The Research Institute for Nature and Forest (INBO) is in charge of investigating changes in seabird distribution following the offshore wind farm development at the Belgian part of the North Sea. Since 2005, three years before the construction of the very first offshore wind turbine, INBO therefore performs monthly seabird surveys through the impact and control areas of the two operational wind farms, thus following a BACI approach. To statistically discern seabird displacement effects, these count data were analysed through zero-inflated negative binomial modelling. We further investigated how the statistical power of our impact assessment analysis is related to seabird distribution characteristics as well as to survey length and monitoring intensity.
These power analyses showed that five up to ten years of impact monitoring are needed to obtain sufficient power (90%) to detect a decrease in numbers of 50-75%. At both wind farms, numbers of several species indeed appeared to have changed, but without these changes being statistically significant. With more years of monitoring ahead, our data will allow to better distinguish between true displacement and indifference.
Nevertheless, we already found significant displacement effects. At the Thorntonbank, little gull, sandwich tern and common tern were found to be attracted to the immediate surroundings of the six phase I turbines. These results are highly provisory regarding the one-dimensional configuration of the wind farm at the time of the survey. However, if these attraction effects should persist during the now fully operational and two-dimensional phase of 54 turbines, this is of serious conservational concern given the involved species’ high protection status and the associated risk of increased mortality. Three years after the completion of the wind farm at the Blighbank (55 turbines), it showed that northern gannet, common guillemot and razorbill avoided the wind farm. Otherwise, numbers of lesser black-backed and herring gull increased significantly. The question whether these birds are attracted to the wind farm from a sheer physical point of view – with the wind farm functioning as a stepping stone or a resting place – or whether birds already learned to exploit the hypothesised increase in food availability, is yet to be answered. What we do know for sure is that attraction of seabirds inevitably results in an increased number of collision fatalities. As such, ‘collision risk modelling’ learned that gulls in particular are at risk of colliding with the turbine blades at the Blighbank, with up to 2.4 bird strikes per turbine per year.