Monitoring the effects of the Belgian windmill parks on the epibenthos and demersal fish fauna of soft bottom sediments

A BACI (Before After Control Impact) design is used to evaluate two possible effects of offshore windmill parks in the Belgian part of the North Sea on soft bottom epibenthos and demersal fish, namely the effect of the presence of turbines (hard substrate) and border (fringe) effects due to changing fisheries activities at the concession borders. For the Thorntonbank wind farm (27 km offshore, 18-24 m water depth, 6 gravity based foundations), a lower number and slightly larger individuals of swimming crab Liocarcinus holsatus and brown shrimp Crangon crangon and higher autumn densities of whiting Merlangius merlangus inside the park were noted in 2010 compared to the reference area. VMS (Vessel Monitoring System) data showed an increased fishing intensity in the neighbourhood of the concession. The absence of the smallest size classes of sole Solea solea at the ‘fringe’ stations might indicate an increased indirect fishing mortality (discards) or changes in the local benthic community. For the Bligh Bank windmill park (40 km offshore, 18-40 m water depth, 55 monopiles), clear differences were noted at species level. For example, the green sea urchin Psammechinus miliaris was found at higher densities in the impact area, both on the sandbank top and in the gullies. Also, some larger flatfish individuals (plaice Pleuronectes platessa and turbot Psetta maxima) were encountered inside the concession area. The results suggest fringe, refugium and reef effects on soft bottom epibenthos and demersal fish, related to the presence of the windmill parks.