Those tiny scanes of eggs are for next year. The fish can either stay in the river for another year or head out to the salt and chill till next december. From what I understand these fish do not grow at all over this next year and were actually the biggest they will ever be before they spawned this winter.

He said they get ~7% return spawning on hatchery fish.

Im not a bio, but the fish checker at wilsons recently gave me the story when I asked the same question.