Impact Assessment of an Off-Shore Wind Park on Sea Ducks

• Main Authors: Guillemette, M., Larsen, J. K., Clausager, I.
• Format: Book
• Language: English
• Published: National Environmental Research Institute 1998
• Subjects: vindmøller; fugle; vindmølleparker; havfugle; blåmuslinger; ederfugle; TK; RS; forstyrrelser; kollisioner; Common Eider; Melanitta nigra; Somateria mollissima
• Online Access: https://pure.au.dk/portal/da/publications/impact-assessment-of-an-offshore-wind-park-on-sea-ducks(b16e80b0-7e9c-11dd-a5a8-000ea68e967b).html; http://www.dmu.dk/1_viden/2_Publikationer/3_fagrapporter/rapporter/FR227.pdf

As part of the plan of the Danish government to expand off-shore wind energy production, The Ministry of Environment and Energy, in collaboration with ELSAM (an energy consortium), initiated a three-year study of the potential conflict between the Tunø Knob off-shore wind park and aquatic birds in 1994-97. Danish coastal waters support very large, internationally important concentrations of moulting, migrating and wintering sea ducks which depend on shallow water areas as major feeding habitats. Denmark is committed, in relation to international conventions and EU directives, to protect and maintain these populations of aquatic birds and such shallow coastal areas are precisely the type of areas in which future wind parks are planned. Two general approaches were adopted for the investigation: the before-after-control-impact design (BACI) and After studies conducted around the wind park. The aim of the BACI studies was to compare bird abundance and distribution before and after the construction of the wind park and between the area presumably affected by the development and a control area. This was carried out on three spatial scales: i) conducting aerial surveys in two large zones (about 5,000 ha), Tunø Knob and Ringebjerg Sand, while controlling the total number of birds in Århus Bay (88,000 ha), ii) conducting ground surveys of two areas of about 700-800 ha coverage at Tunø Knob and Ringebjerg Sand, using the latter as a control area and (iii) within Tunø using three subareas (160-250 ha) as controls compared to the construction area. The after experiments were conducted around the wind park with the aim of controlling the confounding effect of food supply and to establish (a) the short-term possible effects of noise and rotor movements generated by the turbines on the distribution and abundance of sea ducks and (b) the long-term scaring effect of the wind park (the impact of revolving rotors and the presence of the standing towers). Finally, an experiment was conducted in order to quantify the scaring effect, if any, of the wind park on flying sea ducks. The wind park is constructed on Tunø Knob, a shoal in Århus Bay, and consists of ten 500 kW turbines erected in 3-5 m of water during the late summer of 1995. All studies were carried out during the winter period, and results are reported for common eider Somateria mollissima and common scoter Melanitta nigra which composed 97.2 % of the bird numbers. The BACI investigations started in the winter before construction of the wind park (base line year = 1994-95), and extended to two winters after its completion (year 2 = 1995-96 and year 3 =1996-97). The main study period was February - April. In addition to counts and localisations of the birds on the study areas, aerial surveys were conducted to monitor the trends in regional bird numbers. Furthermore, the benthic community was sampled each year to assess variation in the potential food supply. The After investigations were conducted during the third winter from November to April, and focused on the area within 600 m of the wind park. Eider numbers declined by 75% and common scoters by more than 90% at Tunø, from the winter prior to construction (base line) compared to the third year of the study. In contrast, eider numbers at Ringebjerg Sand and for the whole Århus Bay showed no decrease during the same period. Amongst common scoter, the pattern depicted for Tunø Knob only slightly deviated from that at Ringebjerg Sand. During the same period, the biomass of blue mussel Mytilus edulis, a favourite prey species, decreased from the base line year to the third year of the study with a marked difference between Tunø Knob and Ringebjerg Sand in the size available. This was associated with a general impoverishment of the whole benthic community. This suggests that the decrease observed in sea duck abundance over the three years was caused by the availability of food supply and not by the wind park. Within Tunø Knob, eider numbers decreased markedly within the subarea which included the wind park from Before to After periods. However, a similar decline was observed in the three other subareas. Furthermore, detailed mapping showed a high degree of annual and seasonal variation in spatial distribution of eiders over the area which was not consistent with an effect caused by the wind park. These results suggest that the observed changes in abundance of common eiders in the wind park area were due to natural variation. In one experiment conducted around the wind park, we used three groups of 30 decoys each at different distances from the wind park to test the impact of the wind park on flying eiders. It was found that rates of landing and flying were significantly lower 100 m from the turbines than at distances of 300 and 500 m. In a second experiment, switching the turbines on and off, no detectable effect of revolving rotors and noise on the abundance and spatial distribution of common eiders was found. This was supported by another set of observations showing that 10 flocks of eiders within 600 m of the wind park did not take off and did not swim away when the turbines were switched on again. In a third experiment, food abundance and eider exploitation over the entire winter were determined for four experimental quadrates (4 ha each) positioned at different distances from the wind park (from 0 to 600 m). The data showed that more than 92% of the variation in common eider numbers in these quadrates could be accounted by food supply. This result alone suggests that the turbines had no effect on the exploitation of food resources around the wind park over a long time scale ( a whole winter). In summary, the decline in the number of common eider at Tunø Knob could not be attributed to the presence of the wind park as no effect of the wind turbines could be detected on the abundance and the distribution of that species. Indeed, the results of benthic surveys suggest that, at intermediate spatial scale, the observed changes in the abundance of eiders were driven by natural variations in the food supply. Since the probability of a negative impact decreases with increasing distance from the wind park, the above interpretation is further supported by the experiments conducted in the immediate vicinity of the wind park. Altogether these results should, however, be used with caution because flocks of eiders were small during that study and it is known that sensitivity to disturbance increases with flock size. Finally, the present study did not address all aspects of the potential conflict between bird life and off-shore wind parks. Further information is needed about (1) other periods of the annual cycle (especially moulting), (2) the effect on other species, (3) the risk of collisions under varying weather conditions, (4) the effect of disturbance from maintenance ships and (5) the effect of large scale wind parks.