Hey Slipstream -
Man, I really wish you were right about the rainbows being sterile but I'm afraid you may not be. The only sterile stocker rainbows are triploids (which have an extra chromosome). There's nothing inherently different about any other kind of stocker from a stream bred rainbow. They have grown to a larger size than would be possible in a Smokies river environment in a shorter period of time, but they are definitely capable of reproduction if given time.
I've caught a lot of wild mountain rainbows in the S.E. In my experience, the males become reproductively active at about 14" long while the females may wait a little while longer. But that's assuming a lot of food, etc. (like a stocker). If you put in a 12" hatchery stocker it would most likely be a couple years before it was ready to spawn, assuming it survived in its native environment.
Once it did spawn though, its offspring would stunt to fit the environment just like any other stream-born trout. They would hit sexual maturity in a few more years (3-4) and at a much smaller size. Thus we're back to where we started.
Those hatchery fish need to be caught out ASAP. Rainbows mostly spawn in spring but they will begin spawning runs in the mountains as early as December. If one of them was 14" long, it might be ready to start this year. Moreover, those fish will be in great shape as the only fish of their size left in the river.
No one has said so but this stocker might have been motivated by simple big fish greed: as the only rainbow in the river, starting at 14", it isn't out of the question for those fish to top 20" or more even in skinny water like Lynn Camp Prong.
Active spawner 14" rainbow (male):
(Caught the first week of January).