I think the magic number is 50*. Here's a link .
http://www.rung.us/chas/rivtemp.pl
That whole 50 degree thing is a bunch of bs... (no insult intended) ..... but i have heard that so many times that i cringe
The herring show up by mid april, and have year after year forever .... it is more of a function of the lengthening days that get the herring to move and not the water temp..... and as soon as there are herring here there are stripers..
My biggest fish ever up here was a 42 inch, 34 lb cow that we caught on 4/21 when the water was 41 degrees.... got it on chunk herring that we had netted in the creek... and caught other fish as well that day.
Now........... i will say that the herring won't hit the jigs much until the water gets over 50 ..... and by the time the bulk of the fish arrive after the initial wave the water has typically warmed to the 50 degree mark and everything really gets going..... but as soon as there are some herring here there are stripers
If you can get your hands on herring early season is one of the best times to drift because the fish are spotty but the bait is really spotty so they really stand out ... later in the year there are herring everywhere