Operation and maintenance phase

Magnitude of impact
  1. Electricity transfer through AC and DC submarine cables generates EMFs consisting of an electric field component and a magnetic field component (Normandeau Associates Inc et al., 2011). The introduction of subsea cabling within the Array marine mammal study area will increase EMFs in the marine environment. These fields have the potential to alter the behaviour and distributions of species that rely on electric and/or magnetic signals for navigation and hunting.
  2. As outlined in Table 10.17   Open ▸ , the Array is designed to have up to 116 km of dynamic inter-array cables within the water column. While EMFs from inter-array cables may be lower than those from offshore export cables due to the reduced amount of power being transmitted (Thomsen et al., 2015a), several factors can influence the strength of EMFs generated from the inter-array cables. These factors include the distance between conductors, the balance of the load and the type of cable (Copping and Hemery, 2020). Different dynamic cable hanging configurations for FOW structures exist, but the specific arrangement for the Array within the Project Description (e.g. catenary, taut, semi-taut; volume 1, chapter 3) is to be determined post-consent. There is consensus among marine renewable energy (MRE) researchers, developers, and regulators that EMFs travelling through cables from single or small numbers of devices will have relatively low EMF intensities and therefore of very localised extent, resulting in low potential for encounter with animals, and therefore pose a low risk to sensitive marine species (Copping et al., 2020, Hasselman et al., 2023). The intensity of EMF from subsea cables decreases at approximately the inverse square/power of the distance away from the cable (Hutchison et al., 2021), and this attenuation is the same for buried, unburied, and dynamic cables (Hutchison et al., 2021). Therefore, levels of EMF are expected to return to baseline levels with a few metres of the cable.
  3. Marine mammals have been observed to be affected more by the magnetic fields (Gill et al., 2014, Kirschvink et al., 1986, Klinowska, 1990, Tricas and Gill, 2011) rather than electric fields, with passive electroreception inferred as a sensory modality in only two species of odontocetes (Czech-Damal et al., 2012, Hüttner et al., 2022). Whales and dolphins are believed to form a useful “magnetic map” which allows them to travel in areas of low magnetic intensity and gradient (“magnetic valleys” or “magnetic peaks”) (Walker et al., 2003). The current lack of data on magnetic fields within commercial scale FOW farms poses challenges for a comprehensive understanding of the associated risks, and knowledge gaps exist around the estimates of cumulative EMF (i.e. repeated exposure through time and space) (Ocean Science Consulting Ltd., 2022). Therefore, at this stage it is difficult to quantify the exact effects of EMF on marine mammals. Even direct calculations of magnetic fields from dynamic cables alone are considered insufficient for a general risk assessment (Tricas and Gill, 2011). Interactions between the cable system and the Earth's magnetic field are site-specific and depend on factors such as the intensity, shape, direction, and spatial extent of the resultant magnetic field (Tricas and Gill, 2011).
  4. Furthermore, the distance between cables can influence the resulting magnetic intensity. The specific mooring arrangement for the Array is yet to be determined and therefore the potential for electric and magnetic fields from dynamic cables in the Array is currently not feasible to assess in detail. The effects of EMFs for arrays may be additive (rather than synergistic) as some cables may be in close proximity to each other directly under the wind turbine at the cable stiffener, with each cable generating its own near-field magnetic field (Hasselman et al., 2023). Any interaction of EMF would therefore only occur in very close proximity to the underside of the floating foundation. However, most cables will be significantly further apart than this, and the minimum turbine spacing at the Array will be 1000 m, as detailed in volume 1, chapter 3. Magnetic fields from cables are anticipated to dissipate in close proximity to the cable. The likelihood of substantial additive effects of EMF or interaction from adjacent cables is therefore considered to be minimal. . At these ranges, any exposures to EMF are expected to be very short (e.g. few minutes) and occur only when the marine mammal swims through the highly localised area with elevated EMF surrounding the cable (CSA Ocean Sciences Inc and Exponent, 2019). The high mobility of cetaceans means they are unlikely to remain under the influence of EMFs for any prolonged period (Ocean Science Consulting Ltd., 2022), and the spacing (1000 m) between wind turbines at the Array allows influence-free spaces. Furthermore, the length of dynamic cables (up to 116 km) is small in the context of the large site boundary and the water depths within it, and the rapid decay (i.e. within metres) of EMFs with horizontal and vertical distance (Bochert and Zettler, 2006) would also reduce the extent of potential impacts. The area in which EMF is elevated will be very small around each cable and represents a very small portion of the available habitat for marine mammals, who utilise large areas of the ocean (evidenced by the large MUs given for marine mammals). So whilst EMF levels from dynamic cables will be higher than those from buried cables, beyond the range of a few metres, levels of EMFs will be expected to be at baseline levels for this part of the North Sea, resulting in potential impacts that would therefore be highly localised. CSA Ocean Sciences Inc and Exponent (2019) considered the area around undersea power cables to be small (less than 10 m) around the cable, and localised and transient.
  5. To date, studies on EMF-receptor interactions in marine mammals have primarily taken place in controlled settings, such as laboratories or field-deployed enclosures (Copping and Hemery, 2020). However, there is a lack of research on EMF-receptor interactions specifically related to multiple subsea cables, particularly in the vicinity of existing FOW farms (Copping and Hemery, 2020). Consequently, there is no available evidence to facilitate assessments for large scale FOW projects, and there is paucity of data on this topic from other industries. Copping and Hemery (2020) concluded that although odontocetes and mysticetes are likely to detect and respond to the EMFs from underwater cables, there is no evidence that species are likely to be adversely affected. EMF-receptive species may respond to low intensity changes, but the range at which animals respond behaviourally (e.g. attraction or avoidance) is unknown and highly difficult to identify (Albert et al., 2020, Hutchison et al., 2020). Similarly, Geelhoed et al. (2022b) investigated the potential effect of EMFs from export cables at the Borssele Offshore Wind Farm to influence acoustic activity in harbour porpoise, and no relationship was observed.
  6. Across all marine mammal species likely to be encountered in the vicinity of the Array, only humpback whales are known for travelling long annual migration distances that could be potentially affected by magnetic fields (Rizzo and Schulte, 2009, Tricas and Gill, 2011). There is evidence that humpback whales are seasonally present in waters off the east coast of Scotland and that these waters may represent a migratory stopover, or a feeding or recovery opportunity during a longer migration (O’Neil et al., 2019). Whilst it has been suggested that adverse effects on humpback whales occurring in Scottish waters could potentially impact populations in Cape Verde, it is not feasible to assess how many individuals are present off eastern Scotland in total over recent years (see paragraph 206 for more details). There is no evidence that the waters in the vicinity of the Array marine mammals study area represent an important feeding ground during their migration for notable proportion of the population. Considering the above, alongside the seasonality of humpback whale encounters (see paragraph 163), it is unlikely that potential effects associated with EMFs could result in measurable population consequences.
  7. The impact is predicted to be of local spatial extent within the geographic frame of reference, long term duration, continuous and the effect is of high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
Sensitivity of the receptor
  1. Sensitivity of marine mammals to EMF is not widely understood (Ocean Science Consulting Ltd., 2022, Taormina et al., 2018). As of now, there is limited information available regarding the existence and functionality of an electric sense in cetaceans. Czech-Damal et al. (2012) observed that the hairless vibrissal crypts on the rostrum of the Guiana dolphin Sotalia guianensis, initially associated with mammalian whiskers, serve as electroreceptors. The reported perception threshold for the dolphin was 4.6 μV/cm. Considering that fish can produce bioelectric fields above this level, the observed sensitivity is well-suited for detecting bioelectric fields generated by prey items (Czech-Damal et al., 2012). Subsequently, Hüttner et al. (2022) conducted a study on the electroreceptive abilities of bottlenose dolphins and proposed the hypothesis that bottlenose dolphins can perceive electric DC fields well below 0.5 mV/cm-1. Both studies suggested that passive electroreception functions as a supplementary sense to echolocation during benthic feeding, also described for bottlenose dolphins as crater-feeding (Czech-Damal et al., 2012, Hüttner et al., 2022).
  2. Many cetacean species migrate seasonally, covering distances of up to thousands of kilometres each year as they travel between summer feeding grounds in northern waters and wintering grounds in southern waters (Tricas and Gill, 2011). Both mysticetes (e.g. humpback whales) and odontocetes (e.g. dolphins and porpoises), have displayed beneficial correlations with geomagnetic field differences (Tricas and Gill, 2011). While the studies have not determined the precise mechanism for magneto-sensitivity, the observational, theoretical (based on correlation studies), behavioural, physiological, and anatomical evidence, including the presence of magnetite, collectively suggest that cetaceans can sense the Earth's magnetic field and potentially use it for long-distance migrations (Kirschvink et al., 1986, Klinowska, 1990, Tricas and Gill, 2011, Walker et al., 2003). Cetaceans seem to use the Earth's magnetic field for migration in two primary ways: as a map by moving parallel to the contours of the local field topography and as a timer based on the regular fluctuations in the field, allowing animals to monitor their progress on this map (Klinowska, 1990). As such, there exists the potential for animals to react to local variations in the geomagnetic field caused by EMFs from a large number of cables (Walker et al., 2003). Depending on the magnitude and persistence of these changes, such effects could result in trivial temporary alterations in swim direction or longer detours during the animal's migration (Gill et al., 2005).
  3. Hutchison et al. (2021) in the context of buried cables, suggested that the proximity of an animal to the seabed is a contributing factor to the distance from the source and will influence the intensity of EMF to which the animal is exposed. If it is assumed to be the same for the dynamic cables in the water column, it is believed that despite being magneto-sensitive, marine mammals have a relatively low likelihood of being affected by inter-array cable EMFs as their high mobility would limit the duration of exposure (Tricas and Gill, 2011). Furthermore, given marine mammals must surface for air to breathe at intervals (deemed necessarily pelagic), the time experiencing EMF would be further limited (US Wind Inc., 2023). The risk of EMF may increase in the event an animal encounters multiple cables, with fewer EMF-free spaces to navigate. However, in a recent literature review on the effects of EMFs from FOW (Ocean Science Consulting Ltd., 2022), the risk from EMF is still considered to be minimal.
  4. To date no studies have provided direct evidence of magnetoreception in pinnipeds (Ocean Science Consulting Ltd., 2022), it is possible they may detect B-fields, with observations of animals correcting to follow constant headings over large distances (Tricas and Gill, 2011).
  5. All receptors, except humpback whale, are deemed to be of high resilience, high adaptability and recoverability and international value. The sensitivity of these receptors is therefore, considered to be low.
  6. Humpback whale is conservatively deemed to be of medium resilience and adaptability and high recoverability. The sensitivity of humpback whale is therefore, considered to be medium.
Significance of the effect
  1. Overall, for all marine mammal species, the magnitude of the impact is deemed to be low and the sensitivity is considered to be low for all species except humpback whale, which is assessed as medium. Due to the variance associated with the magnitude (i.e. low to medium), the effect is assessed precautionarily as being of minor adverse significance (rather than negligible), which is not significant in EIA terms.
Secondary mitigation and residual effect
  1. No marine mammal mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Injury and disturbance from underwater noise generated during the operation of floating wind turbines and anchor mooring lines

  1. Throughout the operation and maintenance phase of the Array, there is a potential for mooring lines as well as wind turbine structures to generate underwater noise. Due to a limited number of operational floating wind farms at the time of writing, and those in operation being of small scale (in terms of wind turbine numbers and size), a representative operational sound source level for use in modelling was not defined (see volume 3, appendix 10.1). As such, the operational noise from wind turbines and mooring lines have been assessed qualitatively, and was agreed with NatureScot following the Marine Mammal Methodology Note ( Table 10.10   Open ▸ ; volume 3, appendix 5.1, annex B).

                        Operation and maintenance phase

Magnitude of impact
Auditory injury (PTS)
  1. The periods of mooring line slackening and tensioning have the potential to produce transient ‘pinging’ or ‘snapping’ noises during the operation and maintenance phase of the Array (Liu, 1973). As described in volume 3, appendix 10.1, presence of snapping transient noise was identified during acoustic underwater noise measurements at the Hywind Demonstrator Project in Norway in 2011 (Martin et al., 2011). The data was subsequently analysed and Stephenson (2015) extrapolated results from a single wind turbine to a theoretical array and it was found that with up to 115 snapping events per day, the resultant potential cumulative SEL over a 24 hour period was 156 dB re 1 µPa2s at 150 m from the wind turbines. This value is below the PTS and TTS onset acoustic thresholds detailed in paragraph 92 (Southall et al., 2019).
  2. Underwater noise measurements were also taken at the completed Hywind site in Scotland (Burns et al., 2022), with analysis of data recorded at 14 km from the site to determine different sound signatures from the Hywind structures. The study reported three distinct transient sounds from mooring systems, characterised as ‘bang’, ‘creak’ and ‘rattle’ (Burns et al., 2022) and their presence was found to be positively correlated with wave height, and to a limited extent with wind speed. Burns et al. (2022) speculated therefore that transient occurrence was related to the greater influence of waves rather than wind speed on the heave motion of the structure (which may lead to a dynamic response of the mooring system. The sounds were shown to originate from close to the wind turbine as opposed to further down a mooring line. There was little evidence of the sharp and impulsive “snap” noise that was found in the Hywind demonstration system recordings from 2011, and this is possibly explained by the lack of ballast weight on catenary chains (to add tension) at the Hywind Scotland site, compared to the Hywind Demo system.
  3. In terms of turbine related noise, Burns et al. (2022) stated the dominant operational noise from the Hywind system appeared to be distinct tonal sounds, which were relatively narrowband and continuous, typically associated with running machinery. Two dominant tones, largely stable in frequency, were apparent: below 100 Hz and between 350 and 460 Hz. However, the study highlighted it remained unclear which component generated this noise.
  4. A quantitative analysis of the impulsiveness of the soundscape at Hywind showed that sounds generated by FOW farms should be considered as non-impulsive i.e. continuous (Burns et al., 2022). Analysis of the combination of tonal, turbine and mooring transient noises for different wind speeds resulted in the lowest derived broadband source level as 156.7 dB re 1 μPa²m² (and occurred in 10 kn wind speed). The highest (95th percentile) was 172.0 dB re 1 μPa²m² in 25 kn wind speed. The dominant turbine-related tonal noise was measured at 24 Hz and 71 Hz. These source levels were then used to define a noise field across the array to determine the potential impact on marine mammals. Burns et al. (2022) found little difference in the daily marine mammal weighted SEL between the Hywind and control site, and no exceedances of the TTS threshold occurred. The maximum distance at which the TTS could occur across all hearing groups was estimated for harbour porpoise at 50 m from a wind turbine assuming that the animal would remain stationary for the 24 hour period (Burns et al., 2022), which is highly precautionary given the mobile nature of marine mammal receptors. Potential TTS ranges for all species are presented in Table 10.51   Open ▸ . The study concluded that even at a wind speed of 25 knots, the noise footprint is negligible and in the relatively noisy soundscape of the North Sea, it does not present any realistic threat of auditory injury (PTS) to marine mammal receptors.

 

Table 10.51:
Modelled Maximum Distances to Weighted SELcum TTS Threshold for 15 Knots Wind Speed (Burns et al., 2022)

Table 10.51: Modelled Maximum Distances to Weighted SELcum TTS Threshold for 15 Knots Wind Speed (Burns et al., 2022)

 

  1. A recent project by Risch et al. (2023a) collected acoustic data from two FOW farms, currently deployed off the Scottish east coast: Kincardine and Hywind Scotland. At Kincardine five wind turbines rated at 9.5 MW were deployed on semi-submersible foundations, while at Hywind Scotland five 6 MW rated wind turbines were deployed on spar-buoys. The study found noise emissions from FOW turbines were concentrated in the frequencies below 200 Hz, similar to the operational noise of fixed offshore wind turbines, and showed distinct tonal features likely related to rotational speed (between 50 Hz and 80 Hz at Kincardine and 25 Hz and 75 Hz at Hywind Scotland). The median one-third-octave band levels below 200 Hz were between 95 dB re 1 μPa and 100 dB re 1 μPa at about 600 m from the closest wind turbine for both wind farms, well below the level of mild disturbance for cetaceans. The study found the biggest difference between fixed and FOW turbines in relation to underwater noise generation is mooring-related noise, rather the operational wind turbine noise. Risch et al. (2023b) found that during higher wind speeds the number of impulsive sounds or transients from mooring-related structures increased at both Kincardine and Hywind Scotland. Source levels for turbine operational noise (25 Hz to 20 kHz) increased with wind speed at both recording locations, with levels ~3 dB higher at Kincardine than Hywind which may be due to power ratings or difference in mooring structure (semi-submersible versus spar-buoy). The study predicted noise fields for unweighted sound pressure levels were above median ambient noise levels in the North Sea for maximum distances of 3.5 km to 4.0 km from the Kincardine five wind turbine array, and 3.0 km to 3.7 km for the five wind turbine array at Hywind Scotland. At both FOW farm locations, recorded harbour porpoise detections were reduced at the recording site closest to the wind turbine compared to the site further away, but Risch et al. (2023a) does highlight these FOW farms have only been operational for a short period and these observed occurrence patterns may change over time as FOW farms become more mature.
  2. Fixed-foundations may be used as a proxy for operational noise from floating wind turbines, with the main source of noise derived from the moving mechanical parts in the nacelle (which is generally below 1 kHz in frequency). Volume 3, appendix 10.1 highlights measurement data for operational wind farms is lacking, with few empirical investigations, and summarises the relevant literature available (Table 8.27 in volume 3, appendix 10.1). The majority of studies at various wind farms (e.g. Horns Rev, North Hoyle, Scroby Sands, Kentish Flats, Barrow, Burbo Bank, Thorntonbank, Bligh Bank and Princess Amalia Wind Farm) concluded sound levels will be audible by marine mammal receptors, but not at a level that would cause injury or behavioural change (Betke, 2006, Jansen, 2016, Nedwell et al., 2007, Norro et al., 2011, Ward et al., 2006). Norro et al. (2011) reviewed a range of foundation and turbine types and found a slight increase in SPL compared to ambient noise measured prior to construction. It should be noted however, that operational noise will be long term (i.e. over the lifetime of the project) and there is very little understanding of this on marine species.
  3. As discussed in volume 3, appendix 10.1, though existing empirical studies on operational noise from offshore floating wind of any foundation type are limited, there is a general consensus that the risk of injury to marine mammals from both structure-borne noise (regardless of foundation type) and additional noise such as those by moving mooring lines, is very low.
  4. The impact (elevated underwater noise from floating wind turbines and mooring lines) is predicted to be of local spatial extent in the context of the geographic frame of reference, long term duration, continuous and high reversibility (the elevation in underwater sound occurs only during the operation and maintenance phase of the Array). The effect of injury, which is highly unlikely to occur, would be of medium (TTS) to low (PTS) reversibility. It is predicted that the impact will affect the receptor directly. Given that animals are highly unlikely to stay within the injury ranges continuously for 24 hours, injury and therefore population-level effects are highly unlikely to occur. The magnitude is therefore considered to be negligible.
Behavioural disturbance
  1. Although the underwater noise study carried out at completed Hywind site makes no attempt to quantify the disturbance (Burns et al., 2022), the semi-qualitative assessment provided in volume 3, appendix 10.1 concluded that the areas of disturbance are unlikely to extend further than those for fixed wind turbine foundations.
  2. The underwater noise from operational offshore wind turbines comes from vibration in the gear box and generator, which is transmitted down the tower and radiated from the tower wall. Given that there is a paucity of qualitative data on sound radiation from the FOW towers, qualitative assessment is presented with respect to fixed wind turbines (considered as maximum design case when compared to floating). The desktop review carried out in volume 3, appendix 10.1 suggests that although sound levels are likely to be audible within the hundreds of metres from the wind turbine, these will not be at levels sufficient to cause behavioural changes in marine mammals. However, these findings are based on data collected for wind turbines with capacity between 2 MW to 5 MW and a hub height of up to 95 m (see Table 8.27 in volume 3, appendix 10.1). The wind turbines for the Array will likely be larger than those in the desktop review ( Table 10.17   Open ▸ ) and therefore it is likely that there will be an increase of a few dB compared to smaller wind turbines. However, considering that the Array will be located in the North Sea with relatively high shipping traffic, the difference in ambient sounds is anticipated to be minimal.
  3. The impact (elevated underwater noise from floating wind turbines and mooring lines) is predicted to be of local spatial extent in the context of the relevant geographic frame of reference, long term duration, continuous and the effect of behavioural disturbance is of high reversibility. It is predicted that the impact will affect the receptor directly. Although noise levels are likely to be audible to marine mammals, animals are unlikely to experience behavioural disturbance including displacement as a result of the increased underwater noise during operational phase. The magnitude is therefore considered to be negligible.
Sensitivity of the receptor
  1. Compared to increasing robust scientific literature on fixed-foundation wind turbines, very little is known about FOW turbines, even less so on the impacts of noise generated during operation (Maxwell et al., 2022) and therefore assessing the responses of marine mammal receptors to these more novel impacts means the assessment is based on highly conservative assumptions (as discussed in paragraph 114). Whilst noise during construction is likely to be less than from pile-driven fixed foundations, noise levels are expected to be similar during operation and it is largely unknown how noise levels differ for floating versus fixed-foundation wind turbines (Maxwell et al., 2022). Farr et al. (2021) conducted a systematic review to estimate potential effects of deepwater FOW farms during operation, which included potential effects of underwater noise on marine species. Effect magnitudes were determined using a four-level classification scheme (negligible, minimal, moderate, and major, defined in Minerals Management Service (MMS (Minerals Management Service) (2007) and suggested noise effects on marine mammals were classified as ‘minimal’. Noise effects are unlikely to pose a risk to marine species as the operation noise is low frequency (with dominant frequencies of ~1 kHz or less) and at low levels (Madsen et al., 2006, NYSERDA (New York State Energy Research and Development Authority), 2017, Thomsen et al., 2015b). Farr et al. (2021) did state empirical measurements are still needed for FOW farms.
  2. While operational noise is continuous, it is unlikely that these noise levels would result in physiological damage (Madsen et al., 2006, Marmo et al., 2013, Tougaard et al., 2009a). Early measurements of underwater noise due to operational wind turbines concluded that the underwater noise from operating wind turbines is limited to low frequencies (below 1 kHz) and of low intensity and would therefore be unlikely to affect marine mammals with main hearing sensitivities at higher frequencies (i.e. VHF and HF cetaceans and PCW) (Madsen et al., 2006). Even so, behavioural responses by marine species to operational wind turbine noise appears to be minimal. Modelled predictions by Marmo et al. (2013) suggested that only a small proportion (<10%) of minke whales and harbour porpoises would display behavioural responses up to ~18 km away from an offshore wind farm, and the majority of animals studied would not show a behavioural response, indicating low potential for displacement.
  3. Monitoring using acoustic recordings (with towed passive acoustic monitoring devices, T-PODs) at Horns Rev Offshore Wind Farm in the North Sea revealed, whilst there was a weak adverse effect on harbour porpoise from the construction on porpoises, no detectable effects were observed on abundance from the operating wind farm (Tougaard et al., 2006). It must be noted however there was a significant difference between when intensive maintenance work took place (termed ‘semi-operation’) in the study, and operation. Acoustic and ship survey data indicated more porpoises in the area as a whole during the operational period than for any other of the periods, baseline included.
  4. However, field measurements and modelling efforts to estimate operational noise levels have predominantly focused on fixed-bottom offshore wind farms in shallow, near-shore environments. Analysis of noise measurements from two Danish (Middelgrunden and Vindeby) and one Swedish (Bockstigen-Valar) fixed-bottom offshore wind farms, concluded that operational noise levels are unlikely to harm or mask acoustic communication in harbour porpoises and harbour seals (Tougaard et al., 2009b). Tougaard et al. (2009a) reported at 100 m distance from 1.5 MW wind turbines, underwater sound would be audible to both harbour porpoise and harbour seal. However, at a greater distance of 1,000 m the signal to ambient sound ratio is too low for detection in harbour porpoise as a VHF cetacean (detection by harbour seal might be possible). Furthermore, the authors caveat these results, as ambient sound values used in this study were extrapolated from measurements obtained in the Baltic and the ambient sound in most parts of the North Sea is much higher and will decrease the radius of detection significantly. The study concluded that the sound is unlikely to exceed injury thresholds at any distance from the wind turbines and was considered incapable of masking acoustic communication by harbour porpoise.
  5. Studies using long term frequency data from wind farms with 5 MW wind turbines (Alpha Ventus, Germany) found that whilst operational sound can be identified, levels hardly exceed beyond ambient sound levels in areas near main shipping traffic routes negligible (Stober and Thomsen, 2021). Therefore, marine mammals in high traffic areas may not be able to discern operational wind turbine sound from background levels. Analysis of individual frequencies predicted a correlation between SPLs and the operational status of the wind turbines as well as the wind speed, but the total impact of the operational sound was mostly negligible (Stober and Thomsen, 2021). Nedwell et al. (2007) analysed measurements of underwater sound inside and outside of four different offshore wind farms in British waters and found operational sound levels were low and only exceeded background levels close to the wind turbines (<1 km).
  6. Risch et al. (2023b) collected acoustic data from two FOW farms (Kincardine and Hywind) and found recorded porpoise detections were reduced at the recording site closest to the wind turbine compared to the site further away for both FOW locations.
Auditory injury
  1. Since there is no empirical information on responses of marine mammals to floating wind turbine and mooring line noise during operation, it has been assumed that the sensitivity of marine mammal receptors to auditory injury (PTS) is the same or less than as a result of underwater noise during piling (paragraphs 217 et seq.) and is not reiterated here.
  2. Therefore, all receptors are deemed have limited resilience to PTS, low recoverability and high international value. The sensitivity of the receptor is therefore considered to be high.
Behavioural disturbance
  1. Since there is no empirical information on responses of marine mammals to floating wind turbine and mooring line noise, it has been assumed that the sensitivity of marine mammal receptors to behavioural disturbance is the same or less than as a result underwater noise during piling (233 et seq.) and is not reiterated here.
  2. Therefore, all receptors are deemed to have some resilience to behavioural disturbance, high recoverability and high international value. The sensitivity of the receptor is therefore considered to be medium.
Significance of the effect
Auditory injury
  1. Overall, the magnitude of the impact is deemed to be negligible and the sensitivity of the receptor is considered to be high. The effect will therefore be of minor adverse significance, which is not significant in EIA terms.
Behavioural disturbance
  1. Overall, the magnitude of the impact is deemed to be negligible and the sensitivity of the receptor is considered to be medium. The effect will therefore be of negligible to minor adverse significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
  1. No marine mammal mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Effects on marine mammals due to entanglement Associated with the Array

  1. To provide stability and the fixed positioning of floating wind turbines within the Array, effective mooring systems will be implemented. Additionally, a connection to dynamic inter-array cables will facilitate interlinking between individual wind turbines.
  2. There are concerns regarding the hazards that mooring lines and dynamic cables may pose to marine mammals, which could inadvertently become entangled or entrapped (MD-LOT, 2023). The entanglement risk can be categorised into two types: primary and secondary (Synthesis of Environmental Effects Research (SEER), 2022). Primary entanglement refers to the direct entanglement of marine life with mooring lines or dynamic cables. Secondary entanglement occurs when marine life becomes entangled with marine debris, such as derelict fishing gear, that has become snagged on a mooring line or dynamic cable (SEER, 2022). According to Benjamins et al. (2014), the entanglement risk is contingent upon various physical and biological parameters.
  3. Physical parameters, integral to the wind farm design, encompass mooring tension characteristics, cable/mooring line diameter, swept volume and curvature. This section will comprehensively assess these parameters in terms of magnitude. In parallel, biological parameters, including body size, the ability of animals to detect moorings, body flexibility and general feeding modes will undergo evaluation in the sensitivity section in paragraphs 522 et seq. below.

                        Operation and maintenance phase

Magnitude of impact
  1. The design of the Array will incorporate dynamic inter-array cables and mooring lines in the water column, introducing the potential for injury or death from entanglement to marine mammals.
  2. As outlined in Table 10.17   Open ▸ , the Array is designed to have up to 116 km of dynamic inter-array cables within the water column. Each wind turbine will be equipped with a mooring system, which introduces the additional potential for entanglement, with up to 1,590 mooring lines. The Project Description for the Array considers various mooring line design options for semi-submersible floating wind turbines, including full chain catenary, semi-taut and taut, both incorporating a top fibre rope section (nylon or polyester) and a bottom chain section (volume 1, chapter 3).
  3. According to Benjamins et al. (2014), tension characteristics in moorings significantly affect entanglement risk, with taut moorings under high tension being less likely to cause entanglement than flexible ones under low tension. The potential impact of dynamic moorings can be assessed by the concept of swept volumes, as it considers the volume of the water column occupied by mooring lines under energetic conditions (Benjamins et al., 2014). A useful physical parameter in the assessment of entanglement is also curvature, as it assesses the bending of mooring lines, with taut configurations exhibiting smaller curvatures compared to catenary configurations (Benjamins et al., 2014). Harnois et al. (2015) found that the catenary moorings with chains configuration shows the highest curvature values.
  4. Benjamins et al. (2014) findings indicate a greater risk of entanglement to marine mammals with catenary moorings, particularly those containing nylon. Across all potential mooring line types considered for the Array, catenary moorings represent the MDS for entanglement risk. It can be anticipated that, especially for catenary mooring type, there will be some horizontal movement of the floating wind turbine and therefore the mooring line may experience stretching (representing the maximum length in the water column) or slackness (representing the maximum length resting on the seabed). To address this, clump weights may be strategically placed around the touchdown point to mitigate the length of the mooring line between the anchor and the wind turbine.
  5. While Harnois et al. (2015) suggest that certain features of mooring systems may influence entanglement risk, the study also concluded that the absolute risk of primary entanglement is low regardless of mooring configuration. Garavelli (2020) suggested that all mooring configurations (catenary/taut) have too much tension to create a loop that could entangle a whale. This has been corroborated by SEER (2022), as the study also concluded that the risk of primary entanglement at FOW farms is very low due to the weight of the cable systems. The potential for heavy mooring gear combined with relatively taut mooring lines to entangle whales has been shown to be negligible (Wursig and Gaily, 2002) and MRE device moorings are unlikely to pose a major threat (Benjamins et al., 2014). Statoil (2015) stated for mooring lines at the Hywind Pilot Park, it was a design requirement that no line should ever go into slack, even in extreme weather conditions, and it was considered effectively impossible for entanglement on a marine mammal to occur. For inter-array cables in the water column cables have a very high bending stiffness and therefore the cable cannot bend around a marine mammal (Statoil, 2015). Therefore, the magnitude assessment of primary entanglement considers the very small likelihood that entanglement can actually occur.
  6. Research on the risk to marine mammals has focussed on injury or mortality by entanglement of fishing gear (e.g. nets of slack lines) or submarine telecommunication cables, however these have loose ends or loops that could ensnare animals (Benjamins et al., 2014, Moore et al., 2006) and therefore mooring lines/cables are not comparable and has not been considered a significant concern (Copping et al., 2020). Evidence of entanglement of marine animals with MRE mooring lines and cables has not been observed to date (Isaacman and Daborn, 2011, Offshore Renewables Joint Industry Programme (ORJIP) Ocean Energy, 2022, Sparling et al., 2013) and even entanglement with offshore aquaculture is rare (Fujita et al., 2023), but it is important to consider absence of evidence is not evidence of absence of risk. However, there is a risk of entanglement in anthropogenic debris caught in mooring lines/cables (Clavelle et al., 2019).
  7. The Array will use fibre rope diameters ranging from 110 mm to 300 mm and chain diameters between 76 mm to 175 mm. Fishing gear, which pose the greatest entanglement risk to marine species, were reported to fall between 1 mm to 9.5 mm in diameter (Knowlton et al., 2016, Wilcox et al., 2015). Thus, marine mammals are more likely to be at risk from secondary entanglement through interactions with fishing gears than through direct entanglement with the large, thick mooring and cable components.
  8. Lost fishing gear is made of synthetic materials, including nylon, polyethylene, and polypropylene, that resist natural biodegradation and can endure in the marine environment for extended periods, promoting the phenomenon known as 'ghost fishing' (Stelfox et al., 2016). This phenomenon occurs when lost or discarded gear continues to catch wildlife from various taxa, including marine mammals. Indirect entanglement in anthropogenic debris caught on mooring lines and inter-array cables, e.g. secondary entanglement, poses the risk of direct injury and is anticipated to result in significant fitness reduction for the affected marine mammals through tissue damage, infection, and mobility restrictions that prevent foraging or migration (Garavelli, 2020, Van Der Hoop et al., 2016). However, the quantification of the actual amount of abandoned, lost, or discarded fishing gear and other anthropogenic debris poses significant challenges due to its elusive nature.
  9. As a part of the designed in measures ( Table 10.22   Open ▸ ), mooring lines and dynamic inter-array cables will undergo regular inspections during the operation and maintenance phase. The inspection frequency for mooring lines and dynamic inter-array cables is anticipated to be more frequent initially (e.g. years 1 and 2), and likely to decline in frequency after this, following a risk based approach. Any inspected or detected debris on the floating lines and cables will be recovered based on a risk assessment which considers impact on environment including risk to marine mammal, risk to asset integrity, and health & safety. In addition, Ossian OWFL will consider new technologies for monitoring of mooring lines/snagged gear and will agree approach to monitoring of mooring lines and associated removal of gear with NatureScot and MD-LOT prior to the operation and maintenance phase. As such, the removal of debris from mooring lines and cables further reduces the likelihood of secondary entanglement.
  10. The impact (risk of entanglement due to presence of mooring lines and inter-array cables in the water column) is predicted to be of very local spatial extent in the context of the geographic frame of reference, long term duration, continuous and the effect is irreversible when entanglement does occur. It is predicted that the impact will affect the receptor directly in the case of both (rare) primary entanglement and secondary entanglement, however the risk of potential secondary entanglement is sufficiently reduced with the application of the designed in measures ( Table 10.22   Open ▸ ) regular inspections of mooring lines and dynamic inter-array cables and removal of marine debris recovered based on risk assessment, and any population-level effects are highly unlikely. As such, following application of designed in measures, the magnitude of primary and secondary entanglement is considered to be low.
Sensitivity of the receptor
  1. In line with the approach applied in Benjamins et al. (2014), for the purpose of assessing marine mammal sensitivity to primary entanglement, animals were classified into broad groups based on taxonomic relationship as well as body size:
  • odontocetes – harbour porpoise, bottlenose dolphin, white-beaked dolphin;
  • mysticetes – minke whale and humpback whale; and
  • pinnipeds – grey seal.
  1. Due to paucity of empirical evidence for secondary entanglement associated with FOW farms components, sensitivity to secondary entanglement will be assessed based on potential entanglement with fisheries materials (mostly nets, lines) that are most likely to be caught on the Array infrastructure.
Primary entanglement
  1. The Array will utilise mooring lines and inter-array cables in exceedance of 76 mm and 110 mm, respectively, e.g. dynamic components will be relatively large in diameter, particularly compared to fishing gear/submarine telecommunications cables, on which most entanglement evidence has been based upon (Benjamins et al., 2014, Moore et al., 2006).
  2. When considering the size of marine animals, mooring lines and cables may pose a reduced risk to smaller animals compared to larger ones simply because smaller animals ‘cannot physically become entangled’ (Benjamins et al., 2014). Consequently, odontocetes as well as pinnipeds, face a lower risk of primary entanglement with mooring lines and inter-array cables compared to larger mysticetes.
  3. In terms of flexibility, marine mammals exhibit variations in the degree to which they flex their bodies while swimming. Benjamins et al. (2014) made an assumption that animals with greater flexibility would be able to avoid entanglement more easily compared to those with more rigid bodies. The study assigned a consistent entanglement risk based on body flexibility for both mysticetes and odontocetes. Pinnipeds, presumed to be relatively flexible, were consequently assigned a lower score for the risk of entanglement when compared to odontocetes and mysticetes (Benjamins et al., 2014). As discussed in paragraph 516, it is highly unlikely that the mooring cables will be flexible enough (they have high bending stiffness) however to loop around passing marine mammal receptors.
  4. Due to the size of mooring lines and inter-array cables considered for the Array (see paragraph 518), they are detectable at considerable distances for echolocating odontocete cetaceans. Various mooring components are likely to influence audibility, with chain, for instance, being inherently noisier than fibre rope due to metal-on-metal movement and a larger surface area that can generate turbulence (Benjamins et al., 2014). The smoothness of mooring elements surface will also impact the amount of turbulence produced, which is likely to be detectable by pinnipeds (Benjamins et al., 2014). Nevertheless, detectability at a distance may be altered under adverse conditions such as storms or turbid waters, regardless of the sensory modality used or the extent of device motion. Benjamins et al. (2014) assessment of the entanglement risk across marine mammal groups, based on their ability to detect moorings, revealed that odontocetes who possess echolocation are more likely to detect mooring components at larger distances than mysticetes and pinnipeds which rely on passive acoustic detection or pressure wave detection. Pinnipeds however possess acute mechanosensitivity through their vibrissae or whiskers (Dehnhardt et al., 2001, Hanke and Dehnhardt, 2013) which may allow them to detect wakes formed downstream of a mooring or cable.
  5. Foraging behaviour appears to be an important risk factor contributing to entanglement in fishing gears. Entanglements in ropes often occur as the rope wraps around animals' extremities or passes through their mouths, particularly during foraging activities (Benjamins et al., 2014). Mysticetes are at a higher risk of entanglement when lunge feeding as opposed to filter feeding (Benjamins et al., 2014), noting that studies have been based upon entanglement in fishing gear (Knowlton and Kraus, 2020), rather than mooring lines. The substantial thickness of mooring lines and inter-array cables associated with the Array, in comparison to the ropes used in fishing gears, may largely prevent such entanglements except in very specific cases (Benjamins et al., 2014). Considering the mode of foraging alone, odontocetes and pinnipeds are assessed to be at a low risk of primary entanglement.
  6. It must be noted that it is considered that marine mammals are highly unlikely to get entangled in the first place, given their advanced hearing and echolocation which would allow them to detect any noise from cables (such as ‘bangs’, ‘creaks’, ‘rattle’, ‘snapping’ or ‘pinging’ as described in Burns et al. (2022) and Liu (1973). Statoil (2015) assessed the sensitivity of marine mammal entanglement as low, given the risk of entanglement is considered highly unlikely. Furthermore, the evidence base for sensitivity is largely based off fishing gear or submarine telecommunications cables and therefore it is unlikely that the design of cables (see paragraphs 516 to 517) will physically allow primary entanglement of marine mammals to an extent that would entrap them and cause drowning. Thus, on the basis that primary entanglement is considered highly unlikely and the lack of any evidence for entanglement from MRE, there is considered to be some resilience and survivability largely due to avoidance behaviour of MRE structures.
  7. Taking into account all biological parameters and the difficulty in attributing sensitivity to such a novel impact with no direct evidence from scientific study, mysticetes are precautionarily identified as being at a slightly higher risk of direct entanglement with mooring lines and inter-array cables, which is attributed to their large size and distinctive foraging techniques.
  8. As such, the mysticetes are deemed to have some resilience to primary entanglement (largely due to avoidance and design of mooring lines/cables), some adaptability, limited recoverability and are of international value. The sensitivity of the receptors (minke whale and humpback whale) is therefore, conservatively, considered to be medium.
  9. Odontocetes and pinnipeds are perceived to be at a lower risk of inadvertently becoming entangled in moorings (as discussed in paragraph 528) and inter-array cables associated with Array infrastructure.
  10. As such, the odontocetes and pinnipeds are deemed to have some resilience to primary entanglement (largely due to avoidance and design of mooring lines/cables), high adaptability, limited recoverability and are of international value. The sensitivity of the receptors (harbour porpoise, bottlenose dolphin, white-beaked dolphin and grey seal) is therefore, considered to be low.
Secondary entanglement
  1. The primary source of small cetacean bycatch is thought to be gillnets (Read et al., 2006). One hypothesis explaining cetacean entanglement in gillnets suggests that these animals may either be incapable of detecting the nets due to low target strength or may detect the nets too late to avoid entanglement (Mackay, 2011). Limited information is available regarding how odontocete cetaceans utilise echolocation in the wild and the ecological as well as behavioural contexts in which the echolocation is used (Mackay, 2011). Bottlenose dolphins, for example, have been observed to use echolocation sparingly in the wild, predominantly relying on passive listening to detect prey (Gannon et al., 2005). In contrast, free-ranging harbour porpoises have been documented to echolocate frequently (Akamatsu et al., 2007).
  2. Cox and Read (2004) reported that harbour porpoises are often found in the vicinity of commercial gillnets more frequently than actual entanglement events occur. Kastelein et al. (1995) examined the circumstances in which three captive harbour porpoises reacted to gillnets in a pool. The initial encounters of the animals with standing gillnets resulted in entanglement, and the harbour porpoise would have faced the risk of drowning if not rescued. Subsequent to these experiences, the animals in the study learned from one or more encounters and developed behaviours that reduced their chances of colliding with or becoming entangled in the gillnet (Kastelein et al., 1995). It is important to note that this learning process may not occur in the wild, where animals do not have the opportunity to be rescued. The authors also suggested that harbour porpoises learned to detect the gillnet by using echolocation in complete darkness, highlighting the adaptability of their sensory capabilities in response to the new environmental challenge posed by the gillnet (Kastelein et al., 1995).
  3. Read et al. (2003) investigated the fine-scale movements of bottlenose dolphins around commercial Spanish mackerel gillnets and found that the most commonly recorded interaction was avoidance, wherein dolphins altered their course to navigate around the net and then resumed their original path once past it. Avoidance behaviours were observed at distances of up to 100 metres from the net (Read et al., 2003). The authors concluded that bottlenose dolphins frequently interact with gillnets but rarely become entangled (Read et al., 2003). When entanglement does occur, it is attributed to dolphins being either unaware of the net or distracted by other stimuli in the net's vicinity, such as fish (Read et al., 2003).
  4. The welfare assessment of free-swimming white-beaked dolphins off the coast of Northumberland in the North Sea revealed that the majority of the recorded injuries were caused by interactions with fishing devices (Van Bressem et al., 2018).
  5. Between August 1990 and September 1995, a comprehensive examination of 422 cetacean carcasses representing 12 species that had died around the coasts of England and Wales was conducted (Kirkwood et al., 1997). Among the examined specimens, there were 234 harbour porpoises, 138 common dolphins, and 50 individuals from ten other species of dolphins and whales. In both harbour porpoises and common dolphins, the most frequent cause of death was entanglement in fishing gear (Kirkwood et al., 1997). A more recent study by Reeves et al. (2013) showed that bycatch continues to affect many odontocete species, as 61 of 74 studied species (82%) have reportedly been bycaught in some kind of fishing gear within their range between 1990 and 2011. Harbour porpoise faces significant challenges due to high bycatch rates in coastal gillnet fisheries across its range, leading to conservation concerns for several populations (Kindt-Larsen et al., 2023).
  6. Based on sighting records and a photo-identification catalogue from a haul-out site in south-west England, Allen et al. (2012) reported that over the period from 2004 to 2008, the annual mean entanglement rates fluctuated between 3.6% and 5%. Among the 58 entangled cases in the catalogue, 64% exhibited injuries classified as serious and in 15 cases where the entangling debris was visible, 14 were found to be entangled in fisheries materials (Allen et al., 2012).
  7. In waters around the coast of Scotland, entanglement in fishing gear is a particular welfare and conservation concern for mysticetes, particularly minke whales and humpback whales (Leaper et al., 2022, MacLennan, 2018, MacLennan, 2021). These entanglements can occur through various body parts such as the mouth, around the body, pectoral fins, and the tail (MacLennan, 2018). It can impact an animal's ability to feed, swim, and reproduce, depending on the part of the body that becomes restrained. Ropes can cut through baleen and blubber and lead to amputation of fins and flukes (MacLennan, 2018, Rolland et al., 2017). More than half of the post-mortems conducted on mysticetes found dead around the Scottish coast have identified entanglement as the cause of death (Northridge et al., 2010).
  8. According to estimates by Leaper et al. (2022), approximately six humpback whales and 30 minke whales become entangled in Scotland each year. The data from reports from strandings, live disentanglements and interviews with Scottish inshore creel fishers indicated that 83% of minke whale entanglements and 50% of humpback whale entanglements occurred in the groundlines between creels (Leaper et al., 2022). Whales become entangled in ground lines because the buoyant rope used in creel fishing floats in loops between pots rather than lying on the seabed. Leaper et al. (2022) reported that while disentanglement efforts have seen success in several incidents involving humpback whales, the majority (84%) of minke whale discovered were already deceased. The behaviour of whales swimming away with gear is more commonly associated with humpback whales than minke whales as humpbacks are known to be powerful swimmers capable of towing gear over substantial distances (Knowlton et al., 2016). This ability may contribute to the relative success of disentanglement efforts for humpbacks. Minke whales are considered particularly vulnerable to gillnet entanglement for various reasons (Reeves et al., 2013). These include their near-shore and shelf occurrence, their tendency to prey on fish species targeted by net fisheries, and their smaller size, making it more challenging for them (compared to larger mysticetes) to rescue themselves once entangled (Reeves et al., 2013).
  9. As presented in paragraph 206 humpback whales photographed off eastern Scotland have been matched with a non-recovering population breeding in Cape Verde and Arctic feeding grounds (Scottish Humpback, 2023). Although it has been suggested that adverse effects on humpback whales occurring in Scottish waters could potentially impact populations in the north-east Atlantic (Leaper et al., 2022), only individual humpback whales were recorded seasonally and there is no evidence that the waters in the vicinity of the Array marine mammals study area represent an important feeding ground during their migration.
  10. Statoil (2015) considered the risk of marine mammal entanglement in mooring lines and inter-array cable to be unlikely, but concluded that it is possible for smaller whales and dolphins (e.g. minke whale and smaller cetaceans using the offshore area) to become entangled in lost or derelict fishing gear which may become entangled in mooring lines and cables Based on the species most likely to be at risk, the sensitivity of marine mammals to entanglement was concluded to be low (Statoil, 2015). It must be noted that these smaller species (such as bottlenose dolphin and grey seal) are found in lower densities in the Array marine mammal study area, though small cetaceans such as harbour porpoise and white-beaked dolphin may be present in greater numbers around the Array. Quantifying sensitivity on the basis of little scientific evidence is difficult, with only few examples of sensitivity given to date (Statoil, 2015).
  11. It is important to consider that mooring lines and dynamic inter-array cables will undergo regular inspections during the operation and maintenance phase. The inspection frequency for mooring lines and dynamic inter-array cables is anticipated to be more frequent initially (e.g. years 1 and 2), and likely to decline in frequency after this following a risk based approach. Any inspected or detected debris on the floating lines and cables will be recovered based on a risk assessment which considers impact on environment including risk to marine mammal, risk to asset integrity, and health & safety. In addition, Ossian OWFL will consider new technologies for monitoring of mooring lines/snagged gear and will agree approach to monitoring of mooring lines and associated removal of gear with NatureScot and MD-LOT prior to the operation and maintenance phase. This is considered to further reduces the potential risk to marine mammals from secondary entanglement.
  12. Therefore, marine mammals are deemed to have some resilience to secondary entanglement (largely via avoidance), medium adaptability, limited recoverability and are of international value. Although the risk of entanglement is likely to be low, and some species (particularly bottlenose dolphin and grey seal) are expected to occur around the Array in very low abundance, due to the paucity of information on secondary entanglement and the irreversible nature if entanglement does occur, the sensitivity of all marine mammals is conservatively considered to be medium.
Significance of the effect
Primary entanglement
  1. Overall, the magnitude of the impact (primary entanglement) is deemed to be low for all marine mammals and the sensitivity of the receptor is considered to be low for odontocetes/pinnipeds and medium for mysticetes. Considering that studies on primary entanglement assessed it as low risk, especially for odontocetes and pinnipeds (Benjamins et al., 2014, Harnois et al., 2015), the effect will therefore be of minor adverse significance for all marine mammal species, which is not significant in EIA terms.
Secondary entanglement
  1. Overall, the magnitude of the impact (secondary entanglement) is deemed to be low for all marine mammals and the sensitivity of all receptors (odontocetes, pinnipeds and mysticetes) is considered to be medium. The effect will, therefore, be of minor adverse significance for all marine mammals, which is not significant in EIA terms.
Secondary mitigation and residual effect
  1. No marine mammal mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Effects on marine mammals due to altered prey availability

  1. Potential effects on fish and shellfish during the construction, operation and maintenance and decommissioning phases of the Array, as identified in volume 2, chapter 9, could lead to indirect effects on marine mammals. The assessment includes temporary and long term habitat loss/disturbance, colonisation of hard structures, underwater noise and increased SSCs and associated deposition.
  2. The key prey species for marine mammals include sandeels, gadoids (including cod Gadus morhua, haddock Melanogrammus aeglefinus, whiting Merlangius merlangus), clupeids (including herring Clupea harengus) plaice Pleuronectes platessa, Pleuronectiformes and Scomber scombrus (see volume 3, appendix 10.2 for detail on marine mammal feeding ecology). These prey species have been identified as being of regional importance within the fish and shellfish ecology study area, except for sandeel which is deemed to be of national importance (see volume 2, chapter 9). For example, volume 2, chapter 9 reports that the Array overlaps with low intensity spawning grounds for lemon sole Microstomus kitt, cod, whiting, sandeel, mackerel, Norway pout Trisopterus esmarkii, sprat Sprattus sprattus and plaice based on Coull et al. (1998) and Ellis et al. (2012). For herring, volume 2, chapter 9 states that no high intensity spawning grounds identified by Coull et al. (1998) directly overlap with the Array (noting low intensity grounds overlap). Volume 2, chapter 9 also presents the outputs of recent modelling conducted by Langton et al. (2021) within the Array, this modelling shows that the whole Array has extremely low probability of sandeel presence, corresponding to the low-intensity nursery/spawning grounds presented in Coull et al. (1998) and Ellis et al. (2012), with areas where predicted density is high closer to the coasts or towards the Firth of Forth.
  3. Consequently, adverse effects on fish receptors that are key prey species for marine mammal receptors (detailed in paragraph 550) may have indirect adverse effects on marine mammal receptors.

                        Construction phase

Magnitude of impact
  1. Potential impacts on marine mammal prey species during the construction phase have been assessed in volume 2, chapter 9 using the appropriate maximum design scenarios for these receptors. Construction impacts on prey species include temporary habitat loss/disturbance, long term habitat loss and disturbance, injury and/or disturbance to fish and shellfish from underwater noise from piling and UXO clearance.
  2. The installation of infrastructure within the Array will lead to temporary habitat loss/disturbance as a result of a range of activities including boulder and sand wave clearance, disturbance from inter-array and interconnector cables, and use of jack-up vessels for the OSP installation. There is the potential for temporary habitat loss/disturbance to affect up to 40.95 km2 of the seabed during the construction phase, which equates to 5.82% of the Array and represents a relatively small proportion of the fish and shellfish ecology study area. Temporary habitat loss and disturbance has the potential to affect spawning, nursery or feeding grounds of fish and shellfish receptors, and therefore impact prey availability for marine mammals. Due to the highly localised nature of the effects (i.e. spatially restricted to within the Array) and the small proportion of habitats affected as a proportion of the northern North Sea and medium term duration with recovery beginning immediately following cessation of the construction activity, temporary habitat loss/disturbance during the construction phase was assessed as being of low magnitude in volume 2, chapter 9.
  3. As outlined in volume 2, chapter 9, only a small proportion of the maximum footprint of temporary habitat loss and disturbance may be affected at any one time during the construction phase with areas starting to recover immediately after cessation of construction activities in the vicinity allowing mobile species, such as sandeel and other fish and shellfish species, to repopulate the areas of previous disturbance. Additionally, habitat disturbance during the construction phase will also expose benthic infaunal species from the sediment (see volume 2, chapter 8), potentially offering foraging opportunities to some fish and shellfish species (e.g. opportunistic scavenging species) immediately after completion of works. Most fish and shellfish receptors found within the fish and shellfish ecology study area are deemed to be of low vulnerability, high recoverability and local to international importance and therefore sensitivity of these receptors was considered to be low. However, some species including larger crustacea (e.g. Nephrops, European lobster Homarus gammarus) and sandeel were assessed as having medium sensitivity to the impact. As described in paragraph 553, he overall magnitude of the impact was considered to be low. Consequently, the impact temporary habitat loss and disturbance was assessed as being of minor adverse significance.
  4. Long term habitat loss within the fish and shellfish ecology study area will occur during construction (i.e. through placement of infrastructure) although effects will extend throughout the operation and maintenance phase. Long term habitat loss will occur under mooring lines and anchors on the seabed, OSP foundations inter-array and interconnector cable protection and cable crossing protection, inter-array junction boxes and associated scour protection.
  5. The presence of infrastructure within the Array will result in long term habitat loss of up to 19,270,958 m2, which represents 2.25% of the total site boundary. Additionally, up to 812,808 m2 of long term seabed disturbance may exist due to the movement of foundation mooring lines, which is subject to movement and, as such, seabed disturbance. Many species of fish and shellfish are reliant upon the presence of suitable sediment/habitat (notably herring and sandeel) for their survival and therefore seabed habitats removed by installation of the infrastructure will reduce the area available for foraging, spawning and nursing. However, the area that will be impacted represents a very low proportion of the available habitat (2.25% of the total site boundary). The Array fish and shellfish ecology study area is located over low intensity spawning and low intensity nursery grounds for sandeel, and a mix of preferred, marginal and unsuitable habitat type, with the preferred habitat types in the north-west of the Array (see volume 3, annex 9.1). Herring spawning habitat is largely unsuitable within the fish and shellfish ecology study area, with core spawning grounds existing outside the site boundary. Therefore, the area of herring spawning grounds affected by this impact is expected to be very limited, in the context of available favourable sediments habitat outside the fish and shellfish ecology study area.
  6. Monitoring at Belgian offshore wind farms has reported that fish assemblages undergo no drastic changes due to the presence of offshore wind farms (Degraer et al., 2020). They reported slight, but significant increases in the density of some common soft sediment-associated fish species within the offshore wind farm (Degraer et al., 2021). There was also some evidence of increases in numbers of species associated with hard substrates, including crustaceans (including Cancer pagurus), Dicentrarchus labrax and common squid Alloteuthis subulata (potentially an indication that foundations were being used for egg deposition (Degraer et al., 2021). The sensitivity of fish and shellfish receptors ranged from low (most fish and shellfish) to medium (sandeel) with the majority of fish receptors deemed to be of low vulnerability, high recoverability and local to international importance. The magnitude of the impact was considered to be low. Consequently, the effect of long term habitat loss was assessed as being of minor adverse significance.
  7. There is the potential for underwater noise during construction (from piling and UXO) to result in injury and/or disturbance to fish and shellfish communities (see volume 2, chapter 9). For SPLpk and the maximum design scenario assessed in volume 2, chapter 9, the maximum recoverable injury range is estimated at 226 m to 414 m from the piling location. The potential for mortality or mortal injury to fish eggs would also occur at distances of up to 414 m. However, this is considered to be highly conservative due to the implementation of soft starts during piling activities which will allow fish to move away from the areas of highest noise levels, before the received noise reaches a level that would cause an injury. As such, the maximum injury ranges predicted for soft start initiation (i.e. of the order of tens of metres) are likely to be more realistic. Using the SELcum metric, underwater noise modelling showed that injury may occur out to ranges of tens to a few hundred metres (e.g. mortality ranges for the 3,000 kJ hammer energy of 15 m to 50 m for fleeing receptors and 328 m to 1,460 m for static receptors). TTS, from which animals will recover, was predicted to occur out to a maximum distance of 26,960 m (based on static fish) for single piling scenario at 4,400 kJ. The potential onset of behavioural effects (such as elicitation of a startle response, disruption of feeding, or avoidance of an area) may occur to ranges of approximately 33 km to 49 km. A qualitative assessment of behavioural effects in fish to underwater noise suggested, however, that responses will differ depending on the sensitivity of the species and the presence/absence of a swim bladder (Popper et al., 2014). For the least sensitive species (e.g. flatfish), the risk of behavioural effects is moderate to high in the nearfield (tens of metres) and intermediate field (i.e. hundreds of metres). For more sensitive species (e.g. herring, gadoids, sprat etc.) behavioural effects may occur further away from the source (i.e. over several kilometres or more from the source). The magnitude of underwater noise effects was considered to be low and the sensitivity of the fish and shellfish receptors was assessed as low to medium. Therefore, as detailed in volume 2, chapter 9, the effect of underwater noise from piling and UXO on fish and shellfish receptors was minor adverse significance.
  8. With respect to indirect effects on marine mammals, no additional indirect effects other than those assessed for injury and disturbance to marine mammals as a result of elevated underwater noise during piling (see paragraph 140 et seq.) have been predicted. This is because if prey were to be disturbed from an area as a result of underwater noise, it is assumed that marine mammals would be disturbed from the same or greater area, and so any changes to the distribution of prey resources would not affect marine mammals as they would already be disturbed from the same (or larger) area. Whilst there may be certain prey species that comprise the main part of their diet (as discussed in volume 3, appendix 10.2), all marine mammals in this assessment are considered to be generalist opportunistic feeders and are thus not reliant on a single prey species. Given that marine mammals are wide-ranging in nature with the ability to exploit numerous food sources, there would be a variety of prey species available for marine mammal foraging.
  9. Marine mammals forage over extensive distances and exploit a wide range of different prey items, with the ability to switch prey sources depending on season and availability. The impacts resulting from the construction of the Array on fish and shellfish receptors will be highly localised and largely restricted to the boundaries of the Array. In context of the wider available foraging habitat within the northern North Sea, the area of impact is very small. Marine mammals within the regional marine mammal study area may also have the potential to be directly affected as a result of effects such as injury and disturbance from elevated underwater noise impacts during piling/UXO clearance and it is likely that the effects to prey resources (e.g. behavioural displacement) are likely to occur over a similar, or lesser, extent and duration as those for marine mammals. It is therefore considered that there would be no additional displacement of marine mammals as a result of any changes in prey resources during the construction phase, as marine mammals would already be potentially disturbed as a result of underwater noise during piling/UXO clearance. In addition, as fish and shellfish receptors are likely to be displaced from the array marine mammal study area, marine mammals are expected to also move to other areas in order to exploit these prey resources.
  10. On the basis of the assessments presented in volume 2, chapter 9, negligible or minor adverse effects have been predicted to occur to fish and shellfish species (marine mammal prey) as a result of the construction of the Array, which are not significant in EIA terms.
  11. Therefore, the impact on marine mammals is predicted to be of local spatial extent in the extent of the geographic frame of reference, medium term duration, intermittent and the effect on marine mammals is of high reversibility. The magnitude is therefore, considered to be low.