For quite some time there have been indications of bat movements over the North Sea. Observers of bird migration at the Dutch coast regularly report bats flying in from sea (Lagerveld et al. 2014b). Bats have also been observed during surveys at the North Sea and have been found on oil & gas platforms, ships and remote islands (Walter 2007, Boshamer & Bekker, 2008). In 2013 a Nathusius’ pipistrelle Pipistrellus nathusii was found in the Netherlands, which was banded three years earlier in the United Kingdom (UK; pers. comm. Teddy Dolstra), providing the first evidence that bats are able to cross the North Sea.
To what extent and how bats use the North Sea is a relevant question, considering that the number of offshore wind farms in the North Sea is increasing and that several (onshore) studies have shown that wind turbines can cause high fatality rates among bats.
We therefore conducted studies in 2012 and 2013 to monitor offshore bat activity with passive acoustic ultrasonic recorders (Jonge Poerink et al. 2013, Lagerveld et al. 2014a). During these studies one recorder was installed at the meteorological mast at the Offshore Wind Farm Egmond aan Zee (OWEZ) and a second recorder was attached to the entrance platform of wind turbine number 22 at Princess Amalia Wind Farm (PAWP), respectively 15 and 23 km from the shore.
In 2012 monitoring has been conducted exclusively in autumn; from 29 August until 20 October at OWEZ and from 4 until 23 September at PAWP. The monitoring period in 2013 ran from 4 April until 15 October at OWEZ and at PAWP from 6 until 16 June and from 5 August until 2 October. Bats were recorded regularly at both locations in both years during the autumn migration period. In 2013 there were occasional recordings in spring and no bats were recorded during June and July. The observed pattern of occurrence (observations in the migration season and apparently absent in June and July) in combination with the recorded species indicate that our observations referred to migrants.
In the follow-up project reported here, more data on the offshore occurrence of bats was collected in 2014. Using the same methodology as in 2012 and 2013, bat activity was monitored from spring to autumn in both wind farms (OWEZ & PAWP) and two additional locations: the IJmuiden meteorological mast (85 km west of Callantsoog) and at the coast near Egmond aan Zee.