coastline () at 137 metres. This ancient coastline corresponds to the coastline of the Baltic Ice Lake during the Younger Dryas. These are near the lake's eastern shores with patterned ground which presumably formed during the periglaciation of the area during the Younger Dryas. The biogeography of various aquatic species deemed glacial relicts that are found in Sommen is likely related to a different geography during the early history of the lake. One theory claims that aquatic species were transferred from the Baltic Ice Lake through a natural lock system in connection with a temporary advance of the ice-front during the Younger Dryas. On land, the unusual occurrence of dwarf birch near Sund is also judged to be a leftover from a cold geological past. At present the post-glacial rebound of the crust in the lake area is uneven. At the lake's outlet in the northwest it is 2.36mm per year while in the eastern Svanaviken it is 2.05mm. This means the lake is being slowly tilted and the southeastern shores drowned while near the outlet land is being dried up. Wildlife Birds Large birds that breed in the area include fish hawk, European herring gull, heron and black-throated loon. Inventories of these four birds were taken in 1974, 1980, 1994 and 2012. According to this data, populations of European herring gulls and heron appear to have declined. Other water birds that breed in the area include oystercatcher, common gull, black-headed gull, common tern, common sandpiper, goosander, great crested grebe, mute swan and Canada goose, together with two duck species; mallard and common goldeneye. Various pairs of peregrine falcon nest in the lake area; this species was almost extinct in Sweden in the 1970s. An inventory made in 2018 showed that during summer about 1,000 cormorants live in the lake area. As cormorants feed on fish, their population is thought to impact on the amount of fish, possibly producing an ecological disturbance. Hunting of cormorants is allowed under certain conditions from 2019 to 2023. Fish Twenty-two species of fish are found in Sommen. Common fish in the lake are Eurasian perch, northern pike, brown trout, common whitefish, Sommen charr, smelt, burbot, European eel, ruffe and vendace. Less common fishes include common roach, common bream, tench, vimba and common bleak. Sommen charr Lake Sommen is home to a subspecies of charr called Sommen charr (). This subspecies originated in the distant