Abstract
As part of the energy transition there is an increasing demand for a further expansion of offshore wind energy. However, there are concerns that a rapid expansion of offshore wind farms can have adverse effects on the marine eco-systems and individual species within it. This study aims to assess the reaction of Common Guillemots and Razorbills to the presence of operational offshore wind farms. Seabird behavioural responses to wind farms range from attraction to avoidance. One of the species groups that is potentially affected are auks and among them the Common Guillemot, one of the most abundant offshore seabird species in the German North Sea and the sympatric species, Razorbill. The German North Sea is an important foraging and resting habitat for auks breeding on Helgoland and in the non-breeding season additionally to large numbers from other breeding colonies, mostly in Great Britain.
This study investigated the effects of wind farms on the distribution of Common Guillemots and Razorbills in the German North Sea, with particular focus on differences between autumn and winter, regions and the two species. It is based on a large data set of high-quality data from aerial and ship-based surveys collected over 8 years during post-construction monitoring of all 22 wind farms in operation in the German North Sea by 2021 and during scientific monitoring projects. Data were analysed applying the approach of integrated nested Laplace approximations (INLA) to do approximate Bayesian inference for latent Gaussian models, which yields robust estimates of seabird distributions in space and time. In addition, it does not require pre-construction data and allows model validation.
Common Guillemots and Razorbills showed a very similar spatial distribution in autumn, with areas of higher densities in the northwestern part of the study area and in the southeast, in the vicinity of the breeding colony on the island Helgoland. In winter, overall numbers of auks were substantially higher and the distribution more widespread than in autumn.
Both species showed avoidance effects towards wind farms, although both species regularly occurred within the wind farms as well. We found high variability in the distribution and effect radii between seasons and regions for both species. The calculated effect radius around wind farms was consistently larger in autumn (Common Guillemots: between 6-12 km, Razorbills: 6-11 km) than in winter (Common Guillemots: between 0.4-2 km, Razorbills: no significant avoidance), for the total area analysed. The theoretical habitat loss, a theoretical value assuming a total loss of habitat for all individuals due to avoidance, in autumn was calculated as a radius of 0-4.5 km around the OWF for Guillemots and 0-3 km for Razorbills. In winter, the effect radius was too low to result in a definable habitat loss outside the OWF.
The analysis of the regions separately revealed a similar seasonal pattern for both species. Model results revealed that in both species a large part of the individuals avoided the area inside the wind farms and up to 1 km distance in autumn, with Common Guillemots (reduction of 65-76%) showing a much higher reduction than Razorbills (reduction of 41-56%). Within an area of up to 5 km around the wind farm, densities were still noticeably reduced in Common Guillemots (reduction of 49-66%) and Razorbills (reduction of 31-50 %). In winter, the reductions were much lower and seemed to be confined to the wind farms and up to 1 km distance in both species.
When comparing the different regions, our results hint at a possible increase in effect size with distance from shore and a potential connection with density of turbines within the offshore wind farm, however, more studies are necessary to determine a true connection.
This study provides evidence for a seasonal difference in avoidance of wind farms by Common Guillemots and Razorbills and suggests possible differences between the two species and between different regions.