This will ensure that there's enough insects for the fish to eat. Space is needed so that there are enough rivers with enough water to protect salmonids against a catastrophe. For example, if all the salmonids were in one or two rivers and a disease spread or pollution ruined those rivers, all the salmonids would be gone. If they were living in many healthy rivers, they could overcome the tragedy on any one stream and recover. Spawning alone requires space.
Salmonids require a lot of clean, loose gravel.
The average size of a chinook salmon redd is approximately 60-80 square feet. Another way to think of the need every creature has for space is to imagine that a whole class of students lived on a school bus. Everything they needed they would have to have room for on the bus. They would need a garden or maybe some livestock. They wouldn't have space to exercise and keep healthy.