Long-term effects of the Horns Rev 1 offshore wind farm (OWF) on fish abundance, diversity and spatial distribution were studied. This OWF is situated on the Horns Reef sand bank in the North Sea. Surveys were conducted in September 2001, before the OWF was established in 2002, and again in September 2009, 7 yr post-establishment. The sampling surveys used a multimesh-size gillnet. The 3 most abundant species in the surveys were whiting Merlangius merlangus, dab Limanda limanda and sandeels Ammodytidae spp. Overall fish abundance increased slightly in the area where the OWF was established but declined in the control area 6 km away. None of the key fish species or functional fish groups showed signs of negative long-term effects due to the OWF. Whiting and the fish group associated with rocky habitats showed different distributions relative to the distance to the artificial reef structures introduced by the turbines. Rocky habitat fishes were most abundant close to the turbines while whiting was most abundant away from them. Species diversity was significantly higher close to the turbines. Overall, these results indicate that the artificial reef structures were large enough to attract fish species with a preference for rocky habitats, but not large enough to have adverse negative effects on species inhabiting the original sand bottom between the turbines.

Offshore wind farms (OWFs) are being constructed at a high rate due to a high demand, both economically and politically, for sources of renewable energy. We investigated the short-term and long-term effects of an OWF situated in the North Sea off western Denmark (Horn Rev I; global position: 7.84°E, 55.48°N) on 3 ecologically important species of sandeel. Since sandeels display a distinct preference for sand habitats with a weight fraction of silt+clay < 2%, we expected changes in habitat quality to provide a causal explanation for the potential effect of the OWF on the sandeel community. A Before-After-Control-Impact (BACI) analysis was applied. A baseline survey from March 2002 (prior to construction) was combined with surveys conducted in March 2004 (short-term effects) and March 2010 (long-term effects) plus an additional survey in September 2009. Sandeels were collected using a modified scallop dredge and sediment samples using a van Veen grab. The results from an analysis on all species combined revealed a positive short-term effect on the densities of both juveniles and adults, which was consistent with a reduction in the fraction of silt+clay. In the long term, a negative effect on juveniles was found; however, this effect was neither consistent with the additional survey in 2009 nor the silt+clay fraction. Subsequent analysis at the species level revealed that the effects detected were driven by Hyperoplus lanceolatus, which dominated the study area in all years. Habitat quality was high in both the affected and control area throughout the study period.