It could be offspring from hatchery fish spawning naturally in the river or just unclipped hatchery fish that someone just didn't clip ,I know that at the Snoqualmie hatchery they hired some people that thought for 7.00 hr. it was too much trouble to clip All the fish. when they swam into the trap they were checked to see if they were true natives and none of them were so they used them for spawning.