Originally Posted by al_a
Don't know when you're talking about by saying "spring", but hope you know that fishing season within the park doesn't open until Memorial Day weekend.
Exactly. I was there June 10-20 this year. The Firehole (especially) changes weekly as it warms. Local weather matters a lot; this year the Firehole stayed good longer than a year ago.
Best to check on this forum, fishing reports on the web, and ask at shops in the area when you arrive.