Let me take a stab at it.
1. Eggs have to survive to hatch. (clean water, remain wet)
2. Juveniles need habitat to hide and food to eat. (clean habitat shelter from predators)
3. Smolt need to out-migrate to the ocean. (avoid predators and don't get the bends )
4. Fish need to eat in the ocean. (avoid predators/harvest, abundant food supply)
5. Fish need to migrate back up rivers. (avoid predators/harvest, adequate passage)
6. Fish need area to spawn. (gravel, clean water)

...and repeat.

Improving any one of these items does not necessarily boost the population as a whole, but hindering any one of these items will decline the population.