There are others on here that are more knowledgeable about their movements, I primarily will only target stripers when they are on top. My personal opinion is that for top water fishing on the dam end, the prime time is all of May into about mid-June.
Every year seems to be a little different, I never know what I'll find when I go out looking for them. Get a nice warm May morning with flat water and you can see them from a long ways off when they come up on top. It really helps if you can get out and look for them, usually a good indicator is seeing some guide boats in the area.
I would not surprise me to see some stripers around Starkey in April, might be a bit early, but who knows?