In order to meet the targets set by the European Directive 2009/29/EG on renewable energy, the European Union is aiming at a total offshore capacity of 43 GW by the year 2020. Meanwhile, the offshore wind industry is growing fast and by the end of 2011, 1371 offshore wind turbines were already fully grid-connected in European waters, totalling 3.8 GW (European Wind Energy Association, 2011). The Belgian government has reserved a concession zone comprising almost 7% of the waters under its jurisdiction for wind farming (an area measuring 238 km²). In 2008, C-Power installed six wind turbines (30 MW) at the Thorntonbank, located 27 km offshore, and in 2009, Belwind constructed 55 turbines (165 MW) at the Blighbank, 40 km offshore. In the first coming years at least 175 more turbines will be installed in this part of the North Sea (MUMM, 2011).
Possible effects of offshore wind farming on seabirds range from direct mortality through collision, to more indirect effects like habitat change (including positive effects of increased food availability and resting opportunities), habitat loss and barrier-effects (Exo et al. 2003, Langston & Pullan 2003, Fox et al. 2006, Drewitt & Langston 2006, Stienen et al. 2007). Whereas several studies investigated the effects of offshore turbines on migrating or local seabird communities (Desholm 2005, Petterson 2005, Petersen et al. 2006, Larsen & Guillemette 2007), only a few papers focussed on the monitoring protocol to assess these effects (Maclean et al. 2006 & 2007, Pérez-Lapeña et al. 2010 & 2011).
The Research Institute for Nature and Forest (INBO) is in charge of monitoring the effects of these wind farms on the local seabird distribution. Therefore, it designed a BACI monitoring program and delineated impact and control areas for both wind farm projects. INBO performs monthly seabird surveys in these areas, and developed an impact assessment methodology accounting for the statistical problems inherent to ‘seabirds at sea’ (SAS) data.