John, what I suspect happens is that in the pools that have several of the intermediate trout, one becomes dominant and runs off the others which, lacking the protection of the pool, are more easily taken by predators such as herons and mink. There were numbers of fry and fingerlings. Some were even active in some very skinny riffles. I am intrigued by having never seen any bream larger than about 4" in this stream, and those were of an unusual color. They appeared to be more wan and almost jaundiced.

I have seen things similar to this in several streams. I picked an extreme example for my question. I wonder just how isolated the gene stocks of such streams are. Even the sunfish often seem different from fish downstream after the streams have gotten bigger or merged. I suspect that there are the seeds of PhDs in some of these places, just waiting to be grown and harvested...

Thanks,
Ed