Abstract
This report presents the results of the first year of bat surveys conducted from April 2023 to April 2024 as part of the environmental pre-investigations for the North Sea I area. The bat baseline surveys included passive acoustic monitoring from offshore stations on buoys, offshore stations on existing wind turbines, and from vessel surveys carried out as part of the bird baseline surveys, all within a survey area covering the 1.400 km2 project area and a 20 km buffer zone around it. Bats were recorded on 11 out of 22 offshore buoy stations, on four out of 10 deployments on wind turbines in Horns Rev 3, and on one out of 45 nights where bat activity was surveyed during vessel surveys of birds.
There were few records of bats offshore. Most of the recordings were from the buoy and wind turbine stations, were identified to Nathusius’ pipistrelle (Pipistrellus nathusii), and occurred from mid-August to mid-September 2023. Bat occurrence generally coincided with temperatures > 15°C (estimated at 2 m above mean sea level) and mean wind speeds below 8 m/s (estimated at 10 m above mean sea level). Nearly all bat passes were recorded on stations no more than 40 km off the coast, except for a single record of two bats interacting socially at one of the buoy stations furthest west, approximately 80 km from the coastline.
The survey programme for bats also included passive acoustic monitoring from 11 stations on land along the coastline of western Jutland. There was significant bat activity along the coast during the one-year survey period. For Nathusius’ pipistrelle, most activity was recorded at Husby and Kammerslusen, and activity of this species increased notably for nearly all stations at the end of August and was pronounced at seven stations throughout September. The activity peaks observed at the land-based stations likely relate to migration near-shore or over land but were not reflected in similar peaks offshore.
Thirteen Nathusius’ pipistrelles were caught in northern and western parts of Jutland and tagged with radio-transmitters in autumn 2023 as part of the survey programme. Two of the individuals tagged in Thy, in the northern part of Jutland, were since registered on receivers in the northern part of Germany. The tagging effort documents migration of Nathusius’ pipistrelle over at least 270 km and 400 km, respectively, between northern Jutland and Germany. Since no further registrations were made on additional receiver stations in between the site of tagging and the receiver where each of the two individuals was registered, the tagging results did not provide detailed information about the migratory flight paths but based on the most direct line of flight, the two individuals appear to have stayed over or close to land, with one potentially following the coastline.
If autumn migration of bats occurs over the area of the North Sea surveyed as part of the North Sea I pre-investigations, it is either sporadic or focused through flight paths outside the detection range of the passive acoustic monitoring stations deployed during the bat baseline surveys. Based on the results of the bat surveys, it is equally likely that the observed activity offshore reflects exploratory flights during a period of mainly land-based intense foraging activity which may be related to migration activity. Notably, however, 99% of the bat recordings from the buoy stations were from autumn 2023, starting from mid-August. The buoy stations were active earlier in August but recorded no offshore activity before this date, although no major weather changes occurred. In contrast, 93% of the activity recorded on PAM stations on wind turbines in Horns Rev 3 occurred in spring 2023 and included numerous feeding buzzes. Offshore activity thus occurred specifically during the expected migration periods. The addition of data from the second year of bat baseline surveys currently underway will help elucidate if this is a consistent pattern.