John -- here is the reason that I asked about the largemouth bass management program. I'll try to keep this short, although it can be a complex subject. Also, pardon me if I sound like I am lecturing -- I'm aware that you are a lake manager and mean no disrespect. :-)
Fishery biologists typically apply a protected slot length limit (in this case, 13-16 inch largemouth bass must be released) to a higher density, slower growing bass population. The goal is to reduce abundance of smaller bass, increase their growth rates, and increase/improve the size structure of largemouth bass in the impoundments. This can be a VERY effective tool in small impoundments where the management goal is production of larger bass, and it sounds like it has been effective for you.
However, there is also a trade-off on bluegill sizes when you use this protected slot length limit. Many people, and you'll hear similar comments on this forum, believe that larger largemouth bass better control bluegill populations through predation on intermediate size bluegills (say 4-6 inches). It makes common sense, but it is not correct. High density bass population with lots of small bass (< 12 inches) really think the small bluegills. That's what creates a high quality bluegill population.
Ok, so here's my long-winded point. By successfully using the protected slot length limit on largemouth bass, you should expect a decline in the average size of bluegills in your impoundment. Don't get me wrong: angler overharvest can certainly affect the size structure of your bluegill population, and I'm not excluding that problem. However, your management strategy is designed to increase the size structure of bass at the expense of the size structure of bluegills.
