Are offshore wind farms in the Netherlands a potential threat for coastal populations of noctule?

Summary

Offshore wind farms likely cause mortality amongst migratory bats. Yet it remains unknown whether resident coastal bat populations may be affected by offshore wind developments. We performed an analysis to assess the potential risk of offshore wind farms in the Dutch North Sea for local coastal populations of noctule (Nyctalus noctula). First, we assessed the potential overlap between their foraging range and areas with operational and planned offshore wind farms. Subsequently, we tracked 14 noctules from a coastal population during late summer and autumn and analysed their movements. In general, it seems unlikely that offshore wind farms in the Netherlands will significantly affect coastal populations of noctule since offshore wind developments take place beyond their regular foraging range. In some cases however, noctules do perform distant flights (‘swarm flights’), possibly in response to migrating insects. We recorded six distant foraging trips both over land and over sea with a maximum distance of 18.5 km from their roost and 12.7 km from shore. Acoustic records confirm that noctules are occasionally present in offshore wind farms at distances of 15-25 km from shore. During such an event, noctules face the risk of a collision as virtually all their flight activity occurs at heights within the rotor swept area of offshore wind turbines.