Several European studies have demonstrated that fish are attracted to artificially created hard substrates. Most attempts to quantify fish stocks near hard structures as well as natural reefs have used visual techniques. In order to improve results, the use of hydroacoustics has been used to quantify fish stocks around oil fields and in lakes. This methodological approach has been applied to assess impact on fish communities from introduced hard structures such as wind turbine foundations at Horns Rev Offshore Wind Farm. This study is a continuation of studies carried out from 2004 on behalf of the Environmental Group.
The aim of the present study was:
- To investigate the regional effects from the wind farm by studying differences in distribution patterns in local pelagic and semi pelagic fish communities between areas inside and outside the wind farm area.
- To investigate local effects from turbines on fish distribution patterns to demonstrate attraction or avoidance behaviour.
Dynamic, horizontal hydroacoustic survey were carried out along transects inside and outside the wind farm in autumn, 2005. Hydroacoustic data was collected using a split beam transducer mounted on a pan & tilt unit mounted to the side of a survey vessel. In order to describe the species composition and calibrate the acoustic signals, supplementary fishing was performed simultaneously with the acoustic surveys. The supplementary fishing was carried out with the use of survey gill nets and a small specially designed pelagic trawl. Post processing and analysis of hydroacoustic data was performed using Sonar5-Pro data application software.
General findings During the supplementary fishing, a total of 21 different species were registered. Nine species were categorised as semi pelagic or varying between pelagic/semi pelagic and semi pelagic/benthic. The remaining 11 species were categorized as inhabiting benthic habitats. Sandeels and gobies were the most numerous with sand gobies dominating the smallest length group.
According to the analysis of the hydroacoustic data, a total of 12,099 fish were registered along the six surveyed transects. Most of the fish, 7,892 individuals, were classified as fish with a swim bladder (other fish) and the remaining 4,207 individuals were classified as sandeels.
Regional effects No general and unambiguous regional effects were demonstrated by the presence of the wind farm during the hydroacoustic surveys at Horns Rev Offshore Wind Farm. No distinct, significant, temporal or geographic patterns in densities, biomass or length distribution could be found in sampling periods, diurnal variations, or transects inside and outside of the wind farm area. Different species composition might be responsible for the variances found in the fish communities. Abiotic factors, like the area with coarse sand south of the wind farm, aggregated fish to a much higher extent than the presence of the wind farm itself.
Local effects Fish density was expected to be higher inside the wind farm and especially higher in the vicinity of the turbine foundations because of a potential attraction effect on reef fish. However, no statistical evidence was found confirming that densities of pelagic and semipelagic fish near the vicinity of the turbines were different from between the turbines.
In conclusion, it is very difficult or impossible to achieve statistically useful representative replicates and geographical representative reference areas due to the high variability in the spatial and temporal distribution of both pelagic and semi pelagic fish populations. No statistically significant results were obtained for a regional or local impact on fish communities from the wind farm or the turbine foundations due to pronounced variability in biotic and abiotic factors influencing the fish communities.