I've always thought that fish respond to a combination of water temperature and photoperiod. When fish are brought together, to attractive spawning grounds, around the same time, then they likely use visual cues to "seal the deal". The red fringe on a redear eartab, the white spot on a walleye tail...these sorts of things in combination with length of day and water temperature converge and intertwine to initiate spawning. While the water temperature can be tied to approximate spawning times of some species, fish such as yellow perch have been shown to have gamete development even in tanks based on increase in daily light duration. It is also likely that not all fish of a certain species fire up at the same time, giving us our "rolling spawn" phenomena that would seem to give evolutionary advantage, since fry produced over a period of time are less likely to experience a year where every fish dies off due to an extreme weather event or the presence of a particular predator right at the time of spawn.

I also believe that the one overriding factor as to year class strength is availability of appropriate sized forage during first few months of life. Since one female can produce enough fry for a full year class in most ponds, it seems logical that the year class strength is dictated by how quickly each YOY class can reach sizes that discourage predation, and good enough size to survive first winter, especially in the north.

Holding a redear sunfish is like running with scissors.