Iteration cycle: Dealing with peaks in counts of birds following active fishing vessels when assessing cumulative effects of offshore wind farms and other human activities in the Southern North Sea

This report is an additional note to IMARES report number C166/14.

This report is an addition to the report of Leopold et al. (2014) that evaluates the effects of offshore wind farm development in the southern North Sea, on birds and birds. In that report, unacceptably high mortalities were predicted for several large gull species. However, it was felt that this might be an effect of local, short-lived concentrations of these birds, e.g. around fishing vessels. If this were the case, such concentrations of gulls in the vicinity of future offshore wind farms would unduly increase the estimated number of victims at these locations. Here, we explore a method to decrease the impact of high peaks in gull densities recorded in the past, by redistributing the gulls attracted to a temporal feeding hotspot over the supposed area from which they were attacted. This results in a marked (ca 37-41%) reduction of estimated numbers of victims among the bird species supposedly most impacted, the Lesser Blackbacked Gull. The reduction of predicted numbers of victims among Great Black-backed Gulls was lower (14-21%), while predicted numbers of victims among Herring Gulls increased by 32-42%). The latter is probably an artifact of the coastal habits of Herring Gulls and a redistribution extending too far into offshore waters.