Operation and maintenance phase
Magnitude of impact
248. The potential of an increase in SSCs may arise because of mooring lines or cables making contact with and moving on the seabed, disturbing seabed materials and causing scouring and increased 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. Any increase in SSCs and associated deposition will include native material only, and although comprises predominantly mobile sand material, the low rates of sediment transport, will ensure it is redeposited close by after a short period of suspension, thus not impacting significantly on seabed morphology. Any significant changes to the seabed morphology will not recover immediately, due to the low rates of sediment transport, however the evidence of mobile sediments implies any impacts will be fully recoverable after some time (volume 2, chapter 7).
249. In line with the physical processes assessment, the first MDS was considered to be the number of foundations with the greatest length of mooring line on the seabed per foundation, rather than over the site boundary as a whole, as the effects are considered to be very localised, with no interactions between adjacent foundations. This was assumed as up to 130 semi-submersible turbine foundations with up to 9 catenary mooring lines each ( Table 9.13 Open ▸ ). This first MDS is hereafter referred to as the ‘130 turbine MDS’ for clarity. The second MDS considered was based on up to 265 semi-submersible turbine foundations with up to 6 catenary mooring lines each ( Table 9.13 Open ▸ ) and is hereafter referred to as the ‘265 turbine MDS’ for clarity. This was included in the assessment for fish and shellfish ecology as the 130 turbine MDS represents a potentially higher impact to fish and shellfish IEFs and at a localised level (due to a higher number of mooring lines per foundation), but it does not consider the overall footprint of impact over the Array fish and shellfish ecology study area as a whole. Thus, the 265 turbine MDS represents a higher overall length of mooring lines in contact with the seabed over the Array fish and shellfish ecology study area as a whole, but a lower potential for impact associated with fish and shellfish IEFs in the immediate vicinity of individual turbines.
250. The mooring line radius for both MDSs is 700 m, with a touchdown distance of between 25 m and 150 m from the foundation, and overall length of 750 m. During operation, approximately 680 m of the catenary mooring line will be in contact with the seabed which amounts to up to 6,120 m per foundation for the 130 turbine MDS and up to 4,080 m per foundation for the 265 turbine MDS ( Table 9.13 Open ▸ ). Overall, up to 795,600 m of mooring line may be in contact with the seabed under the 130 turbine MDS, and up to 1,081,200 m under the 265 turbine MDS ( Table 9.13 Open ▸ ), highlighting the differences between the two MDSs. The tidal range at the Array fish and shellfish ecology study area is less than 4 m; therefore it is not anticipated that tidal movements will result in substantial horizontal and vertical movements. As such, the mooring lines are not considered to notably increase the SSCs under standard operating conditions for both the MDSs.
251. Under harsher weather conditions, the dynamic interaction between the mooring lines and the seabed will increase with intensity and direction of the storm. Horizontal movement of the floating foundations may result in the lifting of the mooring lines located on the windward side of the turbine, as tension on these mooring lines increases. Mooring lines on the leeward side would experience the opposite effect, whereby the length of mooring line in contact with the seabed increases as they slacken, up to a maximum of 710 m for some mooring lines in the most extreme storm conditions. The length where disturbance is likely to occur will be less, as this will be greater closer to the touchdown point and negligible towards the anchor point. Furthermore, the dimensions of the mooring lines are small, with a chain thickness of 185 mm, and horizontal diameter of 620 mm, which will limit the volumes of seabed material they have the potential to disturb, even if they were to become completely embedded.
252. Movement on the seabed by inter-array cables will be limited to a small section between the touch down point and the point where the cable becomes static, resulting in minor increases to SSCs in the vicinity of the touchdown point only. Regarding inter-array cables, the total length of the dynamic inter-array cables will be 116 km with a maximum external cable diameter of 300 mm for both MDSs considered. Movement of the inter-array cables may be reduced using buoyancy modules and clump weights (subject to engineering design) thus limiting movement on the seabed to a very small proportion of the total dynamic cable length between the touchdown point and where it transitions to a static cable. Static inter-array and interconnector cables on the seabed will be buried or fixed with cable protection where target burial depths cannot be achieved. Thus, the potential disturbance area is restricted to small areas in the vicinity of up to two dynamic cable touchdown points per turbine. Increased SSCs would therefore be spatially limited, smaller, and adjacent to any disturbance resulting from the mooring lines.
253. The spacing between the floating foundations is a minimum 1.4 km for the 130 turbine MDS and a minimum of 1 km for the 265 turbine MDS ( Table 9.13 Open ▸ ). These spacings are large enough for any impacts to SSCs to be considered as isolated, considering the low current speeds and sediment transport rates in the physical processes study area. Any dynamic interactions between the seabed and mooring lines or dynamic cables will likely be experienced similarly at adjacent foundations under tidal and storm conditions, with the foundations moving in the same direction and orientated the same way as their neighbouring foundations. Thus, storm conditions will not impact upon minimum foundation spacing and seabed disturbance areas from mooring lines are considered sufficiently far apart to be isolated even under storm conditions for both MDSs considered.
254. Horizontal movement of the floating foundations may result in the lifting of the mooring lines located on the windward side of the turbine, as tension on these mooring lines increases. Mooring lines on the leeward side would experience the opposite effect, whereby the length of mooring line in contact with the bed increases as they slacken, up to a maximum of 710 m for some mooring lines in the most extreme storm conditions. The length where disturbance is likely to occur will be less, as this will be greater closer to the touchdown point and negligible towards the anchor point. Furthermore, the dimensions of the mooring lines are small, with a chain thickness of 185 mm, and horizontal diameter of 620 mm, which will limit the volumes of seabed material they have the potential to disturb, even if they were to become completely embedded.
255. Regarding inter-array cables, the total length of the dynamic inter-array cables will be 116 km with a maximum external cable diameter of 300 mm. Movement of the inter-array cables may be reduced through the use of buoyancy modules and clump weights (subject to engineering design) thus limiting movement on the seabed to a very small proportion of the total dynamic cable length between the touchdown point and where it transitions to a static cable. Static inter-array and interconnector cables on the seabed will be buried or fixed with cable protection where target burial depths cannot be achieved. Thus, the potential disturbance area is restricted to small areas in the vicinity of up to two dynamic cable touchdown points per turbine. Increased SSCs would therefore be spatially limited, smaller, and adjacent to any disturbance resulting from the mooring lines, of which there are up to nine per floating foundation.
256. A small proportion of the dynamic cable between the touchdown point to the point where it becomes static may move on the seabed. However, installation of clump weights and buoyancy modules, or alternative solutions as required, will reduce the movement of the dynamic component of the cable from the touchdown point to the transition point to minimise wear.
257. The impact is predicted to be of local spatial extent, long term duration, intermittent, and of high reversibility. The magnitude is therefore considered to be low.
Sensitivity of the receptor
258. The sensitivity of the fish and shellfish IEFs, for both marine and diadromous species, can be found in the site preparation and construction phase assessment (see paragraph 237 et seq.).
Significance of the effect
Marine fish and shellfish species
259. Overall, for all marine fish and shellfish species considered as IEFs, 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 negligible to minor adverse significance, which is not significant in EIA terms.
Diadromous species
260. 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 negligible to minor adverse significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
261. No secondary fish and shellfish ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.
Decommissioning phase
Magnitude of impact
262. Decommissioning of infrastructure associated with the Array may lead to increases in SSCs and associated deposition. The MDS is represented by the removal of all infrastructure, as this represents the largest potential for increased SSCs and associated deposition ( Table 9.13 Open ▸ ). Note, the decommissioning strategy is not defined, and cables, cable protection, and scour protection may potentially be left in situ. If some infrastructure remains in situ, the MDS presented here will be an overestimation, and SSCs will be lower.
263. Decommissioning activities are assumed to result in increased SSCs and associated deposition that are lesser than or equal to those produced during construction. The impacts of decommissioning activities are therefore predicted to be no greater than those presented in paragraphs 232 et seq. for the site preparation and construction activities. In actuality, the release of sediment in the decommissioning phase will be lower as it doesn’t include activities such as seabed preparation and DEA installation.
264. Therefore, this impact is predicted to be of local spatial extent, short term duration, intermittent, and of high reversibility. The magnitude is therefore considered to be low.
Sensitivity of the receptor
265. The sensitivity of the fish and shellfish IEFs, for both marine and diadromous species, can be found in the preparation and construction phase assessment (see paragraph 237 et seq.).
Significance of the effect
Marine fish and shellfish species
266. Overall, for all marine fish and shellfish species considered as IEFs, 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 negligible to minor adverse significance, which is not significant in EIA terms.
Diadromous species
267. 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 negligible to minor adverse significance, which is not significant in EIA terms.
Secondary mitigation and residual effect
268. No secondary fish and shellfish ecology mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.
Effects to fish and shellfish Receptors due to EMF from subsea electrical cabling
269. Effects to fish and shellfish ecology due to Electromagnetic Fields (EMFs) from subsea electrical cabling EMF may arise due to the operation of inter-array and interconnector cables during the operation and maintenance phase as outlined in Table 9.13 Open ▸ . The conduction of electricity through subsea power cables will result in emission of localised EMFs which could potentially impact the sensory mechanisms of some species of fish and shellfish, particularly electrosensitive species (including elasmobranchs) and diadromous fish species (Centre for Marine and Coastal Studies (CMACS), 2003). This section also involves the assessment of the impacts of EMFs from the dynamic inter-array cables in the water column on fish and shellfish IEFs within the fish and shellfish ecology study area.
Operation and maintenance phase
Magnitude of impact
271. As shown in Table 9.13 Open ▸ , the MDS assumes there may be up 1,261 km of 66 kV or 132 kV inter-array cables installed within the site boundary. Of these, a maximum of 116 km of these inter-array cables will be in the water column as dynamic cables, with the rest of these installed on the seabed. There may be up to 236 km of 275 kV AC or 525 kV DC interconnector cables with total length buried to a minimum depth target burial depth of 0.4 m (subject to a CBRA).
272. It is common practice to block the direct electrical field using conductive sheathing, meaning that the only EMFs that are emitted into the marine environment are the magnetic field and the resultant induced electrical field. It is generally considered impractical to assume that cables can be buried at depths that will reduce the magnitude of the magnetic field, and hence the sediment-sea water interface induced electrical field, to below that at which these fields could be detected by certain marine organisms on or close to the seabed (Gill et al., 2005; Gill et al., 2009). By burying a cable, the magnetic field at the seabed is reduced due to the distance between the cable and the seabed surface as a result of field decay with distance from the cable (CSA, 2019).
273. A variety of design and installation factors affect EMF levels in the vicinity of the cables. These include current flow, distance between cables, cable orientation relative to the earth’s magnetic field (DC only), cable insulation, number of conductors, configuration of cable and burial depth. Clear differences between AC and DC systems are apparent: the flow of electricity associated with an AC cable changes direction (as per the frequency of the AC transmission) and creates a constantly varying electric field in the surrounding marine environment (Huang, 2005). Conversely, DC cables transmit energy in one direction creating a static electric and magnetic field. Average magnetic fields of DC cables are also higher than those of equivalent AC cables.
274. The strength of the magnetic field (and consequently, induced electrical fields) decreases rapidly horizontally and vertically with distance from source. A recent study conducted by CSA (2019) found that inter-array and interconnector cables buried between depths of 1 m to 2 m reduces the magnetic field at the seabed surface four-fold. For cables that are unburied and instead protected by thick concrete mattresses or rock berms, the field levels were found to be similar to buried cables.
275. CSA (2019) found magnetic field levels directly over live AC subsea power cables associated with offshore wind energy projects range between 65 mG (at seafloor) and 5 mG (1 m above sea floor) for inter-array cables. At lateral distances from the cable, magnetic fields greatly reduced at the sea floor to between 10 mG and <0.1 mG.
276. While the majority of cables will be buried beneath surface sediments as set out in the MDS ( Table 9.13 Open ▸ ), a small proportion of inter-array cables will be dynamic cables within the water column (up to 116 km length across the Array). EMFs produced by these dynamic cables also have the potential to impact fish and shellfish ecology receptors. As set out above, EMF intensity from subsea cables (which include dynamic cables) decreases at approximately the inverse square/power of the distance away from the cable (Hutchison et al., 2018), and this attenuation is the same for buried, unburied, and dynamic cables (Hutchison et al., 2021). So whilst the EMF levels from dynamic cables and buried cables will remain the same along the entire cable, the surface sediments and cable protection maintain distance between fish and shellfish species and cables on the seabed thus reducing interaction. For dynamic cable portions pelagic species may pass closer to cables within the water column and have the potential to be exposted to increased levels of EMFs. Nonetheless levels of EMF will be returned to baseline levels within a few metres of the cable and therefore the area of effect is highly limited in extent, particularly in the context of the habitats available in the fish and shellfish study area and the water depths within the Array.
277. The impact is predicted to be of local spatial extent, long term duration, continuous and low reversibility during the operation and maintenance phase (impact is reversible upon decommissioning). It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
Sensitivity of the receptor
279. Studies examining the effects of EMFs from AC subsea power cables on fish behaviours have been conducted to determine the thresholds for detection and response to EMFs. Table 9.28 Open ▸ provides an up-to-date summary of the scientific studies conducted to assess sensitivity of EMFs on varying fish species. The overall amount of research into the impacts of EMFs have indicated that marine fish and shellfish species are known to have some level of sensitivity to this effect, and so these have been split out for separate consideration within this assessment.
Table 9.28: Relationship Between Geomagnetic Field Detection Electro Sensitivity, and the Ability to Detect 50/60-Hz AC Fields in Common Marine Fish and Shellfish Species (Adapted from CSA, 2019)
Marine fish species
280. Several field studies have observed behaviours of fish and other species around AC submarine cables in the USA ( Table 9.28 Open ▸ ). Observations at three energised 35 kV AC subsea power cable sites off the coast of California that run from three offshore platforms to shore, which are unburied along much of the route, did not show that fish were repelled by or attracted to the cables (Love et al., 2016). A study investigating the effect of EMFs on lesser sandeel larvae spatial distribution found that there was no effect on the larvae (Cresci et al., 2022), and a prior study concluded the same for herring (Cresci et al., 2020).
281. Elasmobranchs (i.e. shark, skate and ray) are known to be the most electro-receptive of all fish. These species possess specialised electro-receptors which enable them to detect very weak voltage gradients (down to 0.5 μV/m) in the environment naturally emitted from their prey (Gill et al., 2005). Both attraction and repulsion reactions to electrical fields have been observed in elasmobranch species. Spurdog Squalus acanthias, an elasmobranch species known to occur within the fish and shellfish ecology study area, avoided electrical fields at 10 μV/cm (Gill and Taylor, 2001), although it should be noted that this level (i.e. 10 μV/cm is equivalent to 1,000 μV/m) is considerably higher than levels associated with offshore electrical cables. A Collaborative Offshore Wind Research into the Environment (COWRIE) sponsored mesocosm study demonstrated that the lesser spotted dogfish Scyliorhinus canicula and thornback ray were able to respond to EMF of the type and intensity associated with subsea cables; the responses of some ray individuals suggested a greater searching effort when the cables were switched on (Gill et al., 2009). However, the responses were not predictable and did not always occur (Gill et al., 2009). In another study, EMF from 50 Hz to 60 Hz AC sources appears undetectable in elasmobranchs. Kempster and Colin (2011) have noted the physiological capacity for detection of EMFs in basking shark, which may migrate through the fish and shellfish ecology study area (noting abundances of basking shark in the North Sea area generally low), but no current evidence exists on specific impacts of EMFs of any strength on this species, apart from the likely detection capacity of a standard electrical field benchmark level of 1 V/m (Wilding et al., 2020). More generally, Kempster et al. (2013) reported that small shark could not detect EMF produced at 20 Hz and above, and Hart and Collin (2015) found no significant repellent effect of a magnetic field of 14,800 G on shark catch rates, suggesting a low sensitivity to these fields.
282. In summary, the range over which these fish species can detect electric fields is limited to a scale of metres around electrical cables buried to depths of 1 m to 2 m (CSA, 2019). Pelagic species (such as herring) generally swim well above the seafloor, though may still be exposed to the EMFs from the dynamic cables in the water column. 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 EMFs from these cables is likely to only be detected within a matter of metres. Beyond this range, levels of EMFs will be expected to be at baseline levels for this part of the North Sea, resulting in impacts that would therefore be highly localised.
283. Demersal species (e.g. elasmobranchs) that dwell on the bottom, are more likely to come into the ZoI of subsea power cables and thus encounter higher EMF levels when near the cable. Demersal species are also likely to be exposed for longer periods of time and may be largely constrained in terms of location. However, the rapid decay of the EMF with horizontal and vertical distance (Bochert and Zettler, 2006) (i.e. within metres) reduces the extent of potential impacts.
284. Most marine fish ecology IEFs in the fish and shellfish ecology study area are deemed to be of low vulnerability, high recoverability and local to national importance. The sensitivity of the receptor is therefore considered to be low.
285. Elasmobranch species in the fish and shellfish ecology study area are deemed to be of medium vulnerability, high recoverability, and local to national importance. The sensitivity of the receptor is therefore considered to be low.
Shellfish species
286. Crustaceans, including lobster and crab, have been shown to demonstrate a response to B fields, with the Caribbean spiny lobster Panulirus argus shown to use a magnetic map for navigation (CSA, 2019). EMF exposure has been shown to result in varying egg volumes for edible crab compared to controls. Exposed larvae were significantly smaller, but there were no statistically significant differences in hatched larval numbers, deformities, mortalities, or fitness (Scott, 2019). Exposure to EMF has also been shown to affect a variety of physiological processes within crustaceans. For example, Lee and Weis demonstrated that EMF exposure affected moulting in fiddler crab species (Uca pugilator and Uca pugnax) (Lee and Weis, 1980).
287. Observations of crab movement and location inside large cages off south California and in Puget Sound were reported by Love et al. (2016) and these were reported to be unaffected by proximity to energised AC subsea power cables, indicating crab also were not attracted to or repelled by energised AC subsea power cables that were either buried or unburied. Similarly, no significant change in distance or speed of travel over time when American lobster Homarus americanus were exposed to magnetic fields of 53 to 65 μT (Hutchison et al., 2020). However, studies on the Dungeness crab and edible crab have reported behavioural changes during exposure to increased EMF and both species showed increased activity when compared to crab that were not exposed (Scott et al., 2018; Woodruff et al., 2012). Crab may also spend less time buried, which is normally a natural predator avoidance behaviour (Rosaria and Martin, 2010), and some species have been noted not to cross subsea cables (Love et al., 2017), potentially reducing habitats available for predation.
288. It is uncertain if other crustaceans including commercially important European lobster are able to respond to magnetic fields in this way. Limited research undertaken with the European lobster found no neurological response to magnetic field strengths considerably higher than those expected directly over an average buried power cable (Normandeau et al., 2011; Ueno et al., 1986). A field study by Hutchison et al. (2018) observed the behaviour of American lobster (a magneto-sensitive species) to DC and AC fields from a buried cable and found that it did not cause a barrier to movement or migration, as both species were able to freely cross the cable. However, lobster were observed to make more turns when near the energised cable. Adult lobster have been shown to spend a higher percentage of time within shelter when exposed to EMF. European lobster exposed to EMF have also been found to have a significant decrease in egg volume at later stages of egg development and more larval deformities (Scott, et al. 2020).
289. Scott et al. (2020) presents a review of the existing papers on the impact of EMF on crustacean species. Of the papers reviewed, three studied EMF effects on fauna in the field, the rest were laboratory experiments which directly exposed the target fauna to EMF. These laboratory experiments, while giving us an indication of crustacean behaviour to EMF, may be less applicable in the context of subsea cables in the marine environment. Of the field experiments, one demonstrated that lobster have a magnetic compass by tethering lobster inside a magnetic coil (Lohmann et al., 1995), one focussed on freshwater crayfish and put magnets within the crayfish hideouts (Tański et al., 2005), and the last one looked at shore crab Carcinus maenas at an offshore wind farm and found no adverse impact on the population. The two former papers may not be directly applicable to offshore wind farm subsea cables and the latter found no adverse impact on the population of shore crab from the offshore wind farm (Langhamer et al., 2016).
290. Further research by Scott et al. (2021) found that physiological and behavioural impacts on edible crab occurred at 500 μT and 1000 μT, causing disruption to the L-lactate and D-glucose circadian rhythm and altering total haemocyte count, and also causing attraction to EMF exposed areas and reduced roaming time. However, these physiological and behavioural impacts did not occur at 250 μT. Seeing as even in the event of an unburied cable the maximum magnetic field reported was 78.27 μT (Normandeau et al., 2011), it can be assumed that the magnetic fields generated by the cables will be lower than 250 μT, and therefore will not present any adverse impacts on edible crab. Harsanyi et al. (2022) noted that chronic exposure to EMF effects could lead to physiological deformities and reduced swimming test rates in lobster and edible crab larvae. However, these deformities were in response to EMF levels of 2,800 μT and therefore are considerably higher than EMF effects expected for buried cables. The report recommends burying of cables in order to reduce any potential impacts associated with high levels of EMF in line with the designed in measures outlined in section 9.10.
291. As with marine fish species discussed above, the range over which these species can detect electric fields is limited to a scale of metres around electrical cables buried to depths of 1 m to 2 m (CSA, 2019). Demersal shellfish species (e.g. decapod crustaceans) that dwell on the bottom, are more likely to come into the ZoI of subsea power cables and thus encounter higher EMF levels when near the cable, are likely to be exposed for longer periods of time and may be largely constrained in terms of location. However, the rapid decay of the EMF with horizontal and vertical distance (Bochert and Zettler, 2006) (i.e. within metres) reduces the extent of potential impacts.
292. Most marine shellfish ecology IEFs in the fish and shellfish ecology study area are deemed to be of low vulnerability, high recoverability and local to national importance. The sensitivity of the receptor is therefore considered to be low.
293. Decapod crustaceans in the fish and shellfish ecology study area are deemed to be of medium vulnerability, high recoverability, and local to national importance. The sensitivity of the receptor is therefore considered to be low.
Diadromous species
294. EMFs may also interfere with the navigation of sensitive diadromous species. Species for which there is evidence of a response to E and/or B fields include river lamprey, sea lamprey, European eel, and Atlantic salmon (Gill et al., 2005; CSA, 2019). Effects of EMFs surrounding subsea cables on allis shad, twaite shad and sparling are currently poorly researched, with recommendations made to investigate these potential effects in future (Gill et al., 2012; Sinclair et al., 2020; noting that shad species are pelagic and therefore unlikely to interact with EMF from installed cables on the seabed). As with marine fish, however, diadromous fish species may be exposed to EMFs from the dynamic cables in the water column. EMFs emitted from these dynamic cables is likely to only be detected within a matter of metres; beyond which, baseline levels will be established. As such, impacts from EMFs from the dynamic cables are highly localised. Lamprey possess specialised ampullary electroreceptors that are sensitive to weak, low frequency electric fields (Bodznick and Northcutt, 1981; Bodznick and Preston, 1983), which are hypothesised to be used for prey-detection, although further research is required in this area (Tricas and Carlston, 2012). Chung-Davidson et al. (2008) found that weak electric fields may play a role in the reproduction of sea lamprey and it was suggested that electrical stimuli mediate different behaviours in feeding-stage and spawning-stage individuals. This study (Chung-Davidson et al., 2008) showed that migration behaviour of sea lamprey was affected (i.e. adults did not move) when stimulated with electrical fields of intensities of between 2.5 mV/m and 100 mV/m, with normal behaviour observed at electrical field intensities higher and lower than this range. It should be noted, however, that these levels are considerably higher than modelled induced electrical fields expected from AC subsea cables (see Table 9.29 Open ▸ ). There is currently no evidence of lamprey responses to magnetic B fields (Gill and Bartlett, 2010).
295. Atlantic salmon and European eel have both been found to possess magnetic material of a size suitable for magnetoreception, and these species can use the earth’s magnetic field for orientation and direction-finding during migration (Gill and Bartlett, 2010; CSA, 2019). Mark and recapture experiments undertaken at the operational Nysted Offshore Wind Farm showed that eel did cross the interconnector cable (Hvidt et al., 2003). Studies on European eel in the Baltic Sea have highlighted some limited effects of subsea cables (Westerberg and Lagenfelt, 2008), with evidence of direct detection of EMF through the lateral line of this species (Moore and Riley, 2009). The swimming speed during migration was shown to change in the short term (tens of minutes) with exposure to AC electric subsea cables, even though the overall direction remained unaffected (Westerberg and Langenfelt, 2008). The authors concluded that any delaying effect (i.e. on average 40 minutes) would not be likely to influence fitness in a 7,000 km migration, with little to no impact on migratory behaviour noted beyond 500 m from wind farm development infrastructure (Ohman et al., 2007). Research in Sweden on the effects of a High Voltage Direct Current (HVDC) cable on the migration patterns of a range of fish species, including salmonids, failed to find any effect (Westerberg et al., 2007; Wilhelmsson et al., 2010). Research conducted at the Trans Bay cable, a DC subsea cable near San Francisco, California, found that migration success and survival of chinook salmon Oncorhynchus tshawytscha was not impacted by the cable. However, as with the Hutchison et al. (2018) study on lobster, behavioural changes were noted when these fish were near the cable (Kavet et al., 2016) with salmon appearing to remain around the cable for longer periods. These studies demonstrate that while DC subsea power cables can result in altered patterns of fish behaviour, these changes are temporary and do not interfere with migration success or population health.
296. Table 9.29 Open ▸ provides a summary of the scientific studies conducted to assess sensitivity of EMF on varying diadromous fish species.
Table 9.29: Relationship Between Geomagnetic Field Detection Electro Sensitivity, and the Ability to Detect 50/60-Hz AC Fields in Diadromous Fish Species (Adapted from CSA, 2019)
297. Diadromous fish IEFs in the fish and shellfish ecology study area are deemed to be of low vulnerability, high recoverability and national to international importance. The sensitivity of the receptor is therefore, considered to be low.
Significance of the effect
Marine fish and shellfish species
298. For most fish and shellfish IEF species, 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 negligible to minor adverse significance, which is not significant in EIA terms.
Diadromous species