Potential impacts and sensitivity of marine mammals

In the following potential impacts of the planned Thor OWF and cable corridors for marine mammals are assessed. The Danish Energy Agency has prepared a scoping report, which determines which environmental issues are to be included in a later SEA and at what level they are to be assessed (Energistyrelsen, 2020). In the delimitation report, the environmental issues that are likely to be affected by the implementation of the offshore wind farms are identified.

This initial assessment deals with the potential sources of impacts at an overall level, as the expected impacts will depend on the specific project including choice of turbine types and number, the locations for the individual turbines, foundation methods, etc.

The potential effects on marine mammals caused by the establishment of Thor OWF and the relevant project phases are:

- Disturbance, reversible and/or permanent threshold shifts in marine mammals during piling (and to a lesser extend during decommissioning)

Minor impacts are:

- Disturbance caused by vessel traffic

- Disturbance cause by suspended sediments during cable laying - Permanent habitat loss by footprint of foundations

- Temporary habitat loss (scour protection, cable protection and construction activities) - Habitat change

The degree of impact ranks impacts in three levels (“high”, “medium” and “low”) whereas impacts leading to a potential injury of animals is always ranked in the category “high” while effects

leading to disturbance or behavioural changes can be ranked between “high” and “low” depending on the number of affected animals, the sensitivity of animals and a potential effect on population level.

As pointed out above, effects with major impact on marine mammals are expected during the construction phase especially during piling of offshore foundations. Though additional noise by emissions from the turbines during the operational phase can be measured, these emissions are at very low frequencies and of low energy and thus have very short ranges. Because the emitted frequencies are mostly in the lower Hz range, they are more relevant to larger, low-frequency marine mammals (Marmo et al. 2013). Studies could show, that harbour porpoises return to windfarm areas after the construction period and occur to the same extend within the windfarms as outside, with effects of piling lasting from a few hours up to several days (Nabe-Nielsen et al.

n.d., Brandt et al. 2018, Rose et al. 2019). In addition, earlier studies show that offshore wind farms might provide a valuable habitat due the exclusion of fishing activities and species richness due to the artificial structures as stony reefs and vertical piles (Andersson & Öhman 2010).

Compared to the potentially positive effects of the new structures and the overall size of the marine mammal’s habitat, the habitat loss due to the footprints of the scour-protection or the monopile itself is neglectable. Disturbance by vessel traffic can have a negative effect on marine mammals, especially on harbour porpoises. It has been shown, that harbour porpoises react to an approaching vessel with either a reduction of vocalisation and a reduction of activity or fleeing (Wisniewska et al. 2018, Roberts et al. 2019). Thus, vessel traffic should be carefully assessed and managed, and measure taken e.g., to reduce vessel speed or channel the traffic on regular lanes. Sediment spill and thus reduced visibility in water is not regarded as a major effect because all marine mammal species have the option to avoid areas with high levels of dissolved sediments or orientate using their acoustic senses. Harbour porpoises and seals are adapted to life in coastal waters and are able to locate prey at low visibility (Dehnhardt et al. 2001, 2003, Verfuß et al.

2009). Harbour porpoises have shown that they not only use their echolocation to identify prey objects and home in on these, but also use their echolocation for spatial orientation and for identifying objects as landmarks (Verfuß et al. 2005).

The focus in this assessment thus lies on the only major negative effect of the construction period:

To assess the range of potential injuries or disturbance and the number of affected individuals the range of the threshold criteria given in the Underwater Noise Report are translated into affected areas and affected number animals is calculated. Disturbance is included because this is a stressor leading to behavioural changes as leaving an optimal habitat or reducing the hunt for food and the nutritional status of animals (effects are modelled e.g. Nabe Nielsen et al (Nabe-Nielsen et al. 2014, 2018, n.d.). This eventually leads to animals in a reduced physical state, reduced reproduction rates and can thus have effects on the population level. A graphical example taken from a virtual position from the Underwater Noise report (ITAP 2020) of the affected area for seals and harbour porpoise is shown in Figure 6-2 and Figure 6-3. Values for the range of threshold noise criteria are taken as well from the Underwater Noise Report for the Thor OWF gross area as shown in Figure 6-1 (ITAP 2020). For eliciting a PTS or TTS a cumulative sound exposure level SELcum) is used, assuming a noise dose which a marine mammal receives during a complete piling event. For disturbance, a sound exposure level (SEL) of a single hammer blow is used to calculate the impact range. This leads to very similar impact ranges for TTS and

disturbance.

39/49 Figure 6-1 Impact ranges (km) for harbour porpoises (HP) and seals for receiving PTS, TTS or eliciting behavioural responses ((ITAP 2020)

Figure 6-2 Effect ranges and area covered by the noise radii when piling a monopile (13 m diameter) a location WTG 03 in the northern part of the gross area for Thor OWF for PTS, TTS and Disturbance for harbour porpoises

Figure 6-3 Effect ranges and area covered by the noise radii for PTS, TTS and Disturbance for seals when piling a monopile (13 m diameter) at location WTG 03 in the northern part of the gross area for Thor OWF

The effect ranges in which harbour porpoises and seals potentially experience PTS are covering an area of 806 km² for harbour porpoises and 27.4 km² for seals.

Table 6-1 Effect ranges, areas and threshold levels based on the underwater noise modelling.

Species Impact Distance Area

Harbour Porpoise PTS 16.2 km 805.96 km²

Harbour Porpoise TTS 49.95 km 6164.46 km²

Harbour Porpoise Disturbance 48.18 km 5798.57 km²

Seals PTS 2.95 km 27.38 km²

Seals TTS 47.34 km 5628.18 km²

Seals Disturbance 43.3 km 4839.9 km²

The densities of harbour porpoises calculated for the investigation area are lying between 0.47 individuals per km² in Spring 2020 and 0.41 individuals per km ² in Summer 2020. This is slightly higher compared to the densities found in earlier years (2018: 0,38 individuals per km² in the Danish area Southern North Sea in late August 2018 (Hansen & Høgslund 2019)). This is also higher than densities calculated for the block M (the area covering the Danish and German EEZ and thus including the Thor gross area) during the scans III flight surveys (0,277 Ind/km²

(Hammond et al. 2017)). However, the finding of this study for the Thor OWF gross area lie within the range of former counts between 2011 and 2019 (compare chapter 4.1).

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Seal densities reach maximum densities of 0,05 Ind/km² pooled for both species present in the area in spring 2020 and 0.005 Ind/km² pooled for both species in summer 2020. Based on the very low and random counts of grey seals, the estimated population size of 42.000 harbour seals is used as size of the biogeographical population.

Table 6-2 gives an overview of the affected number of individuals and percentage of the total biogeographical population as an indicator for the effect level for PTS, TTS and Disturbance. As a conservative approach the higher density from Summer 2020 is used for the calculation for harbour porpoises and from Spring 2020 for seals. The densities calculated for June 2020 for harbour porpoises are high compared to earlier findings, thus the number of affected individuals is most likely lower than given in Table 6-2. Also, the densities used for the estimates of affected seals in spring is high, so the number of affected animals is most likely lower as given in this conservative approach.

Table 6-2 Effect areas, affected number of individuals and affected percentage of the population per species and impact.

Species Impact Area (km²⁾ Ind/km² Affected individuals

Based on the calculation in Table 6-2 a piling event under the prerequisites given in the

underwater noise modelling (e.g., a monopile with 13m diameter and a maximum blow energy of 3000kJ and no noise mitigation) would not cause PTS on more than one percent of the population of harbour porpoises and seals. However, the number of harbour porpoises eventually

experiencing PTS is calculated to be up to 379 individuals and virtually all porpoises in the investigation area can be affected by TTS or at least disturbance. Under these assumptions up to 1.6% of the population receive a TTS or are disturbed by piling noise. Due to the very low number of seals in the area, seals are at very low risk of experiencing PTS, but are affected by potential TTS or disturbance. The options to reduce the proportion of affected individuals by applying mitigation measures are described in chapter 6.7.