Sensitivity of the receptor
  1. The fish and shellfish communities found within the fish and shellfish ecology study area (see volume 2, chapter 9) are deemed to be characteristic of the fish and shellfish assemblages in the wider northern North Sea. It is considered highly likely, and therefore reasonable to assume that, considering the highly mobile nature of marine mammals, there will be similar prey resources available in the wider norther North Sea region for marine mammals. Foraging over greater distances could however result in an energetic cost with this effect being particularly pertinent for harbour porpoise. Harbour porpoise has a high metabolic rate and only a limited energy storage capacity, which limits their ability to buffer against diminished food. Despite this, if animals do have to travel further to alternative foraging grounds, the impacts are expected to be short term in nature and reversible (i.e. elevated underwater noise would occur during piling only).
  2. Minke whale has the potential to be particularly vulnerable to potential effects on sandeels, particularly if there is a potential for reduced abundance. Studies analysing the stomach contents of minke whale found that in the North Sea this species is their key food resource, followed by clupeids Clupeidae and to a lesser extent mackerel (Robinson and Tetley, 2005, Tetley et al., 2008); see volume 3, appendix 10.2 for more details. However, as outlined in paragraph 550 (as presented in volume 2, chapter 9) modelling by Langton et al. (2021) shows that the Array marine mammal study area has extremely low probability of sandeel presence, with areas where predicted density is high closer to the coasts or towards the Firth of Forth. For sandeels, volume 2, chapter 9 concluded that all impacts would be of minor adverse significance, which is not significant in EIA terms, therefore minke whale are not considered to be affected indirectly through impacts to sandeel.
  3. All receptors are deemed to be of high resilience and adaptability, high recoverability and high international value. The sensitivity of the receptor is therefore considered to be low.
Significance of the effect
  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be low. The effect will, therefore, be of minor adverse significance (though could be minor beneficial for some species dependent on the reef effect), 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.

                        Operation and maintenance phase

Magnitude of impact
  1. Potential impacts on marine mammal prey species during the operation and maintenance phase have been assessed in volume 2, chapter 9 using the appropriate maximum design scenarios for fish and shellfish receptors. The assessment includes temporary and long term habitat loss/disturbance, colonisation of hard structures, underwater noise, increased SSCs and associated deposition, and EMF.
  2. The maximum design scenario is for up to 51,411,500 m2 of temporary habitat loss/disturbance during the operation and maintenance phase. This equates to 5.99% of the total site boundary and therefore this represents a very small proportion of the fish and shellfish ecology study area. The maximum design scenario is for up to 19,270,958 m2 of long term subtidal habitat loss representing 2.25% of the total site boundary, however long term subtidal habitat loss is assessed above in paragraph 556 for the construction phase. Given that these impacts will be similar to those identified for temporary habitat loss/disturbance the construction phase (as discussed in paragraph 554) and will be highly restricted to the immediate vicinity of these operations, the magnitude was assessed as negligible. The sensitivity of fish and shellfish receptors ranged from low to medium with the majority of fish receptors deemed to be of low vulnerability and high recoverability. Consequently, the effects of temporary habitat loss/disturbance on fish and shellfish IEFs during the operation and maintenance phase were assessed as being of negligible to minor adverse significance.
  3. Increased SSCs and associated deposition may arise from mooring lines or cables making contact with and moving on the seabed, disturbing seabed materials and increasing SSCs within the water column. The greatest potential for the increase in SSCs is from catenary moorings which have the greatest length of mooring lines in contact with the seabed. The MDS is considered to be the foundations with the greatest length of mooring line on the seabed per foundation, rather than over the site as a whole, as the effects are considered to be very localised. The length of each mooring line on the seabed is 680 m, which amounts to 6,120 m per foundation. With a mooring line radius of 700 m, the maximum potential volume of sediment disturbance resulting from the movement of mooring lines was estimated to be over a surface area greater than 1,539,380 m2, therefore the magnitude of the increase in SSCs and associated deposition was deemed to be low. Disturbed materials are more likely to move along the seabed, rather than becoming fully suspended in the water column, and due to the low current speeds near the seabed, will not be transported for any significant distance before being re-deposited on the seabed.
  4. In terms of potential increases in SSC, adult fish species are more mobile than many of the other fish and shellfish IEFs, and therefore would be likely to show avoidance behaviour within areas affected by increased SSC (EMU, 2004), making them less susceptible to physiological effects of this impact. Juvenile fish are more likely to be affected by habitat disturbances such as increased SSC than adult fish, which is well researched for commercially important salmonid species (Berli et al., 2014, Bisson and Bilby, 1982). However, as outlined in paragraph 550, for herring, volume 2, chapter 9 states that no high intensity spawning grounds were identified by Coull et al. (1998) within the site boundary. Low intensity nursery grounds were reported to be present within the site boundary and low intensity spawning grounds nearby (Coull et al., 1998). With respect to the effects of sediment deposition on herring spawning activity, it has been shown that herring eggs may be tolerant of very high levels of SSC (Kiørboe et al., 1981, Messieh et al., 1981). Detrimental effects may be seen if smothering occurs and the deposited sediment is not removed by the currents (Birklund and Wijsman, 2005), however this natural removal by the currents and tidal physical processes would be expected to occur quickly in this case (i.e. within a couple of tidal cycles). The impact of increased SSCs and associated deposition is predicted to be of local spatial extent, long term duration, intermittent, and of high reversibility. The magnitude is therefore considered to be low.
  5. Increased SSC could also occur as a result of repair or remedial burial activities during the operation and maintenance phase. The maximum design scenario assessed in volume 2, chapter 9 for increased SSC and associated deposition is for the repair of cables of up to 30,000 m in length and reburial of cables of up to 10,000 m in length for inter-array cables; and repair of cables of up to 4,000 m in length and reburial of cables of up to 4,000 m in length for offshore export cables, using similar methods as those for cable installation activities (e.g. jet-trenching) undertaken at intervals over the 35 years operation and maintenance phase. The assessment in volume 2, chapter 9 considered that any suspended sediments and associated deposition will be of the same magnitude, or lower as for construction, with the sensitivity of the receptors similar to that assessed for the construction phase (see paragraph 560). The overall significance of the effect was therefore deemed to be of negligible to minor adverse significance.
  6. The presence and operation of inter-array and interconnector cables will result in emissions of localised electrical and magnetic fields, which could potentially affect the sensory mechanisms of some species of fish and shellfish. Species for which there is evidence of a response to electrical and/or magnetic fields include elasmobranchs (sharks, skates and rays), river lamprey Lampetra fluviatilis, sea lamprey Petromyzon marinus, European eel Anguilla ecommis, plaice and Atlantic salmon Salmo salar (CSA Ocean Sciences Inc and Exponent, 2019, Gill et al., 2005). A range of their life functions is supported by either electric or magnetic sense, including detection of prey, predator avoidance, social or reproductive behaviours, orientation, homing, and navigation (Gill et al., 2005, Normandeau Associates Inc et al., 2011). Given that the range over which species can detect EMF will be very localised to within a few centimetres of the cable, with rapid decay of the EMF with increasing distance, the magnitude of the impact was assessed as low. Most fish and shellfish species were considered to be of low sensitivity, with the exception of elasmobranchs and decapod crustaceans, which were of medium sensitivity. The significance of the effect was considered to be negligible to minor adverse.
  7. Artificial structures introduced to the marine environment, such as wind turbine anchors, mooring lines and scour/cable protection, provide hard substrate for settlement of various organisms, including small crustaceans and polychaete worms. These communities can provide a valuable food source for fish species and therefore, hard substrate habitat is likely to be colonised within days after construction by demersal and semi-pelagic species. The maximum design scenario assessed in volume 2, chapter 9 assumes up to 10,198,971 m2 of habitat created due to the installation of jacket foundations, associated scour protection and cable protection associated with inter-array cables, OSPs/Offshore convertor station platform interconnector cables and offshore export cables. The dominant natural substrate character (e.g. soft sediment or hard rocky seabed) will determine the number of new species found on the introduced vertical hard surface and associated scour protection. When placed on a soft seabed, most of the colonising fish tend to be associated with hard bottom habitats, thus the overall diversity of the area is expected to increase. If infrastructure is introduced to the area of rocky substrates, few species will be added to the area, but the increase in total hard substrate could sustain higher abundance (Andersson et al., 2009). The magnitude of the impact was assessed as low. Most fish and shellfish species are deemed to be of low to medium vulnerability and high recoverability, therefore the sensitivity of the receptor was assessed as low. The effect is expected to be of negligible to minor adverse significance.
  8. The impact on marine mammals is predicted to be of local spatial extent in the context of the geographic frame of reference, long term duration, continuous and the effect on marine mammals is of high reversibility. The magnitude is therefore, considered to be low.
Sensitivity of the receptor
  1. Following placement on the seabed, submerged infrastructure (e.g. anchors and mooring lines) provide hard substrate for the potential colonisation by various marine life. Faecal deposits from animals colonising structures, such as suspension feeders are likely to alter the surrounding seafloor communities by increasing food availability in the locality of the Array (Degraer et al., 2020). This increased food availability is likely to attract higher trophic levels, such as fish and marine mammals, who can exploit the increased foraging opportunities in the Array.
  2. However, there is still a considerable amount of uncertainty around marine mammal behaviour and distribution within the vicinity of offshore anthropogenic structures. Species such as harbour porpoise, minke whale, white-beaked dolphin, harbour seal and grey seal have been frequently recorded around offshore oil and gas structures (Delefosse et al., 2018, Lindeboom et al., 2011, Todd et al., 2015).Fernandez Betelu et al. (2022) deployed an array of C-PODs within the vicinity of four offshore structures. The probability of porpoise occurrence and foraging activity was found to decrease with distance from offshore structures. A significant increase in porpoise occurrence and foraging was detected during night-time compared to daytime around all four offshore structures (<200 m). These findings demonstrated that marine mammals are attracted to man-made structures and that porpoises modify their diel patterns of occurrence and foraging activity around them (Fernandez-Betelu et al., 2022). Acoustic results from a T-POD measurement within a Dutch wind farm found that relatively more harbour porpoises were found in the wind farm area compared to the two reference areas (Lindeboom et al., 2011, Scheidat et al., 2011). This study concluded that the presence within the wind farm area was due to increased food availability as well as the exclusion of fisheries and reduced vessel traffic in the wind farm (shelter effect). Further evidence suggesting that wind farms are used for foraging includes a study by Russell et al.(2014) where the movements of tagged harbour seals commonly exhibited grid-like movement patterns within two active wind farms in the North Sea. However, other studies have detected no statistical differences in the presence of harbour porpoises inside and outside a Danish wind farm (Brandt et al., 2009). Brandt et al. (2009) suggested, however, that a small increase in detections during the night at hydrophones deployed in close proximity to single wind turbines may indicate increased foraging behaviour near the monopiles. Whilst there is some mounting evidence of potential benefits of man-made structures in marine environment (Coolen et al., 2020), the statistical significance of such benefits and details about trophic interactions in the vicinity of artificial structures and their influence on ecological connectivity remain largely unknown (Elliott and Birchenough, 2022, Inger et al., 2009, McLean et al., 2022, Rouse et al., 2020). Additional details about inter-related effects on marine organisms are provided in section 10.15.
  3. Overall, the sensitivity of marine mammals during the operation and maintenance phase is not expected to differ from the sensitivity of the receptors during the construction phase described in paragraph 560 et seq. The sensitivity of the receptor is therefore, considered to be low.
Significance of the effect
  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be low. Given that marine mammals can exploit a wide range of prey species but travelling longer distances may be associated with higher rate of energy expenditure, the effect will, therefore, be of minor adverse significance, which is not significant in EIA terms.
  2. This is likely to be a conservative prediction as there is some evidence (although with uncertainties) that marine mammal populations are likely to benefit from introduction of hard substrates and associated fauna during the operation and maintenance phase.
Secondary mitigation and residual effect
  1. No marine mammal mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond designed in measures outlined in section 10.10) is not significant in EIA terms.

                        Decommissioning phase

Magnitude of impact
  1. Potential impacts on marine mammal prey species during the decommissioning phase have been assessed in volume 2, chapter 9 using the appropriate maximum design scenarios for these receptors. These impacts include temporary subtidal habitat loss/disturbance, long term subtidal habitat loss and increased SSCs and associated sediment deposition.
  2. Magnitude of impacts are as described for the construction phase in paragraph 552 et seq. The impact on marine mammal receptors is therefore predicted to be of local spatial extent, medium term duration, intermittent and of high reversibility. The magnitude is therefore, considered to be low.
Sensitivity of the receptor
  1. The sensitivity of marine mammal receptors during the decommissioning phase is not expected to differ from the sensitivity of the receptors during the construction phase described in paragraph 560 et seq. The sensitivity of the receptor is therefore, considered to be low.
Significance of the effect
  1. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be low. Given that marine mammals can exploit a wide range of prey species but travelling longer distances may be associated with higher rate of energy expenditure, the effect will therefore be of 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 further mitigation is not significant in EIA terms.