I've looked, but haven't really found any new studies. I did find a book, Trout, by Stolz and Schnell, that says Rainbows are pretty comfortable up to 70, and can survive up to 83. The same book says Browns grow best between 55 and 60, but can survive to 80. And when another study shows that trout grow best between 65 and 75 F, and another says he won't fish anything above 65 because it kills trout, well, somebody's wrong.
So I'm left to try to form the best opinion I can. I think temperature is a strong factor to consider, and so is extremely low water. And more important than both of these is which stream-or-region-specific factors limit the trout biomass in a stream to begin with. As I pointed out earlier, it seems that in the Smokies, it is all about water nutrients, and whatever fish anglers kill has little or no long-term impact.
The most logical personal policy that I can attempt to create is this: I don't think that temps below 75 hurt the fish much. I don't think that low water hurts the fish much, but extremely low water does. Combined, maybe they do hurt the fish a good bit. But these factors may be irrelevant since the only fish I'm gonna catch during such conditions are small fish, which will easily be replaced next spawn when trout biomass will quickly shoot up to match the available food supply, which again in the Smokies is the limiting factor. And on top of all this, the Browns and Rainbows are invasives anyway, so it doesn't really matter much if I'm wrong about those two.
So the question boils down to this: Does it make you feel bad to fish for stressed trout, even though, in this circumstance, they should rebound? I guess my own answer is no, because the very act of angling for them even under the best of circumstances stresses them out, and kills a few. When they are really stressed, it surely kills more. But in the case of the Smokies, it shouldn't matter very much. Maybe it might make the fishing this Fall somewhat tougher, so maybe it is selfish of me to have this view. But those remaining fish should grow better, and should have more food to eat, and because they are more fit, should spawn better.
So all that is a long-winded way of defining my personal beliefs about fishing in the Southern Appalachians: If wild Rainbows and Browns, go fishing, and don't worry about it. Release them fast and use barbless hooks (like I always do anyway). If wild Brookies, don't fish them when the water is extremely low and extremely hot (above 70) unless the stream has a ton of tiny fish, in which case it doesn't matter. If stocked (any species), then fish regardless of anything.
I feel that some may think my views mean, but so would the PETA-types for us forum-ites fishing at all. I've not attempted to be mean, only to be logical. That's the best I can do.
This has been a fun discussion, although it would have been nice to have a heavyweight, like a trout biologist, share some opinions. I am glad for the opportunity to interact with all yall. You've helped me come to a belief that I can have some comfort with, at least until someone produces some better numbers.