past in connection with the deglaciation of the lake basin and the formation of various ephemeral ice-dammed lakes (Sydsvenska issjökomplexet). Subsequently, the population was left isolated for thousands of years. Populations of Sommen charr declined over the 20th century, leading to the fish being declared endangered in 1970. The causes of the decline are likely to include overfishing, fishing of immature individuals, unnatural lake level changes (as the lake is regulated), and competition from introduced species. The largest known Sommen charr weighed almost 9 kg, and was for a while the largest ordinary charr () fished in Sweden. The three largest arms of Sommen, Tranåsfjärden, Asbyfjärden and Norra Vifjärden, host most of the Sommen charr population. The bays near Malexander and Norravifjärden (the central and southeastern part of the lake) host most of the fish. Lek locations lie chiefly along the eastern shores of Norravifjärden and around Malexander in the north-central parts of the lake. Compared to the charr of Lake Vättern, only large individuals of Sommen charr breed; this may be the result of overfishing. In Sommen, charr reach sexual maturity at around 7 years of age. At 6 years Sommen charrs are estimated to reach an average length of 54 cm, while at 7 years of age average length is 60 cm. For comparison, charr in Lake Vättern reach sexual maturity (lek for the first time) at 6 to 8 years when the females have reached lengths of 40–55 cm and the males 35–45 cm. The implication of this difference is that Sommen charr either reach maturity later than charr in Vättern, or that they grow faster. Crustaceans The two large crustaceans of the lake, the signal crayfish and the noble crayfish, are not as abundant as fish. The signal crayfish is an introduced species. The lake stands out for having three crustacean species that are relicts from the time of the Fennoscandian Ice Sheet deglaciation about 12,000 years ago. These species are Pallasea quadrispinosa, Mysis relicta and Limnocalanus macrurus. However, the lake lacks the most common relict crustacean found in the lakes of southern Sweden, the Mysis relicta. Environmental concerns While the eastern parts of the lake have higher eutrophication values than the west, it does not qualify as an environmental problem in the lake as a whole. Neither is there any considerable acidification as seen in other Swedish lakes; however high cadmium and fluoride contents are