Ah-ha!! Now I understand your concern, Ty. I believe you can expect annual spawns of bluegills, so I think that should not concern you. However, I believe you are right to wait to harvest your "brood stock."
I have an old (1970) report from the MN DNR (W.J. Scidmore, Section of Fisheries, Special Publication No. 1) that has Minnesota statewide average growth rates for various fish species. Now, the DNR certainly has newer information than this; however, it's nice to use this for comparison purposes for situations such as yours. Bluegills in the North Country tend to get off to a slow start.
Scidmore's report says that the Minnesota average bluegill lengths (inches) at ages 1-10 are 1.9, 3.4, 4.9, 6.1, 7.1, 7.8, 8.3, 8.6, 9.1, and 9.6 inches. So, even if your 10-12 inch bluegill broodstock came from a more fertile water body, you can bet they are pretty darn old.
Now, newly stocked waters (i.e., less intraspecific competition) often have faster growth rates than "average." However, you and I both obviously have some concern over fertility in a gravel pit. Lower fertility probably means slower than average growth. This summary also contains northern MN populations as well as southern populations, and "stunted" populations as well as high quality populations. So, the range in lengths at age is lost by the use of averages.
In your case, your first spawn of bluegills may not be able to spawn themselves for another year or two, which is a good reason to keep your adults in the pond and not on the table. I would not harvest bluegills for at least another couple of years [I'm pretty conservative on these things
]. If you really want to eat some bluegills, could you find some intermediate bluegills to add this year, and bolster your brood stock with a little diveristy in size?
Hope this helps.
