Were your BG pure-strain, or did you order Hybrid BG? If you don't know/remember the particulars, perhaps check your supplier's website and see what they DO sell?
Were you there when the fish were delivered? I did help a Pond Boss member a while back transfer a lot of Red Ear Sunfish (shellcrackers) from his grow-out pond to his main pond. At his (Kansas) pond, the RES were pretty easy to identify at pretty small sizes. If you were on location, you might have been able to see that shellcrackers were going into your pond - even if you were not an expert. Have you caught some "for sure" shellcrackers from your pond?
I agree with ewest, about that fish possibly being a Hybrid BG. It should be BG heavy spawning time in Georgia right now! The male BG will typically get even richer colors during the spawn! Hybrid BG are usually about 80-90% male. Therefore, if you DO have Hybrid BG, and THAT fish is an example, then you might catch many fish that look similar to that over the course of this summer.
Have you caught any pure Green Sunfish in the pond? When you say, " I have big population of a fish that I’m not familiar with in the pond", does that mean there were already fish in the pond prior to your stocking? If so, then lots of different types of sunfish hybrids are theoretically possible.
Sorry to hear that your BG are not doing well. Perhaps read some of the old threads about feeding commercial pellets to your fish. Your BG and shellcrackers both might thank you.
Good luck bringing your pond around!
