The juvenile fish were competing with each other even more for the limited amount of food available in the lake. Once at sea, these small juveniles presumably were more vulnerable to marine predators, because there was no increase in the number of adult salmon returning a few years later. This reinforces the idea that predators are needed to keep the balance in an ecosystem but too few or too many predators can cause harm to a natural system. Every creature needs space. Salmonids need rivers with plenty of water so they can migrate and spawn.
Space is needed so that enough plants can grow in and along the rivers for the insects to eat.
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.