Used to take a trip to the pothole about this every year, usually coincided with MDC doing the night shocking for brood stocking. I am not sure temp is the key to trigger spawn but suspect a combination of day length and temp play an important key plus not all of them spawn at the same time but over several weeks, just my 2 cents wished I lived closer and knew the area better because the walleye fishery on the rock is really picking up.