Since 2005, the Research Institute for Nature and Forest (INBO) performs monthly BACI-designed surveys to study seabird displacement following the construction of offshore wind farms (OWFs) in the Belgian part of the North Sea. Here we report our findings for the C-Power wind farm at the Thorntonbank after four years of post-construction monitoring. Following the concern on potentially high levels of collision mortality among large gull species, we also report the first results of our behavioral study, making use of our transect count data, GPS tracking data and observations with a fixed camera installed on turbine I5 in Thorntonbank OWF. As expected, considering the rather small amount of data added during the monitoring year 2016, our displacement study results are highly similar to those reported in the previous monitoring report (Vanermen et al. 2016). The impact area appeared to be avoided by four species, being northern gannet, little gull, black-legged kittiwake and common guillemot, these having dropped in numbers by no less than 97%, 89%, 75% and 69% respectively. The Thorntonbank OWF attracted great black-backed gulls, numbers of which increased by a factor 6.6 compared to the control area and the period before impact. Sandwich tern too was attracted to the OWF at the Thorntonbank, the effect being significant for the buffer zone only, where we observed a factor 5.7 increase in numbers. Only for herring gull there was a shift in the estimated wind farm effect since the latest report. While the OWF coefficient for herring gull was estimated to be close to zero after three years of monitoring, it now showed a (borderline) significant increase in numbers (factor 2.9). The buffer zone, however, saw a significant decrease in numbers of herring gull.Though it is still too soon to draw any definite conclusions out of our behavioral monitoring, there were already some indicative results. Great black-backed gulls for example clearly favor outer turbines for roosting, suggesting a partial barrier effect. Based on our tracking data, lesser black-backed gulls seemed to spend half of their time inside the OWF area roosting on the jacket foundations, and spent less time flying inside compared to outside the wind farm. While mostly observed roosting, with the fixed camera we assessed that 9% of the large gulls observed on the jacket foundations were actually foraging. Sustaining the current effort throughout 2017 will allow us to analyse tidal and diurnal patterns in the presence and behavior of large gulls inside the Thorntonbank OWF. Importantly, the results of this behavioral study might shed new light on the currently expected collision risk of large gulls at OWFs, and may highlight the need for proper post-construction monitoring. Because next to a possible post-construction change in numbers, any behavioral shift (i.e. a decrease in time flying) will have a strong effect on the anticipated collision mortality among large gulls. |
