Majority, not all, but majority of these salmon are from hatcheries, its kinda sad actually that we can only sustain the population through hatcheries. However, that being said, most of these fish are unsuccessful in their spawn, and they die right after spawning, so it's a put and take fishery. Your license dollars go towards putting more of them back into the lake!
EDIT- and also, as Terry said, most rivers or creeks have an area well known for spawning which is closed off, most areas being dams where the fish gather in large numbers. I go to school at Fleming College and we are very actively involved in salmon rehabilitation, especially of Atlantic Salmon. We obtain eggs from salmon directly in the rivers and hatch them at our school, we stock different levels of fingerlings and yearlings back into the creeks and rivers of Lake Ontario. We are hoping for a self sustaining population by 2020.