10.15. Inter-Related Effects (and Ecosystem Assessment)

  1. A description of the likely significant inter-related effects arising from the Array on marine mammals is provided in volume 3, appendix 18.1 of the Array EIA Report.
  2. For marine mammals the following potential impacts have been considered within the inter-related assessment:
  • injury and disturbance from underwater noise generated during piling;
  • injury and disturbance from underwater noise generated during UXO clearance;
  • injury and disturbance due to site-investigation surveys (including geophysical surveys);
  • injury and disturbance from underwater noise generated during vessel use and other noise producing activities;
  • injury due to collision with vessels;
  • effects on marine mammals due to EMFs from subsea electrical cabling in the water column;
  • injury and disturbance from underwater noise generated during the operation of floating wind turbines and anchor mooring lines;
  • effects on marine mammals due to entanglement associated with the Array; and
  • effects on marine mammals due to altered prey availability.
  1. Table 10.64   Open ▸ assesses the likely significant inter-related effects (project lifetime effects) that are predicted to arise during the construction, operation and maintenance phase, and decommissioning of the Array and also the inter-related effects (receptor-led effects) that are predicted to arise for marine mammal receptors.
  2. Marine mammals also have the potential to have a secondary effect on other receptors and these effects are fully considered in the topic-specific chapters. These receptors and effects are:
  • fish and shellfish ecology:

–           changes in the marine mammal community could have indirect effects on fish and shellfish populations.

Table 10.64:
Summary of Likely Significant Inter-Related Effects for Marine Mammals from Individual Effects Occurring Across the Construction, Operation and Maintenance and Decommissioning Phases of the Array (Array Lifetime Effects) and from Multiple Effects Interacting Across all Phases (Receptor-led Effects)

Table 10.64: Summary of Likely Significant Inter-Related Effects for Marine Mammals from Individual Effects Occurring Across the Construction, Operation and Maintenance and Decommissioning Phases of the Array (Array Lifetime Effects) and from Multiple Effects Interacting Across all Phases (Receptor-led Effects)

 

Stressor 1: injury or disturbance from elevated underwater noise (from piling, UXO clearance, site-investigation surveys, vessels, operational noise from turbines/mooring lines)
  1. During the construction phase activities resulting in elevated underwater noise include piling, UXO clearance, site investigation surveys and vessel movements could occur. These activities are likely to result in disturbance to marine mammals which may be additive in nature if activities are synchronised, as it could lead to a larger area disturbed at any one time. Disturbance is likely to occur as short term, localised events for each activity within the construction phase. Prior to piling, for example, UXO clearance could result in no more than 15 single clearance events ( Table 10.17   Open ▸ ), and disturbance occurring mainly during secondary mitigation (ADDs and soft start) rather than the UXO clearance event itself which would be no more than seconds for each. There is also a small potential that animals could experience injury during UXO clearance (due to an accidental high order detonation). Site investigation surveys are likely to occur over a total duration of up to five months (over a three year period) whilst disturbance during vessel activity will occur intermittently throughout this phase with timings linked to the pre-construction activities (UXO and site-investigation surveys).
  2. During the construction phase, activities resulting in elevated underwater noise include piling, other construction activities and vessel movements could occur. Since injury to marine mammals will be mitigated through the outline MMMP (volume 4, appendix 22) ( Table 10.22   Open ▸ ), the key focus is on disturbance effects. Disturbance could occur intermittently on a total of 602 days over the construction phase of 96 months. Other construction activities (e.g. drilling and cable laying) and vessel movements would occur intermittently within the eight year construction phase. When piling occurs the disturbance effects are likely to be greater than for any of the other activities contributing to elevated underwater noise so there is less likely to be an additive or synergistic effect during piling. There may, however, be an additive effect spatially where two or more noise-producing activities occur in different parts of the Array, or temporally due to ongoing disturbance from activities throughout the construction phase (e.g. if they occur consecutively).
  3. During the operation and maintenance phase, activities resulting in elevated underwater noise include vessel activity, geophysical surveys and operational noise from floating turbines and mooring lines. These activities have the potential to result in disturbance to marine mammals which may be additive if activities are synchronised, as it could lead to a larger area disturbed at any one time. Disturbance is likely to occur as short term, localised events for vessel activity and geophysical surveys and the disturbance from operational noise is expected to be minimal, but there may be an additive effect spatially where two or more noise-producing activities occur in different parts of the Array, or temporally due to ongoing disturbance from activities throughout the operation and maintenance phase (e.g. if they occur consecutively).
  4. During decommissioning, vessel movements associated with decommissioning activities, as well as underwater cutting and site investigation surveys, will result in elevated underwater noise which could lead to disturbance to marine mammals. Disturbance is likely to occur as short term, localised events and there may be an additive effect spatially where vessels are operating in different parts of the Array area, or temporally due to ongoing disturbance throughout the decommissioning phase.
  5. Therefore, marine mammal receptors have the potential to experience ongoing disturbance due to elevations in underwater noise from different sources at all phases of the Array. The sensitivity of key species will be linked to their ability to tolerate the stressor such that their ability to function normally (e.g. forage, reproduce, communicate, avoid predators) is not impeded. The assessment, which adopts a highly precautionary approach (see paragraph 108 et seq.), has demonstrated that for all impacts, considered in isolation, the residual effects will not be significant (after implementation of secondary mitigation) as either the spatial scale is very localised or where larger scale effects do occur (i.e. during piling or UXO) these will be highly reversible with animals returning to baseline levels rapidly. After implementation of secondary mitigation there is, however, potentially a small residual number of harbour porpoise that could experience auditory injury during UXO clearance activities and would represent only a very small proportion of the NS MU population.
  6. There are, however, uncertainties as to how all activities interact to contribute to an additive effect from underwater noise as a stressor. In a Before-After-Control-Impact design (BACI) study looking at foraging activity of harbour porpoise between baseline periods and different construction phases of the Beatrice and Moray East Offshore Wind Farms (Benhemma-Le Gall et al., 2021) an eight to 17% decline in harbour porpoise occurrence in the impacted area during pile-driving and other construction activities was observed, with probability of detection negatively related to levels of vessel intensity and background noise.
  7. To some extent it is anticipated that animals will acclimatise to or compensate for such increases in underwater noise. Graham et al. (2019), for example, demonstrated acclimatisation in harbour porpoise. The study showed that the proportional response of harbour porpoise to piling noise decreased over the piling phase, with the proportion of animals disturbed at a received level of 160 dB re 1 µPa decreased from 91.5% to 49.2% from the first pile to the last pile. Kastelein et al. (2019b) suggest that harbour porpoise (a species with high daily energy requirements) may be able to compensate for period of disturbance as they can dramatically increase their food intake in a period following fasting within out any detriment to their health. In the Moray Firth, harbour porpoises displaced during wind farm construction of Beatrice and Moray East Offshore Wind Farms increased their buzzing activity, potentially compensating for lost foraging opportunities (although there may be an additional energetic cost from the fleeing and distance travelled to compensate for) (Benhemma-Le Gall et al., 2021).
  8. Therefore, as detailed in paragraphs 1075 to 1081 above, significance is considered to be minor adverse and therefore not significant in EIA terms.
Stressor 2: injury due to collisions with vessels
  1. Injury due to collisions with vessels is associated with increased vessel movement, the impact of which was assessed from different types of vessels and at different phases of the Array. As described in paragraph 1075 et seq., over the lifetime of the Array there will be a longer term risk to marine mammal receptors however, with designed in measures in place (section 10.10) the potential of experiencing injury is likely to be reduced and therefore it is not anticipated that an additive effect will occur. Additionally, to some extent the noise from the vessels themselves (Stressor 2, paragraph 1075 et seq.) would act antagonistically with this impact by deterring animals away from vessels and thereby further reducing the risk of injury due to collision. Furthermore, marine mammals in this area are already accustomed to high level of vessel activity (see paragraph 405 et seq.). For example, Buckstaff (2004) demonstrated that bottlenose dolphins increased their rate of whistle production at the onset of a vessel approach, and then decreased production during and after it had passed. This increased whistle production may be a tactic to reduce signal degradation to ensure that information is being communicated in elevated noisy environment, but it also demonstrates that animals are aware of approaching vessel from a distance. This corroborates previous research of Nowacek et al. (2001) found that bottlenose dolphins swim in tighter aggregated groups during vessel approaches, therefore if a vessel is loud enough to be detected by an animal for which it adjusts its behaviour, the likelihood of collision decreases.
  2. Therefore, as detailed in paragraph 1083, significance is considered to be minor adverse and therefore not significant in EIA terms.
Stressor 3: EMF
  1. EMF is highly localised and there is limited information on the effect of EMF on marine mammal receptors. It is unlikely to be additive with other stressors, given it will be confined to very specific locations in close proximity to the cables. There may be some synergistic effects if animals moving away from other disturbance activities (such as vessels) dive down and therefore move closer to the inter-array cables. Therefore, significance is considered to be minor adverse and therefore not significant in EIA terms.

 

Stressor 4: entanglement
  1. The risk of entanglement is highly localised. The possibility of primary entanglement is very unlikely given design factors such as the taut mooring lines with high bending stiffness (Statoil, 2015) and low weight of the cable systems (SEER, 2022). It is noted there is limited information to assess entanglement of marine mammal receptors in offshore wind development to date. Injury from entanglement is very different to other types of injury (e.g. injuries from collision, PTS) and therefore there is not considered to be any additive effects. As is the case for stressor 2, to some extent the noise (pinging or snapping) from operational noise from turbines/mooring lines and any vessels utilised during the operation and maintenance phase themselves may act antagonistically with this impact by deterring animals away from the mooring lines. Therefore, significance is considered to be minor adverse and therefore not significant in EIA terms.
Stressor 5: changes in prey communities.
  1. The EIA considered overall effect on fish and shellfish communities from multiple stressors (i.e. habitat loss, SSC, underwater noise, EMF etc) (see volume 3, chapter 9) and therefore, in this respect, has taken an ecosystem-based approach. For some, stressors such as underwater noise effects on fish and shellfish, will be over the same timescales as marine mammals whilst for others, such as temporary habitat loss, timescales may be different to those assessed for marine mammals (e.g. low mobility or sessile species may recover slowly). The assessment of effects, however, demonstrated that due to the high mobility of marine mammals, generalist feeding strategy and ability to exploit different prey species, combined with the small scale of potential changes in context of wider available habitat, the changes to fish and shellfish communities are unlikely to have an effect even from multiple stressors. Therefore, significance is considered to be minor adverse and therefore not significant in EIA terms.
Multiple stressors: inter-related effect of all stressors
  1. Arrigo et al. (2020) studied synergistic interactions among growing stressors to an Arctic ecosystem and found that synergistic interactions amplify adverse stressor effects, and the impact of synergy is predicted to increase with the magnitude of stressors. Arrigo et al. (2020) suggests that large organisms at higher trophic levels, such as marine mammals, tend to be generally negatively impacted by increasing stressor interaction strength but the variability in the response to stressor is small and therefore reduces the probability of population collapse.
  2. For stressor 1 (elevated underwater noise), there is the potential for marine mammals to forage in different habitats and to compensate for reduced foraging time. As such the ability of displaced animals will depend on the availability of prey resources in the habitat to which the animals are displaced. Studies have shown that for small, localised marine mammal populations with high site fidelity, there may be biological risks posed by displacement (Forney et al., 2017). For example, due to the importance of the areas for survival (i.e. areas of high resource availability), animals may be highly motivated to remain in an area despite adverse impacts which may increase stress (Rolland et al., 2012). Thus, the inter-related effects of underwater noise and changes in fish and shellfish prey resources needs to be considered. Impacts on fish and shellfish prey resources (stressor 5) were predicted to be localised and short term and therefore unlikely to contribute to an inter-related effect where animals are displaced beyond the boundaries of the Array. Within the boundaries of the Array however, there may be short term inter-related effects of noise disturbance and reduced fish and shellfish prey resources. For marine mammals remaining in proximity to the Array, a substantial disruption in foraging may not be easy to compensate for where there are shifts in the species composition or localised reductions of fish and shellfish communities. It has been suggested it may be possible that damaged or disoriented prey could attract marine mammals to an area of impact due to providing short term feeding opportunities but increasing levels of exposure (Gordon et al., 2003) however, there is currently little evidence available to investigate such indirect effects on marine mammals.
  3. The assessment has largely described potential adverse effects but there is also potential for some beneficial effects on marine mammal receptors. Construction of offshore wind farms can lead to the introduction of hard substrates which can lead to the establishment of new species and new fauna communities, and this may in turn attract marine mammals (Fowler et al., 2018, Lindeboom et al., 2011, Raoux et al., 2017). Consequently, even where there is potential for an inter-related effect between ongoing vessel noise during the operation and maintenance phase this may be compensated for, to some extent, by an increase in available prey resources. Russell et al. (2014) and Russell and McConnell (2014) demonstrated that harbour seals and grey seals moved between hard structures at two operational wind farms and used space-state models to predict where animals were remaining at these locations to actively forage and where they were travelling to the next foundation structure. Lindeboom et al. (2011) studied the ecological effects of the Egmond aan Zee Offshore Wind Farm and found that even though the fish community was highly dynamic in time and space, with only minor effects upon fish assemblages observed during the operation and maintenance phase, some fish species (e.g. cod) benefited from the ‘shelter’ within the wind farm, although this effect may be reduced for floating wind turbines. This is likely due to reduced fishing activity and the new hard substratum with associated fauna which attracts predator species. Lindeboom et al. (2011) suggested the observed increase in echolocation activity of harbour porpoise within the wind farm may be correlated with presence of additional increased food sources compared to reference areas (Lindeboom et al., 2011).
  4. The potential inter-related effects between underwater noise and collision risk have been discussed previously (in paragraph 1083) and it is considered likely that marine mammals will move away from moving vessels in response to engine noise, therefore reducing the risk of collision (classed as an antagonistic interaction). Alternatively, marine mammals may tolerate and persist in a highly stressed state (as a result of injury caused by underwater noise) while the vessels are approaching (Muto et al., 2018). Animals could also become habituated to vessel noise and not move away from the vessel (McWhinnie et al., 2018) which would result in a synergistic interaction (Weilgart, 2011). Therefore, the outcome will depend on the degree of habituation and prior-experience and a number of acoustical properties that allow an approaching vessel to be detected by a marine mammal species (Gerstein et al., 2005). However, as described in the impact assessment, with measures adopted as part of the Array (e.g. the NSVMP, volume 4, appendix 24) in place it is likely that any risk of injury from collision with vessels will be negligible.
  5. Evidence for the potential long term effects of offshore wind farms on marine mammals (related to all potential stressors) comes from monitoring programmes which baseline levels of abundance to construction and post-construction (operation and maintenance) phases. Limited monitoring studies regarding impacts on marine mammals have been carried out to date.
  6. Aerial survey haul-out counts were conducted before, during and after the construction phases at Scroby Sands Offshore Wind Farm, off the coast of Norfolk, to monitor harbour and grey seal counts at haul-out site, located less than two kilometres away from the offshore wind farm array (Skeate et al., 2012). The two studies reported a decline in harbour seal numbers during construction, with numbers remaining lower over several subsequent years. However, the numbers of grey seals increased dramatically year after year throughout the construction and early operational periods. It has been suggested that it is possible that changes in harbour seal numbers may be linked to rapid colonisation of competing grey seal (Skeate et al., 2012). It was noted regional changes in patterns of haul-outs of harbour seal in the Wash coincided with the construction of the Scroby Sands Offshore Wind Farm, but such changes in harbour seal number could have been part of wider regional dynamics (Verfuss et al., 2016). It should be noted that Scroby Sands Wind Farm is located 2.5 km off the coast of Great Yarmouth whereas the Array is located 80 km offshore and therefore a greater distance from haul-out sites. As a part of marine mammal monitoring at Robin Rigg Offshore Wind Farm, boat-based surveys for cetaceans were conducted before, during, and after construction (Canning et al., 2013). The monitoring data suggested that harbour porpoise were displaced from the wind farm site during the construction phase and operation period when compared to the pre-construction numbers. However, because there was only one year of pre-construction survey, natural variation cannot be ruled out as the reason for the observed change, especially since control survey locations outside of the wind farm also appeared to experience declines in harbour porpoise density.
  7. With the rapid expansion of offshore wind farms, post-construction monitoring programmes are being implemented at various developments in Europe. Tougaard et al. (2003) studied short term effects of the construction of wind turbines on harbour porpoises at Horns Rev Offshore Wind Farm. The study showed a decrease in porpoise acoustic activity within the wind farm at the onset of piling operations and subsequent recovery to higher levels a few hours after each piling operation was completed (Tougaard et al., 2003). (Tougaard et al., 2003) also showed that over the entire construction phase at Horns Reef there was no significant change in the abundance of harbour porpoise in the wind farm area compared to reference areas. Teilmann et al. (2008) also reported that during the operation and maintenance phase porpoise activity was higher in both the wind farm and reference area compared to baseline levels. As a result of monitoring at Nysted Offshore Wind Farm, it was demonstrated initially during construction and the first two years of operation that there were lower acoustic detections of harbour porpoises in the wind farm area, with recovery starting to occur within two years after the end of construction (Teilmann et al., 2006). Teilmann et al. (2006) suggested that animals were gradually habituating and returning to the wind farm area.(Teilmann et al., 2006)
  8. Nabe-Nielsen et al. (2011) suggested, using simulations of the response of harbour porpoise to wind farm construction, that wind farms already existing off Danish coast do not have impact on harbour porpoise population dynamics and that the that construction of new wind farms is not expected to cause any changes in the long term dynamics of the population. Likewise, Edrén et al. (2010) and McConnell et al. (2012) investigated possible interactions between seals and Danish offshore wind farms (Nysted Wind Farm and Rødsand II) and found that although there was a temporary reduction in the number of seals hauled out during construction operations (i.e. piling), there was no long term effect on haul-out behaviour trends.(Edrén et al., 2010)
  9. Therefore, the examples of monitoring studies given in paragraphs 1093 to 1095 suggest marine mammal receptors can quickly recover and return to the impacted area, despite the potential effects from multiple stressors associated with offshore wind farms. Therefore, as detailed in paragraphs 1088 to 1095, significance is considered to be minor adverse and therefore not significant in EIA terms.