Abstract
This report compiles the results from two years of baseline surveys of bats (April 2023 – April 2025) for the 1.400 km2 North Sea I pre-investigation area and a 20 km buffer zone around the pre-investigation area. The eastern and western perimeters of the pre-investigation area are located approximately 20 km and 80 km from the coast, respectively. The report also sums up existing, albeit limited, available information of relevance. The primary data collection effort included passive acoustic monitoring (PAM) from offshore stations on buoys, on the transition piece of existing off-shore wind turbines and an offshore substation in the wind farm Horns Rev 3, and from vessels conducting bird base-line surveys within the pre-investigation area. The offshore PAM was supplemented by 11 PAM stations on shore along the western coast of Jutland. Efforts to catch, tag and radio-track bats were also initially part of the first survey year.
Bats were detected on a total of 18 out of the 22 offshore buoy stations that were included in the bat PAM programme across both survey years. Eleven stations recorded bats during the first survey year, and 17 stations recorded bats during the second survey year. Nine of the buoy stations recorded bats in both survey years. A single bat was recorded on a buoy station during spring, all other bat records on buoy stations were from the autumn, between start of August and end of September. Bats were also present in five out of 20 deployments (10 per year) on wind turbines in Horns Rev 3, on the single OSS PAM station added during the second survey year, and on seven nights out of the 97-day collective period where bat activity was monitored during vessel surveys of birds.
The offshore bat records included several species: Nathusius’ pipistrelle (Pipistrellus nathusii), species assigned to the complexes Eptesicus/Nyctalus/Vespertilio and Myotis (based on a conservative approach not identified to species), common noctule (Nyctalus noctula), soprano pipistrelle (Pipistrellus pygmaeus) and one record identified as either Nathusius’ pipistrelle or common pipistrelle (Pipistrellus pipistrellus).
Nathusius’ pipistrelle, a species well-known for long-distance migration, was the most common species found offshore during the two-year survey period. It was also the only species documented beyond 27 km offshore, including records from one of the buoy stations (NS06) approximately 80 km offshore. Most bat passes, also of Nathusius’ pipistrelle were, however, recorded on stations less than 40 km from the coast.
Bat occurrence offshore coincided with temperatures mostly above 15֯C (estimated at 2 m above mean sea level) and mean wind speeds mostly below 8 m/s (estimated at 10 m above mean sea level). Easterly winds were an important predictor of autumn activity at favourable temperature and wind speeds. The few nights with records from spring (one night from buoys, four nights from wind turbines) included lower temperatures (down to 8֯C) and higher wind speeds (up to 11 m/s).
Overall, the offshore bat activity was low and clustered onto specific dates, compared to the significant bat activity along the coast during the two-year survey period. Activity of Nathusius’ pipistrelle increased for nearly all stations at the end of August. The activity peaks observed at the land-based stations could relate to migration but were not reflected in similar peaks offshore.
During the first year of the baseline surveys, in autumn 2023, 13 Nathusius’ pipistrelles were caught in northern and western parts of Jutland and tagged with radio-transmitters to obtain movement vectors for these individuals. Two of the tagged bats were since registered on radio-receivers along the North Sea coast of Germany, documenting trans-national bat migration over at least 270 km and 400 km, without any further data points to detail their routes. The tagging effort was not included in the second year of bat baseline surveys but a third track is added in this report (Department of Ecoscience, Aarhus University, own data) to show a male Nathusius’ pipistrelle following the North Sea coastline south passing by ten receiver stations in Denmark, Germany and The Netherlands after departing from its capture and tagging point near Skjern, Western Jutland.
It is inconclusive whether the offshore records of bats reflect foraging activity, migration activity or both. Based on the results of the bat surveys, the observed activity offshore could reflect foraging flights from and to the coast during a period of mainly land-based intense foraging activity, which may be related to migration activity mainly along the coastline. The activity recorded by PAM stations on the transition pieces of wind turbines and the OSS PAM station added in the second year included numerous feeding buzzes, suggesting that bats could target permanent structures at sea for such foraging bouts or follow insects to them.
It is noteworthy that all offshore activity coincided with the expected migration periods in autumn (predominantly) and spring, whereas no activity was observed by the offshore survey effort during summer in either of the two survey years. Some bat activity in the pre-investigation area, e.g., higher altitude flight, was undoubtedly missed beyond the detection range of the passive acoustic monitoring stations. Still, the level of activity was low in comparison to the level observed at the onshore stations. Based on the evidence available, the offshore activity of bats in the North Sea I pre-investigation area is likely related to migration but it seems probably that bats are migrating mainly over land and along the coastline, while opportunistically following foraging opportunities offshore into the area, whereas no evidence points to the presence of a major migration corridor through the area. More migration activity at altitudes beyond the acoustic detection range of the PAM stations cannot be ruled out by the baseline surveys.