All North Sea countries have ambitious plans for offshore wind development. Offshore wind turbines are an alien element at sea, a “landscape” that is normally wide and open. Large, turning turbines might affect the local seabirds, that are dependent on the sea. One of the possible effects of offshore wind farms might be that the seabirds will be displaced from the sites, which would mean habitat destruction or at least habitat degradation for this group (Petersen et al. 2006; Arends et al. 2008). All seabirds, being migratory, are protected under the EU Birds Directive. Yet, there are no studies into the question where wind farms should best be built (with respect to seabirds) or how they should be designed to minimize disturbance.
This report describes the results of four years of T-1 Local Bird surveys around the Dutch offshore wind farms PAWP and OWEZ in comparison to the results of a one-year T-0 study. The two wind farms are situated off the Dutch mainland coast, in close proximity to each other and are owned and managed by two different parties, Eneco and NUON/Shell (now Vattenfall). In a unique process of mutual agreements, commissioning and data sharing between these two parties, a single, large-scale, multi-year line of research has resulted in a joint approach to address the difficult question: how do local seabirds react to offshore wind farms? The long-term cooperation between the two commissioning parties is probably unique in this line of research, where all too often different wind farms are studied by different parties and different methods.
The project has been a learning process for all involved, certainly also for the biologists and analysts carrying out the field word and the reporting. Both the survey design and the statistical analyses techniques were adapted (improved) in the course of the study. The initial study design comprised ten parallel and equidistant transects, encompassing a rather large area around the wind farms. Seabirds were counted from survey ships along these ten lines, during series of repeat surveys. After one year of pre-construction surveys (T-0) and two years of T-1 surveys it was realised that more count data were required from within the wind farms themselves and the survey set-up was adjusted accordingly, introducing new survey lines running through the two wind farms and parallel to the isobaths in the general study area. A first major report was produced after three years of T-1 surveys (Leopold et al. 2011) when the work, required for the first wind farm (OWEZ) was completed. In this report, we used only presence/absence data to look for evidence that birds avoided the wind farms, or were attracted to them. Another year of field data, required for the second wind farm (PAWP) was collected subsequently, using the exact methodology of the previous year. In addition, and the core of this report, we developed and deployed new statistical analysis methods allowing analysis of on-site deviations of seabird densities. To this end, we use Generalized Additive Mixed Models (GAMMs) or Zero-Inflated GAMMs.
This study compares the effects of two wind farms of different design in close proximity of each other. PAWP has a much higher turbine density (4.3 turbines / km2) than OWEZ (1.3/km2). This difference in turbine density probably constitutes the main difference in design between PAWP and OWEZ. The turbines deployed in PAWP (n=60) are Vestas V80 - 2 MW, at 59 m above mean sea level (amsl), with a rotor diameter of 80 m. Those in OWEZ (n=36) are Vestas V90 - 3MW turbines at 70 m amsl, with a rotor diameter of 90 m.
However, besides the difference in lay-out, PAWP was built in slightly deeper waters (19-24 m versus 18-20 m) and further offshore (ca 23 km versus ca 15 km) than OWEZ. The latter might seem trivial, but the exact location of the two wind farms turned out the have important consequences on the impact on local seabirds, simply because PAWP is situated just outside the realm of a suit of coastal seabirds, while OWEZ is just touching this zone of coastal avifauna. Birds that keep an offshore wind farm at bay, because their normal habitat does not include that wind farm, will not be disturbed, no matter how sensitive to disturbance they might be. In this particular case, birds that prefer nearshore waters could not be disturbed by PAWP that is situated too far offshore. Indeed, negative effects on coastal species like divers, grebes, seaduck, and several terns, were only found for OWEZ. PAWP did not impact these coastal birds and is thus ecologically better placed than OWEZ in this respect.
Most gull species often follow fishing vessels to obtain food and by doing so, occur very clumped at sea (i.e. around fishing vessels). Clearly, as fishing is banned from the wind farms, large flocks around fishing vessels occur only outside the wind farms, after their construction. This phenomenon, however, is not turbine-related, but follows from regulations. Gulls were also regularly seen inside either wind farm, often resting on its hardware.
One species, the Great Cormorant, was clearly attracted to the wind farms. These birds now use the sites in rather large numbers, as a basis for at-sea feeding where they can dry their plumage after diving for food, by resting on the turbine-poles and other structures available.
Significant avoidance was found in species occurring over most of the study area and in relatively even numbers over this area: Northern Gannet, Black-legged Kittiwake, Common Guillemot and Razorbill. All showed avoidance with respect to PAWP, but only the Common Guillemot and the Northern Gannet were also found to avoid OWEZ. For all species, avoidance was not 100%, as some individuals were observed with the wind farms. The stronger reaction towards PAWP may be related by the less ‘open’ setup of this wind farm.
We did not find statistically significant effects for Northern Fulmar, although we found some indications that these birds might be avoiding wind farms. Their numbers around both PAWP and OWEZ were probably too low for such effects to be detected, but this might be different in more offshore situations.
The results obtained in this study seem to be robust, in that these were largely in line with the earlier evaluation based only on presence/absence data (Leopold et al. 2011a): attraction in Great Cormorants, avoidance in most other seabirds, while results for most gull species remain ambiguous.
Future wind farms are unlikely to be built in nearshore waters in the Netherlands and therefore, studying effects on coastal seabirds, although apparently vulnerable to disturbance, should receive lower priority in future projects. Species with more offshore distributions will be more at risk. The most important conclusion of this study probably is, that the Common Guillemot would seem to be the most promising seabird for future studies of disturbance by offshore wind farms, considering that this species occurs all over the North Sea, occurs often in substantial numbers that would allow for a useful analysis and show some, but not 100% avoidance of wind farms. These properties make the Common Guillemot a suitable species for comparisons of the effects of different wind farms. In the present, first inter-wind farm comparison, admittedly based on only two wind farms, a lay-out with larger, but more spaced-out turbines, disturbed the birds to a lesser extent than a wind farm with smaller but more densely packed turbines. Bigger is better, it would seem, all other things being equal.
In conclusion, most local seabird species showed significant reactions in terms of avoidance or attraction to PAWP and OWEZ. For a large part, this could probably be attributed to the location of these two wind farms: just far enough offshore to be outside the realm of coastal seabirds. That wind farms are capable of displacing local seabirds was shown for a number of species. None of these, however, were totally absent from the wind farms. Looking at the larger North Sea, the Common Guillemot should probably be selected for comparative future studies that should look into the effects of wind farm lay-out on disturbance.
This report was presented at a symposium at Naturalis in Leiden on September 8, 2015. A complete report containing brief abstracts of all studies presented at the symposium can be found here.