Abstract
The Near Shore Wind Farm (NSW) off the Dutch coast is commissioned as a demonstration project, in which knowledge and experience on the construction and maintenance of offshore wind farms should be gained. The Dutch government has set the condition that a monitoring programme on - among other subjects - the impact on the demersal fish community is carried out. The Dutch government is responsible for providing a thorough description of the present demersal fish community. With this information, the effect of the wind farm on the demersal fish community will be assessed (outside of this programme). The area was sampled in June/July 2003 and in January 2004. Sampling stations cover the planned location of the Near Shore Wind Farm and three reference areas. A 6 metre beam trawl with tickler chains was used with two type of nets; a fine mesh size (20 mm) to sample smaller fish and a 40 mm mesh size to sample larger fish.
This final report describes the present situation (2003/2004) of the demersal fish community in the area of the wind farm. The description is supplemented with data from other surveys (from 2001-2004) which allows a more general description of the Baseline situation of the demersal fish community.
In the beam trawl surveys, a total of 41 species were observed along the Dutch coast and most species were found in every season investigated. Their abundance varied greatly among seasons. Flatfish are the dominant ecological group in the demersal community in the Dutch coastal zone. In spring and winter, Dab is the most abundant flatfish, followed by Plaice in most length classes. In summer, Sole is the most abundant flatfish in the smaller length classes, while Plaice dominates the larger flatfish. The commercially unimportant Solenette and Scaldfish are also abundant, especially in summer. Next to flatfish, the group of Gobiids were important in the Dutch coast in all seasons but due to their small size they are not important in terms of biomass. For all species, the annual variation in abundance in the Dutch coastal zone is large. The age structure varied among species: large age ranges within one length class were observed for Lesser weever (individuals of 4-13 years old found within one cm group) and Solenette whereas of Dab only two age classes in total were observed. Spatial variation was generally large but seemed often not random, it was significantly correlated with temperature, depth, pH and salinity. In terms of biomass, there were differences in the species composition between NSW and the other areas but the differences were small.
For demersal fish we expect that the largest effect of the wind farm is the closure of the area to fishing. In two ways, this could have an effect on the demersal fish community; 1) indirectly, via a change in sediment structure and availability of benthic organisms and 2) directly, the wind farm could act as a refuge area because in the middle of an intensively fished area, fishing mortality is locally zero. We think, however, that effects are mainly local and do not extend to the level of North Sea fish populations.
A power analysis showed that the detection of an effect of the wind farm on fish biomass through a monitoring programme is not completely unfeasible. The sampling intensity in the current baseline study (13 hauls in the wind farm area) allows the detection of a 30% downward or 40% upward trend. To strengthen the results of monitoring, and to prove effects, research into the underlying processes is essential. This will also make an impact assessment more efficient.