You can actually GRAPH the past years runs of salmon, for whatever dam you want to fish below, at this site:
http://www.cqs.washington.edu/dart/adultpass.html
It's unbelievable the info that is available on this site I just mentioned.
April 25 seems to be the peak at Bonneville, by the way....so I'm GUESSING April 23 is the peak where you want to fish...but I haven't really checked it out more than this, as such.
But closures might effect it, I don't know.
Good luck.