from the anecdotal evidence Ive seen of stray fish, its a natural occurence. Cedar creek has hatchery fish following the egg box fish into the creek to spawn. I think the fish in the video was a hatchery fish. I dont know when fish decide to pair up. It would appear stream scent isnt the overriding concern. I read in one article the bio for the Issaquah creek tribe took hatchery pairs and put them in creek itself or supplemental creeks to lake samammish. At some point the fish are so ready to spawn, they just want to find the gravel.