Atlantic salmon
The Atlantic salmon is an anadromous fish. They spend their first years in freshwater, and when they are big enough they undergo physiological and morphological changes, and migrate into seawter as a ’smolt’.
The smolt migrates from the rivers, throughout the fjords and into the open seas where they spend one to four years. In this period they grow fast, and when they are sexually mature, they migrate back to the river where they were born, to spawn.
The adult and sexually maturing salmon enters the rivers from early spring until late autumn. The spawning takes place from late autumn until mid winter. In Norway the salmon spawns within the same year they enter the river. In other areas (e.g. Scotland) some salmon enters the rivers more than one year before they spawn.
Normally the salmon return to the river where it was born, and in larger river systems to the specific part of the river where it was born. This is called homing. The mechanism behind this homing it is still not fully understood. It has been proposed that the salmon use its smell to be able to orientate itself during the later part of the migration. The theory is, however, debated and it cannot explain how the salmon finds its way over the seas. New research indicate that complex signals from several stimuli is imprinted by the smolt on its migratory route, making it possible to find the way back to the river.
