As known salmon is a fish that can live in salt and fresh water. What does salmon eat it depends on the species, time in the life cycle of salmon and location. In the sea salmon eat zooplankton or catch smaller fish. While swimming in fresh water streams salmon eats insects, such as flies, stoneflies, mayflies etc.
When salmon decide tu return back to fresh water for spawning salmon try to preserve energy for fishing, so usualy salmon do not eat during the spawning time. Many salmon don’t make it after spawning and die. Other salmon return back to the sea. Salmon that die in fresh water are eaten by other fish.