Abstract
The development of offshore wind farms (OWF’s) plays an important role in the Dutch energy transition to comply with the European aim to be climate neutral by 2050. There are however also concerns about the impact of (offshore) wind farms on biodiversity and protected species like bats, who are found at sea during migration, and in some cases during foraging trips from the mainland. In the Netherlands, the impact of offshore wind farm development on legally protected species and the potential effects on the ecosystem is investigated in a central and long-term research programme; Wozep (Wind op zee ecologisch programma). At the North Sea several species of bats have been recorded of which Nathusius’ pipistrelle (Pipistrellus nathusii) is the most commonly observed species. Nathusius’ pipistrelle is known to migrate from the breeding areas in eastern Europe towards their wintering areas in southern and western Europe. Some of the bats go as far as the UK and thus have to cross the North Sea during their migration. For these reasons this species is the priority species in studying the impact of the Dutch offshore wind development on bats within Wozep. The Wozep bat research programme includes various (sub)projects, all of which are aimed to fill important knowledge gaps and answer the overall research question: what is the relevance of the (presumed) mortality due to offshore wind farms for bats, in particular for Nathusius’ pipistrelle population(s) crossing the southern North Sea. To investigate bat migratory movements at a larger scale offshore passive acoustic monitoring and telemetry research using the Motus Wildlife system are used. Motus is a system that uses automated radio telemetry, specifically designed to track the movements of smaller animals, such as bats and birds. In order to do so, the system uses a network of stationary automated receiving stations. The current report shows the results of a telemetry study aiming to assess the percentage of bats departing over sea off the North-Noord Holland coast in autumn. The choice of this study area was related to the existing wind farms at the time the project started in 2017. From the start of the project a network of receivers on land was developed step by step. Starting in Noord Holland, the network increased every year and currently it extends along the coast from Westkapelle to Ameland in the Netherlands and from Landguard to Caister-at-Sea in the UK. Having a dense enough network is a pre-requisite for successfully being able to follow flight paths of the bats that were tagged in this study in the years 20218 – 2022. Due to the high receiver coverage in the northwestern parts of Noord Holland only this area was considered suitable to investigate the number of departures over sea. Bats were initially mainly trapped from bat boxes and at a later stage also mist nets and harp traps were used to increase the number of captures. A total of 409 Nathusius’ pipistrelles was tagged within the study area over the years, including 205 females (50.1%) and 204 males (49.9%). For all of the tracked individuals flight path estimation was conducted by using floating averages to smooth the tracked lines. The flights of 262 individuals detected during their last night in the study area were considered suitable for speed and heading calculations. Of those, 57% of the movements were classified as directional (migratory) flights and 43% as local flights. Most of the documented migratory movements concerned females (both adult and juvenile), as well as juvenile males. Adult males, which defend mating territories during the autumn migration season, appeared to be mostly sedentary, as expected. A relatively small percentage (6 - 10%) of bats departs over sea directly from the study area, whereas most bats (69%) are heading in a southerly direction. Therefore, from the perspective of the study area, the coastal route - is preferred over the route over sea. This preference may well be explained by the risk bats face to cross an ecological barrier like the North Sea. An additional 7% heads north and 14% east. Bats initially following the coast may decide at a later stage to depart over sea. We observed some individuals probably departing onto sea further south along the coast. Therefore, the overall percentage of 6- 10% of bats -tagged in the study area- departing over sea must be regarded as a conservative estimate. Not all of the individuals departing over sea involve bats heading towards the UK as overseas movements parallel to the coast and subsequent continuation along the coast were also observed. Increased bat presence during the autumn migration in the Dutch North Sea therefore can be expected in areas between Noord Holland and Norfolk, as well as in areas relatively close to shore (up to c. 25 km). On land, increased migratory bat presence can be expected along the coast.