Lutra 51(1)_Boshamer & Bekker_2008

Nathusius’ pipistrelles (Pipistrellus nathusii) and other species of bats on offshore platforms in the Dutch sector of the North Sea

Between 1988 and 2007, 34 reports of bats were received from offshore platforms in the Dutch sector of the North Sea. These reports involved Nathusius’ pipistrelle (Pipistrellus nathusii) (26x), noctule (Nyctalus noctula) (2x), northern bat (Eptesicus nilssonii) (2x), serotine (Eptesicus serotinus) (1x) and parti-coloured bat (Vespertilio murinus) (3x). Their distribution over the 65 offshore platforms in the Dutch sector of the North Sea is described. A population of Nathusius’ pipistrelle on the mainland, monitored in bat boxes located in the north of North Holland Province was used to compare sex ratio, age composition, body condition and biometrics with the bats found on offshore platforms. Since the first report of a bat on a platform in the Dutch sector of the North Sea in 1988, there has been an increase in the number of bats reported from offshore platforms over five yearly periods, with the maximum number (15) occurring between 1998-2002. The records of Nathusius’ pipistrelle and most other bat species (with the exception of the noctule) on offshore platforms show no demonstrable bias towards platforms closer to the shore (most were recorded as distances of 60-80 km from the shore). Eighteen adult Nathusius’ pipistrelles have been recorded on offshore platforms in the Dutch sector of the North Sea, and 6 in their first calendar year. Half of the males (50%) were juveniles, while 87% of the females were adults. The sex ratio of Nathusius’ pipistrelles was biased to males during the autumn migration, whereas in spring most bats were females. No significant correlation was found in the numbers of reported Nathusius’ pipistrelles in autumn or spring and wind speed or prevailing wind directions, suggesting the bats were not blown off course. The body mass of both male and female Nathusius’ pipistrelles from offshore platforms was on average lower than for those from bat boxes in mainland North Holland.