I know there aren't yearly die-offs in the Firehole (some have occurred), but haven't read any documentation of migrations. Certainly many fish move up the tribs. I've been to Yellowstone twice during early August, and caught almost nothing in the Madison and Firehole. But I caught a few.
Also, take binoculars and watch for the wolves at the turn to Slough Creek. Plenty of good waters in that region.
I am leaving this Friday (7/26) for Yellowstone. I've read some reports on line but I wanted to see if you have heard anything from your buddy who guides in the area. I think the weather and water flows for the streams in the NE corner should be perfect. I'll post a report when I get back to let you know how I did.
From the web this week: "Slough Creek Updated July 20
Green Drake, Gray Drake, and PMD hatches may bring up fish midday, but we are actually doing just as well if not better using spent caddis to imitate the egglaying caddis that hatched out the night before. Expect heavy crowds in the First and Lower Meadows."
Would love to be out there fishing right now...
Thanks for the pics!