Abstract
The Dutch government has decided to allow the construction of a Near Shore Wind Farm (NSW) demonstration project under the condition that a monitoring programme on - among other things - the ecological impacts is carried out. The Dutch government is responsible for providing a thorough description of the present ecological situation in order to evaluate future effects of planned wind farms. The Netherlands Institute for Fisheries Research has carried out the baseline study on pelagic fish. During this study, the pelagic fish community was sampled twice in April and October 2003: with a high spatial resolution in the planned location of the wind farm and in two reference sites, and with a low spatial resolution in a larger area in the coastal zone. Because of the absence of long-term datasets and to allow investigation of inter-seasonal patterns, we supplemented the data collected in this project with unpublished data from two other projects in which pelagic fish were recently sampled in the same area but in different seasons (June 2002 and November 2003). The combination of these four surveys enabled a year-round description of the pelagic fish community. This report describes the baseline situation for pelagic fish and presents the sampling approach, processing of the acoustic and trawl data and the analysis of all data.
The occurrence, density, spatial variation and population structure of the pelagic fish fauna in the reference situation were assessed by means of echo integration and reference hauls during hydro-acoustic surveys. Before, very little was known of the pelagic fish community off the Dutch coast but this study provided insight into the structure and temporal and spatial patterns of the pelagic fish assemblage. Although the weather and new methods applied caused some unforeseen problems and limited the number of transects that could be carried out, the general pattern in the distribution of pelagic fish was described adequately. For pelagic species of commercial interest for the Netherlands (Herring, Horse Mackerel and Mackerel), a lot of biological information was already available but this was certainly not the case for Anchovy, Pilchard, Raitt’s Sandeel, Lesser Sandeel and Greater Sandeel. For these species, new information on biological parameters (sex, maturity, weight and age) was collected.
In general terms, the pelagic fish community in the Dutch coastal zone consists of nine species that show a large temporal variation and of which spatial patterns only occur at larger scales along north-south and near shore-offshore gradients. The number of species observed (nine) is small in comparison to the demersal fish assemblage in the coastal zone (more than 32 species), but far more than the two pelagic species (Herring and Sprat) that are usually encountered in annual pelagic surveys on the North Sea.
Generally, the highest biomass of pelagic fish occurred in the deepest area (>20 m) and the species composition showed a large variation among areas and periods. In biomass terms, Mackerel was the most important species in October and April, while in November Herring and Sprat dominated. Sandeel were the commonest group in June. Of all species, mature individuals were observed and of many species also 0-group fish were present in the Dutch coastal zone. For most species, length-frequency distributions did not vary among the different areas but in general, small species and the smaller individuals of Herring were observed more inshore.
Because in the baseline situation, spatial patterns mainly occurred at larger scales (north-south, near shore-offshore) and were absent from the scale of the wind farm, we think that small-scaled effects of wind farms on pelagic fish may be detected if they alter spatial patterns to a large enough extent. It remains, however, difficult to predict this. In the baseline situation, there was no pattern in the spatial distribution of pelagic fish in and just around the planned wind farm area. Any non-random pattern detected after the building of the wind farm, may indicate an effect. The reference areas play important roles in such an assessment. They were chosen based on location, depth and sediment characteristics and because we expected that fish assemblages here were comparable to those in the Near Shore Wind farm area. In the baseline situation, no large differences were observed between pelagic fish communities in the reference areas and the NSW area and therefore the sites are suitable reference areas. In future, significant differences between the spatial patterns in the wind farm and in the reference areas may indicate effects of the wind farm.
For future sampling we recommend to extend transects further offshore than those in the baseline study to provide a better insight into the occurrence of fish along a near shore-offshore gradient. To save time, the acoustic sampling intensity in the wind farm and reference areas can be lower than was initially planned. June is probably the best month to sample pelagic fish in the coastal zone because in that period, species were most disaggregated. To actually assess the impact of a wind farm area it is important to combine monitoring programmes like this baseline study with process-oriented studies such as mark-recapture experiments and telemetry in which the behaviour of fish can be studied by using radio or acoustic transmitters.