Had a trip down there last June. A commercial gude said the NPS pays someone to go down there and kill the trout. They claim they interfere with the native species. Well, duuuhh, removing the DAMN will really help the native fish. Long live the grand canyon trout as long as the DAMN is there. Throw them back just to antagonize the NPS.
I caught quite a few nice rainbows mainly Nankoweap and above,however, I did catch trout as far downstream as mile 110. used #6 brown wolly bugger, 5 wt. fly rod. 6 wt, would be best in the main river. In Tapeats I used beaded nymph #14 or 16 and caught dozens while the others in the group went on a hot hike, I had more fun. Great fishing.