The Arctic charr (also spelled char) Salvelinus alpinus is the fish species that lives the furthest north in rivers and lakes. It is found throughout the northern hemisphere.
Most charrs are anadromous fish, which means they migrate from freshwater to the ocean during periods of their life.
It can become quite large, up to 18 kg has been measured, depending on where it lives. More typical size is 1-2 kg. The charr is characterized and can be identified by red colouring along its underside. Even the red colour depends on where it lives.
Photo: Xepheid, CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0), via Wikimedia Commons)