If you have a copy of Ian Rutter's book- Great Smoky Mountains National Park, Angler's Companion- it is explained on page 11. I've heard that similar data from several other sources. I've decided that next year, I'm going to start keeping a 7-8 inch 'bow for the pot- releasing larger fish to keep their genes in the "pool" I haven't killed a trout in the park in almost 25 years.
I, also, think WE'VE BECOME BETTER FISHERMEN! More sophisticated, better flies, techniques, etc. are being used on the more heavily fished lower waters. There are "tons" of fish in the streams and we are doing a better job of getting to them.
As a retired HS Science Teacher, I'll tell you the Global Warming stuff scares me. CNN today said Greenland lost enough of its Icecap this past year to cover the entire state of Maryland with 10ft. of water. A lot of people want to look at a specific location or incident to prove or disprove it. You CAN'T do that!! But still it scary..I've heard speculation that the pattern will increase dramatically over the next 10+ years to where it could be irreversible, before we really notice widespread problems. Politics aside, what Al Gore has been saying makes sense.