Abstract
The distribution and abundance of waterbirds in the region around the Anholt Off- shore Wind Farm (OWF) Project Area have been analysed using baseline surveys from aircraft and ship and all available historic aerial and ship-based surveys in the region. By modelling the fine-scale distribution of the wide range of waterbird species occurring regularly in the northern and central Kattegat the importance of the Anholt OWF to waterbirds could be demonstrated relative to the rest of this region, which houses the largest concentrations of waterbirds in Danish waters. The models document that during the non-breeding season the waterbird fauna ranges from benthivorous birds (chiefly seaducks), across pursuit-diving piscivores like razorbills targeting schooling pelagic fish, to surface foragers like kittiwakes and generalists like gulls. This conglomerate of avian ecotypes is quite unique at an international level, as it represents a combination of the largest continuous area of shallow off- shore waters below 15 m water depth found in this part of Europe and water masses of Atlantic origin rich in animal plankton and fish dominating the areas deeper than 20 m.
Importantly, in relation to establishing the significance of the Anholt OWF to water- birds the site is actually located outside both of these environments. The area of high carrying capacity for mussel growth stretches north of Anholt and Djursland at distances of approximately 8 and 12 km, respectively, from the wind farm site. This matches exactly the distance to the major and, in international perspective, most sensitive elements of the bird fauna in the Northern Kattegat; the extensive concentrations of seaducks. Despite variability in these mean patterns of mussel growth, the large amount of survey data on which the baseline has been established clearly show that the seaducks do not use the wind farm and associated areas to any great extent. As the wind farm site is located outside the major marine environments as found in the central Kattegat it may be regarded as being embedded in the ecotone marking the transition between the two zones. The ecotone is characterised by relatively strong frontal activity and salinity gradients, - structures which have a pro- found influence on the birds using the site most frequently, notably Red-throated and Black-throated divers.
The wind farm, despite being important to divers, has relatively lower cumulative abundance when evaluated across the entire bird fauna. Evaluation of the cumulative importance of the wind farm to regional bird populations, measured in relation to total bio-geographic populations showed three areas of international significance located at some distance from the wind farm – one located northwest of Anholt, one northeast of Djursland and one south of Læsø. The two former areas have a mini- mum distance to the Anholt OWF of 5 km.
An extensive bird migration study was undertaken by integrated radar and visual surveys from Djursland (Gjerrild Klint) and Anholt (Anholt Harbour). The study was designed to enable descriptions of species compositions, and analyses of profiles in relative migration intensity and altitude along different parts of the potential migration corridor between Djursland and Anholt. The results unambiguously indicate the existence of a migration pathway or corridor of landbirds between Djursland and Anholt in spring 2009. Clearly, the ‘island effect’ of Anholt was reflected, and the island seems to function as a magnet on migrants during spring. Although the Anholt OWF is located on this migration corridor the densities of bird migration at the OWF site can be safely assessed to be below the densities recorded close to Djursland and Anholt. At Gjerrild, between 25% and 40 % of the migration took place at altitudes below 200 m during the night, while during the day between 40% and 60% of the migration was recorded below 200 m altitude, Figure 3-55. Intensities at Gjerrild were lower below 100 m altitude than between 100 and 600 m altitude during all 5- day periods and parts of the day.
Habitat displacement impacts on divers due to Anholt OWF were investigated using a displacement range of 2 km. The proportion of the available high-density areas (> 0.66 birds/km2) within the region from which divers could be displaced was estimated at 24.7 %, equivalent of 260 km2. The displaced population of divers was estimated at 150 birds, thus the number of displaced birds does not have any significant impact at the population level. The physical changes imposed by constructing the Anholt OWF are assessed to have insignificant, if any, impacts on birds in the area. Specifically, no impact is expected in relation to ‘direct habitat loss’ as a result of the physical presence of 88-174 turbines because of the very little area that is actually affected.
Due to the documented presence of a migration corridor to landbirds collision risks were assessed as moderate to this group of birds. Frequent collisions are rare events and have been reported from only a few exposed sites with high migration densities and large numbers of, for example, soaring resident raptors. In such worst-case scenarios mortality rates of raptors as a direct result of collisions with the rotor blades are relatively high in comparison with the size of the affected populations. There is an almost complete lack of experience regarding the behavioural responses of large birds on long-distance migration like raptors and cranes around offshore wind farms, as wind farms have not yet been erected in migration corridors for these species groups. Only monitoring will tell us to what extent the different species of raptors using the Djursland-Anholt corridor will change their flight route on approach to the structures or get attracted to the wind farm due to their aversion to migrate over open sea. As the raptor migration along the Djursland-Anholt corridor includes raptor species with small population sizes listed in the Annex I of EU Birds Directive like Golden Eagle, Osprey and Peregrine Falcon impacts due to collisions (extra mortality) may not be in line with the Directive and may be significant at the level of the affected populations.
Collision risks to waterbirds were assessed as minor. Depending on the lay-out chosen, the cross-sectional diameter of the wind farm will span 10-12% of the width of the strait between Djursland and Anholt. Waterbirds will probably deflect the wind farm at distances of 3-5 km; a minor adjustment and collision risks to migrating waterbirds should be expected to be at a low level with no or minor consequences for the populations passing the strait. Yet, even if the collision frequencies will be smaller than at the Nysted wind farm the number of collisions per season may be higher, as the number of seaducks passing the strait might be several times larger than the number passing Nysted.
The cumulative displacement of divers is expected to exceed the estimated dis- placement on account of the Anholt OWF. Thus, the joint impact of fisheries, ferry services and the Anholt OWF will potentially be displacing divers from a large proportion of the available habitat in the region. The total number displaced, however, is likely to be well below levels which are significant in comparison to the size of the bio-geographic populations involved.
Recommendations regarding mitigation of collision risks to migrating landbirds are limited by the lack of data on behavioural reactions of large species of migrating landbirds (raptors and cranes) on offshore wind farms.