Horns Rev Offshore Wind Farm II: EIA Report Fish


Title: Horns Rev Offshore Wind Farm II: EIA Report Fish
Publication Date:
July 31, 2006
Document Number: 2676-03-001-rev 4
Pages: 68

Document Access

Attachment: Access File
(894 KB)


Jensen, B.; Klaustrup, M.; Skov, H. (2006). Horns Rev Offshore Wind Farm II: EIA Report Fish. Report by BioConsult SH, Carl Bro, Danish Hydraulic Institute (DHI), and ENERGI E2. pp 68.

This EIA report reviews and assesses the possible impacts on fish from the establishment of Horns Rev 2 Offshore Wind Farm.


Horns Rev 2 Offshore Wind Farm will be established in one of two designated areas situated north of the existing Horns Rev 1 Offshore Wind Farm. Like the latter, the new wind farm will be situated in an area characterised by a harsh marine environment with strong tidal currents and a rough sea, both of which cause very dynamic current and sediment regimes. It is against this very variable and fluctuating environment that all human activities and installations should be seen and assessed.


Despite the harsh environment Horns Rev is an important fish habitat. The sandy sediments and the grain size distribution are strongly reflected in the species composition, and the distribution of the individuals is strongly influenced by the current patterns. Regarding abundance and density sandeels (Ammotydidae spp.) dominate the fish fauna at Horns Rev, which is the reason for an intensive commercial fishery for sandeels in the area. Other abundant species are the flatfish plaice (Pleuronectes platesa) and dab (Limanda limanda) as well as sand goby (Pomatoschistus minutus), but many more species are recorded at Horns Rev. Some live permanently at Horns Rev or in the vicinity, while others are occasional or seasonal visitors. Thus, depending on the time of the year the different surveys carried out at Horns Rev rank the species differently regarding abundance. Fish of conservation interest occur only very sparsely and occasionally at Horns Rev.


Noise and vibrations are likely to be the most important impacts on the fish fauna, which is why hearing ability among the fish is an important issue. Based on the literature the most abundant species – sandeels, plaice and dab – are all believed to have low sensitivity to noise and vibrations. Other species are more sensitive due to fact that hearing ability is an important part of the sensory apparatus.


The wind turbines will be founded by use of either monopole or gravitation foundations. Which one of these two foundations will be used is not decided yet, but this report focuses on the monopile foundation since the use of this is associated with the highest levels of impacts, particularly in the form of noise and vibrations. In the case that gravitation foundations are to be used, the impacts on fish are believed to be similar or – more likely – smaller than they will be in the case that monopile foundations are to be used.


The life cycle of the wind farm comprises four phases – the pre-construction phase, the construction phase, the operation phase and the decommissioning phase. Each of these phases comprises a number of impacts – some general and some phase specific.


In the pre-construction phase seismic surveys of the sea floor may give rise to transient emissions of noise and vibrations from seismic guns and vessel activity in the wind farm area. Although unavoidable and associated with high but transient levels of noise, these impacts are considered insignificant to fish. They may flee from the impacted areas or avoid these during the surveys, but no lasting effects are to be expected.


The construction phase is considered the most important to fish in terms of impacts. First of all the erection of the turbines along with the establishment of scour protection is encumbered with high impacts of noise and vibrations, the most important source be the pile-driving (in case of monopile foundation). Although fish to varying extent are sensitive to both noise and vibrations, the assessments lead all to the conclusion that no significant lasting effects on fish are to be expected. Indeed fish may flee from or avoid the areas with the highest impacts, but as the emissions of noise and vibrations come to an end, things are likely to return to normal within short time.


Secondly, the erection of the turbines and establishment of scour protection at each of the turbines will invariably cause a loss of natural habitat to fish. Amounting to only a few percent of the total wind farm area, this loss is considered insignificant, even to the most abundant and important fish species in the area, the sand eels. In terms of fish habitats the loss of sandy habitats is correspondingly associated with an increase in stony and rocky habitats, i.e. artificial reefs will come into existence.


In the operation phase the presence of the artificial reefs will increasingly have positive effects on the fish fauna, a process that is known as “the artificial reef effect”. Species not presently living at Horns Rev will be attracted to the artificial reefs, some because the stones and rocks constitute their preferred habitat, others because they constitute suitable spawning and nursery areas. Thus, due to the artificial reefs, the establishment of the wind farm is likely to cause a significant positive impact on the fish fauna in the form of increased species richness and diversity. However, in the operation phase there will also be negative impacts in the form of both noise and vibrations and in the form of electromagnetic fields around the power cables. Based on existing knowledge, including that from the monitoring of the fish fauna at Horns Rev 1 Offshore Wind Farm, nevertheless no significant impacts on the fish fauna are to be expected.


Decommissioning of the wind farm will take place when the turbines have served their time, expectedly at least 25 years. Decommissioning of the wind farm will to large extent comprise the same activities and thus the same impacts on fish as will the construction, although the emissions of noise and vibrations are believed to be less intensive. Like the establishment of the scour protection will cause a loss of sandy habitats and creation of stony and rocky habitats, so will a complete decommissioning cause loss of the artificial reefs and regeneration of sandy habitats. This reversal of the situation will invariably mean a loss of the richness and diversity associated with the artificial reefs, and although no lasting nor significant effects are expected on the large scale, the scour protections should preferably be left in place if nothing speaks against this.


In conclusion, the establishment of Horns Rev 2 Offshore Wind Farm invariably involve a number of human activities and alterations of the existing environment at Horns Rev, all of which are associated with impacts on the fish fauna. In a systematic review all negative impacts are nevertheless assessed to be of minor importance or insignificant to the fish fauna, spatially as well as temporally. Thus, no significant negative changes of the fish fauna are expected in the wind farm area or in the adjacent areas. On the other hand significant positive changes are expected due to the artificial reef effect.


Likewise no significant cumulative effects are expected, neither for Horns Rev 2 Offshore Wind Farm on its own or for the two offshore wind farms as a whole. But there may be a positive cumulative effect on the developmental pattern of the fish fauna at Hors Rev 2 Offshore Wind Farm due to the presence of already colonised artificial reefs at Horns Rev 1 Offshore Wind Farm.


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