Deployment of offshore wind farms is rapidly expanding in Denmark and in the rest of Europe, due to a high demand, both economically and politically, for renewable energy. At present 12 offshore wind farms are in operation in Danish waters. Offshore wind farms are often placed in relatively shallow waters (<20 m) due to engineering and economic constraints. These shallow areas are often biologically highly productive and function as important nursery and feeding grounds for a number of fish species. The establishment of wind farms is hypothesized to positively impact fish abundance and fish community structures by increasing habitat heterogeneity and through exclusion of trawling activities within the wind farm area.
This report presents results from a field experiment in a demonstration study site (Horns Rev Offshore Wind Farm 1), one of the world’s largest offshore wind farms. The construction of this farm, which is composed of 80 wind turbines and located in the North Sea 14-20 km off the western coast of Denmark, at Blaavands Huk, was completed in late 2002. The aim of the study was to analyze changes in fish community structure, spatial distribution and changes in sandeel assemblages due to the establishment of the wind farm.
The baseline study was conducted in September 2001 and March 2002 before the construction of the wind farm and the impact study was conducted 7 years later in September 2009 and March 2010 respectively. Surveys included multi-mesh gillnets targeting semi-pelagic and demersal (bottomdwelling) species. Furthermore, the impact study included acoustic surveys along latitudinal and longitudinal transects targeting pelagic and demersal fish excluding sandeels, which were excluded from the acoustic analysis due to backscatter interferences with other low acoustic detectable organisms.
The introduction of hard substrate and higher complexity relative to the homogenous sand banks characteristic of the North Sea resulted in minor changes in the fish community and species diversity. Fish community changes were observed after the deployment of the wind farm due to changes in densities of the most commonly occurring fish, whiting (Merlangius merlangus) and dab (Limanda limanda), but reflected mostly the general trend of these fish populations in the North Sea. Due to significant temporal variation and patchiness in the distribution patterns of fish densities and biomass no general significant changes in the abundance or distribution patterns of pelagic and demersal fish were found in the acoustic surveys, neither between the control site and the wind farm site nor inside the impact area between foundations.
The introduction of hard bottom substrate resulted in higher species diversity close to each turbine with a clear spatial (horizontal) distribution, which where most pronounced in the autumn, where most species were registered. New reef habitat fish such as goldsinny wrasse (Ctenolabrus rupestris), viviparous eelpout (Zoarces viviparous) and lumpsucker (Cyclopterus lumpus) established themselves on the introduced reef area. Very few gobies were caught near or at the wind farm, and the near absence of these species was suggested to be related to the hydrographical conditions of the wind farm area and to have implications for the occurrence of pelagic and demersal species.
The fish communities in the Horns Reef area showed significant seasonal variation. Species richness and abundance was low in spring compared to autumn and especially the unusually cold winter 2009- 2010 significantly affected the fish communities both in the wind farm area and in the control area. In general fish abundances and species richness seem to increase with increasing depth, increasing the significance of deployed turbine structures at greater depths as refuge areas for fish. Use of telemetrics in Dutch studies has shown a behavioural response where cod (Gadus morhua) move in and out amongst the hard structures of offshore wind farm foundations.
Horns Rev are a habitat to sandeels which are a highly abundant group of fish species that, due to its vast abundances and high oil content, plays an inevitable key role in the North Sea ecosystem and as a commercially viable species. Although pronounced seasonal and day/night (diurnal) effects on sandeel catchability was found, the results revealed no indication that the construction of the Horns Rev I wind farm had a detrimental long-term effect on the overall occurrence of sandeels. However, a short-term effect was detected in March 2004, mainly due to a temporary increase in juveniles primarily of the greater sandeel (Hyperoplus lanceolatus), which completely dominated the sandeel community at the Horns Reef area.
Sandeels are closely associated to the fraction of fine pure sand in seabed sediments and only seabeds with fractions of finer particles of silt and clay below a critical limit of 2% provide suitable sandeel habitats. Although the highest value for the fine particle fraction was found in the control site, no significant changes in the seabed sediment composition was detected after the construction of the wind farm, except for 2004, where a higher fraction of gravel was found inside the wind farm area. The weight fraction of silt and clay in the sediment was generally below 1%. The present study indicates that wind farms represent neither a threat nor a direct benefit to sandeels in near-shore areas dominated by greater sandeel, although the recruitment of greater sandeel, which are self reproducing in the Horns Reef area, might benefit specifically from the exclusion of fisheries in the wind farm area.
Experiences from post construction studies concerning effects on fish communities from offshore wind farm development are rare or almost missing, why no attempt was made to involve an appropriate Population Viability Assessment (PVA) to appraise effects of increased suitable habitat for certain reef species or effects of exclusion of fisheries on sandeel populations. Cumulative effects of more wind farms in the area may be an increase in recruitment of reef habitat fishes and ecological rehabilitation of habitats due to the exclusion of fisheries in larger areas suitable for sandeels. The cumulative effect of introducing vertical structures in deeper waters may be an aggregation of larger gadoids in this area.