As the years go by and the more and more I fish through winter and into spring, I'm more and more convinced the whole spawning thing has everything to do with the days getting longer more than anything else. Rising temperature, it seems, is simply something that happens along the way, but is not related, other than it increases their metabolism.
I have no facts to back this up. Only my observations on the water. And yes, I've seen bass spawn in 54 - 75 degree water.
I just think they're genetically programmed to spawn during a particular time of the year, based off of length of day. Some are early, some are late. Nature's way of preserving the species.