| Summary: | ‘Seabirds at sea’ count data exhibit extreme spatial and temporal variation, impeding the assessment of the impact of wind turbines on seabird abundance and distribution. We designed a BACI monitoring program to assess the effect of wind farm presence on seabird displacement and used the results of ship-based surveys to simulate a broad range of empirical scenarios. Based upon these, we investigated how the power of detecting a change in seabird numbers is affected by survey length, monitoring intensity and data characteristics. The methodology used for the assessment was revised as compared to the previous reports. The most crucial difference is the application of zero-inflated estimation. Data on 13 seabird species regularly occurring in the assessment of displacement effects caused by wind turbines. The impact modelling at the Thorntonbank study area so far only reveals attraction effects, i.e. for Little Gull, Great Black-backed Gull, Black-legged Kittiwake, Sandwich and Common Tern. These findings are highly provisory since at the time of the study, one line of wind mills was present. Nevertheless, this poses some serious conservation concerns, given the high protection status and the fragility of the populations of both tern species and of Little Gull, combined with the raised threat of collision-mortality. After the turbines were built at the Bligh Bank, numbers of Common Guillemot and Northern Gannet significantly decreased in the wind farm area. In contrast, numbers of Common Gull significantly increased, and the BACI-graphs suggest attraction of Herring Gull as well. Gulls are probably attracted by the wind farm from a sheer physical point of view, with the farm functioning as a stepping stone, a resting place or a reference feature in the wide open sea. During recent surveys in 2012, good numbers of auks and even Harbour porpoises were encountered inside the wind farm. From an ecological point of view, the presence of auks is very interesting, and we wonder if these self-fishing species are already habituating to the presence of the turbines, and if they will profit from a (hypothetical) increase in food availability.
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