My nephew is an outfitter in that area, and packs into the Bechler area in the SE corner of the Park. Here's his website, and if you'll click on the photo gallery section, and then on the Yellowstone/Bechler
area you can see some pictures of the area. Just ignore the people and horseys, and look at the area behind them. The area is accessed from the Bechler Ranger station east of Ashton, Idaho. You might e-mail him for details. I got too old, before I could go in there myself. I've heard the stories of great fishing, scenery, and mosquitos, for years.
http://www.dryridge.com/hosts.htm
Hope this helps.
They're just fish, right? Right?