23 September 1943, in time to collect water needed for the following summer, when the British failed to follow up with additional raids to hamper reconstruction. Nature and environment Lake Möhne is designated as a European bird sanctuary "Möhne Lake" in the EU's Natura 2000 system of protected areas. Lake Heve and the Heve arm in the south of Lake Möhne as well as the inlet of the Möhne into the lake are designated as the Hevearm and Hevesee nature reserve. The Heve forebay and adjacent areas have also been registered as an FFH site. Importance for bird protection Lake Möhne is an important resting place in North Rhine-Westphalia with four to six thousand waterfowl. The important resting birds are, with percentages for the years 2001 to 2006, the tufted duck with 31 percent, the mallard with 24 percent, the coot with 22 percent, the great crested grebe with 9 percent, the pochard with 5 percent and other species with 9 percent. An important reason for greater resting occurrences of diving ducks and coots is the presence of the migratory mussel in the lake. The zebra mussel forms mussel beds on the stony lake bottom with several thousand animals per square meter. At the inflows of the Möhne and the smaller streams into the lake there are occurrences of kingfisher, grey wagtail and dipper. However, their actual breeding sites are not directly at the lake, but at the tributaries. On the shore of the lake breed mallard, tufted duck, moorhen, coot, mute swan, greylag goose, Canada goose, Egyptian goose and great crested grebe. The great crested grebe has its largest breeding population in Westphalia here, with about fifty breeding pairs on the lake. The grey heron has a breeding colony on the Westenberg in Wamel with 15 to 30 occupied nests. The cormorant has so far made only unsuccessful breeding attempts, probably due to human disturbance. The red-necked grebe has also bred unsuccessfully at the lake several times since 2002. The little grebe, on the other hand, has had single successful broods in the past. A special feature was the year-round occurrence of the common eider, a sea duck, from September 2001 to December 2006. In July 2006, the first breeding record for North Rhine-Westphalia was found here: a female with three half-grown young. When the water level rose sharply in December 2006, the eiders could no longer reach the