Abstract
In 2006, the first Dutch offshore wind farm was built 10-18 km from the shore of Egmond aan Zee by a joint venture of Nuon and Shell Windenergy. A Monitoring and Evaluation Program accompanied the plans for the construction and exploitation of this farm. The program contained plans for the monitoring and evaluation of potential ecological consequences in the Dutch coastal zone related to the wind farm and was divided in six topics, of which the topics on fish are presented in this report.
Potential ecological consequences for fish were hypothesized to be linked to the introduction of new habitat, i.e. the monopiles and the scour protection surrounding them, disturbance by the operation of the wind farm (e.g. noise) and the exclusion of fisheries in the wind farm and its surrounding safety zone. To monitor and evaluate these hypothesized effects, five sub-projects performed focussing on different parts of the fish community, their spatial and temporal distribution and their behavioural aspects.
To study the demersal part of the fish community a BACI-approach was used, which means that monitoring was executed prior to the construction of the farm, i.e. the baseline (T0), and after the impact during the operational phase. As baseline (2003/2004) and in one year after (T1 2007/ 2008) and five years after (T5 2011) the construction of the wind farm, surveys with beam trawls were performed in the winter and summer. Fishing with the beam trawls took place in the wind farm and in three reference areas that were considered similar in respect to distance to shore, water depth and seabed morphology. The pelagic part of the fish community was studied using a similar approach. Here, monitoring was done acoustically and for reference purposes fishing was done with a pelagic net. The survey was performed in the wind farm, two reference areas and in transects along the Dutch coast in April (T0, T1, and T5) and October (T0 and T5).
On a smaller scale the species composition and abundance of fish in the vicinity of the monopiles compared to the sandy habitats in the middle of the wind farm were studied using gillnets and the DIDSON (acoustic high frequency camera using sonar). Multi-mesh gillnets were used, which were placed on the scour protection as close as possible to the monopiles and in the middle between monopiles on the sandy bottom, in order to identify species and to estimate fish abundance in the lower part of the water column. The DIDSON surveys yielded abundance estimates for the entire water column, however without information on species. Both the gillnets and DIDSON were executed in three seasons during the T5.
Behaviour of individual fish in and around the wind farm was studied by a tagging and telemetry study for which sole (Solea solea) and cod (Gadus morhua) were selected to represent both sand and hard substrate dwelling species. Tagging of sole was executed in 2007 and 2008. The transmitters for the telemetry study were implanted in a number of sole and cod in 2008 and January 2009, while the detection stations were retrieved in June-July 2009.
The surveys monitoring the species on a larger scale in the Dutch coastal zone did not show significant effects of the wind farm on the abundance of fish in the farm compared to the reference areas. This means that the construction of the farm and the exclusion of fisheries in the area did not lead to detectable changes in abundance of the monitored species. The length compositions of fish caught in the surveys showed only a slight increase in the length of two pelagic species, sprat (Sprattus sprattus) and anchovy (Engraulis encrasicolus). This was hypothesized to be related to a reduction in predation pressure due to the protection by the wind farm.
On the smaller scale of the wind farm clear differences were observed between the new artificial hard-substrate habitat and the sandy bottom. Large aggregations of fish were observed near the monopiles mainly in summer, which can most likely be attributed to the presence of horse mackerel (Trachurus trachurus). Furthermore, a significant higher abundance of cod, edible crab (Cancer pagurus), bib (Trisopterus luscus), bullrout (Myoxocephalus scorpius), sea scorpion (Taurulus bubalis) and common dragonet (Callionymus lyra) was observed on the scour protection near the monopiles. In contrast, a lower abundance was observed for the flatfish species, sole, dab (Limanda limanda) and plaice (Pleuronectes platessa) and also for whiting (Merlangius merlangus). For these last four species the construction of the wind farm might have led to a nearly negligible reduction in habitat. However, for the other species the higher abundance near the monopiles may lead to a protection from fisheries with a potential effect on their populations. This is especially the case for cod, for which the telemetry study showed that part of the cod spends long periods within the wind farm and for which other research suggests that they can also spawn and settle in the wind farm. Overall the presence of the wind farm seems to have limited, if any effect, on the fish community of the Dutch coastal zone. For some species local benefits due to a combination of the creation of new hard substrate habitats and exclusion of fisheries might have occurred.