Abstract
Two autonomous surface gliders, Sailbuoys, were used to monitor the pelagic ecosystem around the Hywind Tampen floating wind farm in May 2023 collecting active acoustic data over a 4-weeks period to assess possible influence of the farm on pelagic fish and zooplankton. Sampling transects were run perpendicular to the main environmental gradient (bathymetry) in two areas: an area within or downstream of the wind farm, and an upstream area, considered a control. The upstream area was expected to show natural variability, while the downstream area would reflect the wind farm's potential influence. Three main scattering features were observed in the upper water column (0-150m): 1) weak sound scattering layers likely composed of zooplankton, consistent across times of the day, 2) strong scattering layers, likely a mixture of zooplankton and fish, moving to the upper water column at night and 3) fish schools. The different groups showed varied vertical behaviours and distributions in the area around the farm, but no systematic pattern was detected based on the distance from the farm. Moreover, no differences larger than the average natural variability were found between the upstream and downstream areas at the level of the farm, in terms of biomass density and vertical distribution. Both approaches support the conclusion of a negligible effect of the wind farm on pelagic fish aggregations or lower trophic levels production. Despite no observed patterns, it should be noted that 7 turbines were installed in 2022 and the remaining (4) in May 2024, during the survey. The longer installations are in the water, the more advanced the biofouling process will be, supporting a more complex "reef" ecosystem. Continued monitoring at later stages will be required to assess the influence on the pelagic of an established benthic "reef" ecosystem on the installations.