Hunting of eiders Somateria mollissima from motorboats is common in Danish marine waters, and to reduce hunting pressure on eiders and other diving duck species in Denmark, motorboat hunting was banned within 42 marine sites covering an area of 2,934 km², although the effects of this regulation have to date not been examined. Our case study analyses the effects of excluding motorboat hunting from an area of 682 km² of the Danish Wadden Sea (hereafter the ‘Study Area’) which also supported a large area of blue mussel beds, the preferred food for eiders. Our study covered the entire Danish Wadden Sea (total area 1,225 km²) during the hunting seasons (October-February) of 1980–2003 using 85 aerial surveys of eiders and motorboats used by hunters. Eider numbers increased by 56% in the Study Area following the ban on motorboat hunting despite a 50% reduction in the eider flyway population over the same period. There was a significant negative relationship between the density of hunter motorboats and that of eiders on a small geographical scale (1.8–2.5 km). Motorboat hunting in the Study Area also affected eider distribution at larger geographical scales (4–12 km), displacing eiders from the Study Area offshore from the Wadden Sea into the North Sea. Following the ban on motorboat hunting, most eiders occurred in the Study Area. Winter (21 December-31 January) body condition of eiders was greater in the Study Area than the body condition of eiders in the Offshore Area during autumn (20 October-20 December). Eider abundance relative to blue mussel biomass significantly increased after motorboat hunting was banned in the Study Area, but there was no such change during winter after the hunting ban. Since the ban on motorboat hunting in the Study Area, eider numbers throughout the entire Danish Wadden Sea seem to be regulated by total blue mussel biomass.