The short answer to this is “to spawn”. Salmon are born in freshwater, usually in cool fast-flowing water. They live in the river for about two years before making their way out to sea. In the case of Atlantic salmon (salmo salar) populations in Britain, they migrate to the seas around Greenland, where they normally spend one to two years. After growing considerably in size, they return to the same river where they were born using chemical cues and battle their way upstream to the spawning areas – the idea being that if the area was good enough for them, it will be good enough for their offspring. Sadly the effort of returning and fighting for territory and digging nests (known as ‘redds’) in the gravel wears them out so much that they usually die after spawning.
Answered by James Maclaine, Department of Zoology, National History Museum, London.