The development of regular seasonal habitat shifts in a landlocked Arctic charr, Salvelinus alpinus L., population

Parts of the Arctic charr population of the subarctic Lake Visjön in north‐west Sweden migrate upstream during the spring to two small, recently eutrophied and very productive lakes. Large repeat migrants arrive first, followed by young first‐time migrants. Charr in the small lakes grow more rapidly than those resident in L. Visjön. In early September mature fish leave the lakes, followed by immature fish later in September and in October. Overwintering and spawning takes place in L. Visjön. Migratory females attain maturity at age 4 years and resident females at age 6 years. The migrant fish return annually until they are 5–6 years old. This limit may be due to reduced relative growth benefits of the habitat shift for larger individuals. The rapid development of these regular habitat shifts could be explained by an internally fixed exploratory behaviour in these Arctic charr that makes the detection and utilization of distant feeding resources possible. Migrants will possess a considerably higher fitness, if survival rates for migratory and resident fish are equal.