The Eolien en Mer SIG is launching the first study of bat transits at sea prior to the construction of an offshore wind farm.

During the initial impact study for the Dieppe Le Tréport offshore wind farm (2014-2017), acoustic detections of bats were made. These observations mainly concerned Nathusius’ Pipistrelles (Pipistrellus nathusii) and undetermined Pipistrelles, more probably common Pipistrelles (Pipistrellus pipistrellus), in spring and autumn.

Following these observations, the Conseil Nationale de la Protection de la Nature (CNPN) recommended that the SIG carries out a study on bat migrations. Following numerous consultations with experts on the subject, the SIG can now launch a study of bat transits at sea using acoustic recordings at sea and comparing them with others along the coastline and further inland (see map below).

Acoustic recordings began in 2023, thanks to the Biotope consultancy, which carried out this fieldwork at 6 locations: Dieppe, Le Tréport, Cayeux-sur-Mer, Tourville-sur-Arques, Yzengremer and Saint-Valery-sur-Mer.

At sea, the recordings were made on the Fly’RSea multi-instrument buoy set up as part of the regulatory environmental monitoring of the Dieppe Le Tréport offshore wind farm.

The recordings were made between May and November, from 2 hours before to 2 hours after sunset, to capture the nocturnal passages of the bats.

This year, the data recorded will be analysed by the Muséum Nationale d’Histoire Naturelle (MNHN). The analysis will provide a better understanding of bat transits both locally, in the area of the offshore wind farm, as well as on a wider scale. The data will be incorporated into the Migratlane project, which looks at the migration of birds and bats throughout the Atlantic-Channel-North Sea shoreline.

This study will be extended during the construction and operation phases of the Dieppe Le Tréport offshore wind farm, in order to determine any changes in bat transits at sea as a result of the wind farm’s construction.

Report on 2023 data

This first report details the analysis of data collected in 2023. It shows that at least nine species were detected at sea, only in autumn and later at night than on land. The differences in observations between land-based sites and the sea buoy may be linked to differences in acquisition parameters, abundance or species behaviour.

These preliminary results will be supplemented by several years of recordings, which will provide a better understanding of the influence of weather conditions and interannual variations in bat transits at sea.