Effects of Marine Windfarms on the Distribution of Fish, Shellfish and Marine Mammals in the Horns Rev Area

Report

Title: Effects of Marine Windfarms on the Distribution of Fish, Shellfish and Marine Mammals in the Horns Rev Area
Publication Date:
May 01, 2000
Document Number: DFU-rapport 117-02
Pages: 42
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Citation

Hoffmann, E.; Astrup, J.; Larsen, F.; Munch-Petersen, S.; Støttrup, J. (2000). Effects of Marine Windfarms on the Distribution of Fish, Shellfish and Marine Mammals in the Horns Rev Area. Report by Danish Institute for Fisheries Research. pp 42.
Abstract: 

The fish and shellfish fauna: The description of the fish fauna in the Horns Rev area is based on eleven years trawl surveys carried out by the Dutch Institute for Fisheries Research. The most common species are dab, plaice, hooknose, whiting, dragonet and grey gurnard. A total of 42 different fish species are listed. The relative abundance of the ten most common species is given for three different areas within and outside the windmill area. These mean figures indicate some systematic differences among the three areas for species like plaice, hooknose, whiting and gobies. However, there have been large fluctuations from year to year in the abundance of the species. A high abundance of the brown shrimp is observed east of the windmill area.

 

The effects of the marine windmills. The effects on fish, shellfish and marine mammals are in the following divided in 1. Effects of the physically presence of the windmills, 2. Artificial reef effects, 3. Effects of noise and 4. Effects of magnetic fields.

 

  1. Effects of the physically presence of the windmills: As for whether and how the changes in environment below the sea surface due to placement of a windmill park and the cable tracé will affect the marine species considered, one should distinguish between short term effects and long termeffects (permanent changes). Short term effects. It is very likely that during the construction period of both the wind mills and the cable tracé many of the fish species as well as marine mammals will be disturbed. They will disappear from the relatively small area due to temporary increased turbidity of the water, underwater water movements, noise and other activities on the sea bottom. If the cable tracé is placed in the Grådyb area the effects in the construction period will be considerable to small flatfish and the seals in the Langli area. The experience is, however, that once such construction activities have finished the species affected will return rather quickly. Long term effects. The underwater changes in the windmill area will be the stone and concrete foundations of mills and possibly some minor changes in local currents. The total area of that part of the seabed, which will be occupied by the foundations of the mills is so small (around 1000 m2), that it can be considered insignificant. Also the total area of the proposed windmill area (27.5 km2) is small, and the effects of any local environmental changes within this area will probably be small regarding the surroundings. The physically presence of the cables and any of the proposed cable tracés buried in the sea bed are not expected to cause any changes in the abundance of fish, marine mammals and crustaceans in the area. Five fish species, grey gurnard, lesser weever, snake pipefish, sea trout and lampern are listed on the current red list of endangered fish species for the Wadden Sea. Taking the biology of the species into concern no impact on their distribution is expected in the Wadden Sea due to the windmills at Horns Rev.
  2. Artificial reef effects. The foundations of a marine windmill will to some extent function as an artificial reef providing hard-bottom on an otherwise even sandy bottom. The impact on the fish fauna will either be through increased productivity or simply through attraction. Productivity: The planned marine windmill park will provide a relatively simple artificial reef complex with equidistant, uniform elements of high profile structures of low complexity. The type of fauna and flora that develop on these structures is dependent on a number of parameters. Structure material and design together with hydrographic parameters are important factors determining the extent and rate of biofouling, as well as development of hard-bottom benthic organisms, which may provide the foodchain basis for fish fauna. These substrates may also attract predators feeding on smaller fish or epifauna attracted to the reef. Considering the hydrography and material and design of the Horns Rev structures, there is no indication that the windmill foundations will provide a significant food-chain basis. Attraction: Fish are highly attracted to underwater structures, their affinity being related to their life styles and requirements. Gadoids (codfish) are particularly attracted to high profile structures but their attraction to the windmill structures, which are relatively low-complex structures, may be limited. The abundance of cod in the area is relatively low and there is a sporadic summer fishery on this species. Providing that fishery is allowed close to and around these structures, and gadoids are attracted to windmill profile structures then the windmill reef complex could be a potential fishing area. Flatfish are also attracted to underwater structures resulting in a redistribution of resources. Because of their relatively high mobility between underwater structures, these species may become more vulnerable to fisheries, increasing the exploitable biomass.
  3. Effects of noise. In the construction phase, noise will be expected to be generated by the construction operations (primarily the jack-up-rig ramming operations), by shipping operations (supply vessels coming and going as well as transportation within the area) and by helicopter traffic. The noise generated by these sources will primarily be of low frequencies with most energy probably below 1 kHz. This is not expected to affect the echolocation abilities of the harbour porpoises. However, it is not clear whether harbour porpoises use sounds with frequencies below 1 kHz for communication. If they do, this could potentially be affected by the noise sources mentioned. These noise sources are all temporary and of a localised nature, and although they will probably displace fish, porpoises and seals from the affected areas, it is expected that this displacement will be temporary. In the production phase, noise will be expected to be generated by the windmills and by helicopter traffic. The windmills are expected to generate noise above ambient levels only in frequencies below 1-2 kHz. Below 500 Hz, noise from the windmills could be considerably above ambient levels. This could potentially affect the communication of porpoises in the area, if they indeed use these frequencies. Since the noise from the windmills will be continuous, the porpoises will probably develop some tolerance to the noise, but the extent of this is impossible to predict. Fish typically respond strongly to low-frequency hydrodynamic/acoustic fields (below ca. 50 Hz). Significant noise contributions in this frequency range are expected to be confined to the immediate vicinity of the windmills, within a radius of no more than some hundreds of meters. However, because of the spatial extent of the low-frequency hydrodynamic/acoustic fields from the mills, fish will perceive them to be very different compared to the low-frequency fields of other animals. Therefore, fish are not expected to be impaired in their ability to detect and interpret the fields from different sources (i.e. windmills or animals). Furthermore, the continuous character of the windmill noise will likely promote habituation in the fish. Noise is also radiated in the frequency range 0.05–2 kHz with source levels up to 74 dB re 1 µPa. However, fish respond only weakly, and the influence of the windmills, especially compared to the level of marine antropogenic noise in general, is most likely minor. Above 2 kHz, no noise is expected from the windmills, and this frequency range may therefore be considered of no concern. In conclusion, it should be expected that harbour porpoises, harbour seals and fish will be displaced temporarily from the area affected by the construction of the park and maybe permanently from a smaller area in the production phase. But unless the affected area is a critical habitat for porpoises, the overall effect is expected to be negligible. There are no haulout sites for seals in the vicinity of the windmill park, and not many seals have been observed in the area. Since the noise from the windmills will be continuous, the seals and fish are expected to habituate to this and the overall effect is expected to be negligible.
  4. Effect of magnetic fields. Magnetic fields from cable tracé, windmills, and the offshore transformer station may be expected to reach geomagnetic field-strength levels only in the immediate vicinity of these structures, at distances no more than 1 m. Cartilaginous fishes (sharks and rays) are, by way of their electroreceptive sense organs, able to detect magnetic fields, and they may use the geomagnetic field for navigation. For bony fishes, a true magnetic sense has been proposed, but the evidence is much less compelling. Thus, the weak magnetic fields from the marine windmill park at Horn Rev are not expected to pose any serious problem for the local fish species. Furthermore it does not appear likely that the magnetic fields generated by the power transmission cables will have any detectable effects on the harbour porpoises and seals in the area.
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