Sommen charr

The Sommen charr (

This population and other Arctic charrs in southern Sweden are regarded as relict populations at the southern edge of the natural distribution of Arctic charrs.^[2] Survival of Arctic charrs at the southern edge of their natural range is explained by Lake Sommen having a great depth (53 metres (174 ft)) with cold and oxygen-rich bottom waters.^[2] Sommen charr may also draw benefits from the lake's richness in fish species and the complex ecology it implies.^[2]

Populations of Sommen charr declined over the 20th century, leading to the fish being declared endangered in 1970.^[3] 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.^[3] The largest known Sommen charr weighed almost 9 kilograms (20 lb), and was for a while the largest Arctic charr fished in Sweden.^[3]

The three largest arms of Sommen, Tranåsfjärden, Asbyfjärden and Norra Vifjärden, host most of the Sommen charr population.^[3] Lek locations lie chiefly along the eastern shores of Norravifjärden and around Malexander in the north-central parts of the lake.^[3] Studies have shown that the substrate in most lek places is bedrock, boulders, gravel and pebbles while only a minority are on sand.^[3]

Other nearby populations of Arctic charr are found in the lakes Vättern, Ören and Mycklaflon.^[4] In Drögen, 10 km to the northeast of Sommen, Arctic charr is considered to have gone extinct.^[4]

Growth and sexual maturity

Sommen charrs are among the fastest growing Arctic charrs in northern Europe.^[5] Albeit experiments show that at low temperatures (4 °C (39 °F)) Arctic charrs from more northern lakes grow faster.^[5]

Compared to the charr of

The Sommen charr originated in the distant 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.^[3] As the Sommen charr has near-identical mDNA to the Arctic charrs of nearby Lake Vättern and Lake Ladoga in Russia, it is thought these Swedish populations arrived from the east.^[6] While genetic studies have shown there is some gene flow between Sommen charr and other charr populations it is understood that strong natural selection keeps population or subspecies different.^[8]

References