pressure as well as particle motion and are susceptible to barotrauma. The frequency sensitivity range of this group is higher than the other groups due to the ability to detect the pressure component of the sound signal as well as the particle motion.
  • Fish eggs and larvae.
  1. These groups are known to be able to detect particle motion. However, it is also likely that marine invertebrates are able to detect particle motion (Popper and Hawkins, 2018; Discovery of Noise in the Sea (DOSITS)). Furthermore, some marine invertebrates can detect the vibrations directly from the substrate. This makes them susceptible not only to the particle motion in the water but also the rolling waves, and associated particle motion, in the substrate. It has been observed that benthic marine invertebrates respond directly to anthropogenic noise that has been generated in the substrate or very close to its surface (Hawkins et al., 2021; Aimon et al., 2021). This is particularly important for construction processes like piling that generate a large amount of noise into the substrate. The repercussions of this are that offshore construction activity may affect the benthic habitat, and many benthic invertebrates have a key role in how the substrate is structured. Considerable disturbance of these creatures for a prolonged period could affect habitat quality in addition to any potential impacts associated with sound pressure. It has also been suggested that some species use the noise that travels through the substrate to communicate or to find food sources, loud noises that mask these noises could make it difficult for them to operate normally (Popper and Hawkins, 2018).
  2. There have been several studies into the hearing abilities of fish for a relatively small number of species. From these studies, the upper limit of detection for particle motion was found to be between 200 Hz and 400 Hz and the lower limit was 0.1 Hz (Sigray and Anderson, 2011). It is considered likely that all teleost fish have a similar extent of ability to detect particle motion (Radford et al., 2012). Elasmobranchs are also considered to have a similar range of detection for particle motion. For piling, specifically, it is currently considered that most fish would be able to detect particle motion from 750 m away (Thomsen et al., 2015). Marine invertebrates are generally not considered to be sensitive to the pressure wave component of noise as they lack an air-filled space in their bodies. Research still needs to be carried out to understand the hearing capabilities of marine invertebrates. The research that has been undertaken so far has primarily focused on crustaceans and molluscs. A need has been identified to develop species specific audiograms to improve the understanding of the detection thresholds.
  3. Hammar et al. (2014) discussed the impact of the Kattegat offshore wind farm (offshore Sweden) on Atlantic cod Gadus morhua in the region. Estimates of operational noise were predicted as 150 dB re 1μPa (rms) at 1 m for the 6 MW wind turbines and 250 dB re 1 μPa (rms) for the pile driving based on measurements on the Burbo Bank offshore wind farm taken by Parvin and Nedwell (2006). Using these estimates Hammar et al. (2014) established that developed Atlantic cod were likely to suffer physical injury within several hundred meters of pile driving. However, studies have shown that fish often group around operational wind turbines (Sigray and Andersson, 2011; Engås et al., 1995; Wahlberg and Westerberg, 2005). This suggests that operational noise is not enough to cause them to vacate the area, however it is not clear if it results in higher stress levels in fish in the area.

9.5. Potential Range of Effects Due to Particle Motion

  1. Due to the current state of understanding and existing (validated) modelling methodologies it is not considered feasible at this time to provide a quantitative assessment of the effects of particle motion on marine species for the Array.
  2. Predicting the levels of particle motion from anthropogenic noise sources is difficult. There is a small amount of measured data available on which to base such predictions and some of these data are not necessarily applicable to full scale industrial procedures such as installation of wind turbine foundations. The measurements that do exist mostly come from small scale tank testing. Some of this testing has been conducted in flooded dock style locations with small scale piles. Other recordings have used play-back speakers to generate a simulated piling noise (Roberts et al., 2016; Ceraulo et al., 2016). There is some debate about the validity of comparing measurements from tank tests or from playback speakers to full scale piling operations, as the way that particles move within a tank or smaller scale system is different to the full scale in the open ocean. Furthermore, the way that a speaker will agitate the particles is different to that of a cylindrical pile with an exposed length in the water column and sediment. However, there is one commonality between all measurements so far: the particle motion attenuates rapidly close to the source and more slowly further from it (Mueller-Blenkle et al., 2010).
  3. One such experiment was studied by Ceraulo et al. (2016), which consisted of measurements during piling at several locations within a flooded dock that incorporated a simulated seabed layer (approximately 3.5 m thick). This allowed the piling to be measured from different ranges. Through this experiment it was found that the noise propagation was close to cylindrical in nature. The levels of particle motion (particle velocity) were found to be 102 dB re 1 nm/s at a distance of 2 m from the pile and this dropped to 86 dB re 1 nm/s at 30 m. There was an interesting observation that the pressure wave appeared to have a cut off frequency at 400 Hz for shallow water and 300 Hz for deep water, although the particle motion does not share this cut off. The study was able to confirm that there is a roughly linear relation between particle motion and pressure although it also found that the particle motion levels were higher than expected.
  4. An added complication in predicting particle motion is the propagation of noise through the substrate. This is particularly prominent in piling operations as the pile being driven into the ground will generate considerable waves through the substrate. This particle motion can impact the benthic species in the area due to behavioural reactions and potential injury. This has been identified as an area that requires more research and should be monitored alongside particle motion within the water column itself. Furthermore, the waves passing through the substrate can add to those in the water column, making the noise field in the water more complex (Mueller-Blenkle et al., 2010).
  5. A study by Thomsen et al. (2015) investigated particle motion around the installation of piles at offshore wind sites. The study found that higher hammer energies elicited higher levels of particle motion and that particle motion levels at 750 m from the pile were higher than baseline ambient levels throughout the frequency spectrum, except at very low frequencies. Thomsen et al. (2015) showed that with mitigation (a bubble curtain) turned on however, particle motion levels reduced considerably. It should be noted that the range cited of 750 m was likely due to the regulatory requirement for monitoring at 750 m from a pile and this number is therefore somewhat arbitrary in terms of the potential range of effect for particle motion (i.e. it is the most common measurement range for sound pressure rather than being the range over which particle motion effects were thought likely to occur).
  6. Nevertheless, the study concluded that, for most fish, particle motion levels at 750 m are high enough to be detected during pile driving of even a mitigated pile. However, for elasmobranchs, the study concluded that detectability of mitigated piles is likely restricted to relatively short ranges from the source depending on the ambient noise in the area. For invertebrates the study concluded that there is even less information on how they perceive particle motion, but the Thomsen et al. (2015) study would indicate that some invertebrates should be able to detect the piling noise at a distance of 750 m, whether mitigated or not.
  7. Taking the above into consideration, it is thought likely that particle motion will be detectable for many fish and invertebrates within the order of 750 m from piling at the Array, although it is not feasible to quantify this further at this stage. Furthermore, it is not possible at this time to determine whether the detection of noise by these species at this range is likely to result in an effect, such as behavioural disturbance or injury. Likewise, it is not possible at this time to define the requirements for, or potential effectiveness of, mitigation for particle motion. However, it is likely that potential injury due to particle motion will be confined to a smaller range than disturbance and detectability. Ultimately, until such a time as considerably more data become available, both in terms of measured particle motion during full scale piling and effects on marine species, it is considered that the assessment of effects as set out in this report represents a robust assessment based on the current state of knowledge.