Different lakes and regions differ. In warmer parts of the US, bluegills can often spawn several times over the course of the warmer months. The first "spawn" is always the biggest, though.

In bigger lakes, and southern lakes, overharvest may not be as big of an issue, but it certainly can be a major problem on smaller waters and northerly waters.

I've fly-fished local public ponds intensively for the past 4 years, and have seen several ponds get totally destroyed by excessive harvest during the ice-fishing season, and during the spawning season. I've seen it so bad in some ponds that the females will be HUGE with eggs, but they can't find any males on nests to complete the act of spawning, because the bulls have been harvested off their nests. In those ponds, I finally did catch a few males at night, but during daylight hours I caught almost exclusively females. It was pretty clear to me what had affected those populations.

Our DNR says they have NO evidence that angler harvest has hurt bluegill populations in any of the State-managed lakes, but last year they passed a new law restricting panfish harvest to 25 bluegills/sunfish and 25 crappies/day.