Aldo Leopold, I believe, is the one who put it most perfectly:
If you completely disassembled an automobile engine, and didn't know what functions certain parts served - would you throw them away???
Most places on this planet where trout - and other fish species - are not doing well is due to the clumsy hands of mankind. Case in point, Tony's discussion of smallmouth on the Susquehanna - in this case it's heavy pollution damaging a renowned native fishery made famous by Bob Clouser amongst others. Where trout are not doing good, it can be because of pollution, but is generally from erosion/siltation, water warming, and introduction of non-native species. Mack's approach of enhancing natural coldwater flows by unblocking springs is probably the best way of dealing with the warming problem. Siltation and pollution are best dealt with by stopping the sources, which is best dealt with through education and in some cases legislation. The non-native species problem is probably the toughest nut to crack, especially since hatchery operations have spread fish around so widely and it's still an ongoing process though it seems to be getting better. Here now in Michigan, fisherman are REQUIRED to kill such non-native species as Asian carp and non-native gobies. I will certainly kill any that I found in trout or any other waters.
Perhaps the toughtest nut of all is the preservation of non-game species which are NOT highly valued by the public. I won't kill any native fish that I find in a trout stream. Here in Michigan, we have a tremendous variety of native gamefish species and they are often found in many different waters, some even in trout streams (I caught a largemouth in the Pine River, which holds mostly brookies and rainbows, and have also caught rock bass and pumpkinseed in there too). There is a case in which perch supposedly ruined a brook trout fishery, the place I call [REDACTED] Pond for security purposes, so any eating-size perch I catch there go home with me, but that's about it as far as killing the competition goes with me. Perch aren't exactly rare around here, though the Great Lakes fisheries for them fluctuate considerably.
OK, kind of a rambling post, but one more interesting point to make. Tony mentioned the impact on elk of reintroduction of wolves in Yellowstone. A few years ago there was a National Geographic article on the impacts caused by this reintroduction. Remarkably, this actually had an impact on trout fishing! Excessively high populations of elk, from lack of predation, were eating riparian vegetation down to the roots and removing cover, food sources, and shade from stream banks, which of course had detrimental effects to the trout populations. Sure, elk hunters and guides were happy, but too many elk were eating too much and not leaving enough food for other creatures. With the reintroduction of wolves and subsequent predation pressure, elk populations decreased but nearly everything else increased, including trout populations because their riparian cover began to grow back! Moral of the story? Let nature take its course, but where an important element is missing from the equation, add that element back and THEN let nature take its course!
Hope this adds domething to the discussion.
Jonathon
P.S. In a couple of weeks I will be going on a backpacking trip to Isle Royale National Park, where there are moose and wolves. Moose made it to the island several decades before the wolves and nearly ate the place bare! A little predation pressure on large herbivores is a good thing...
No matter how big the one you just caught is, there's always a bigger one out there somewhere...