John, from my perspective it looks like those RESxGSF have exaggerated GSF features. Maybe it’s just picture angles, but they look to have larger longer mouths than a normal BGxGSF cross.
I get a range of features. Some show more GSF than others. Had I not seen hundreds and hundreds of hybrids from fingerlings to 9+", it might be tempting to just call that fish a GSF. But all you have to do is get a real GSF beside it and the hybrid features become very evident.
The mouth is large on that fish, but if it were a pure GSF either the mouth would be bigger or the fish would not be as tall for that size mouth. The mouth to height (tallness) of the fish is off for a Pure GSF. Pure GSF will have the same size mouth as a LMB of the same length.
Another thing is the opercular tab. These fish have definite darker orange tab borders than what my GSF have. My GSF have a translucent border anywhere from almost white to an orangeish cast. But it is always translucent. It may not show well in the picture but this fish has a mixture of translucent and solid pigment (RES is solid). Some will have an orange dot in the middle of the translucence. Pure RES will always be solid color and none of mine wrap around the opercular tab. They are on the end only whereas the GSF margin wraps around.
I also get hybrids out of my sediment pond. There it is harder for me to tell for sure if they are CNBG/GSF or RES/GSF. Both potential crosses can be made there as well as potentially northern BG because during high water events they can swim up into this pond (with difficulty but they get there - I have seen them do it). That pond had CNBG and RES originally stocked. The nice thing about this forage pond is that it has never had BG stocked and out of hundreds of fish caught and thousands trapped I have never seen a BG. That gives me confidence that I only have to determine if it is either 1. RES, 2.GSF or 3.a hybrid thereof. That makes it a lot easier for me.
In my main pond I originally had 40 or so HBG as contamination at stocking time. I religiously fished them out and transferred them to my old refurbished pond that already had GSF in it. Got rid of those "things".
About the time I could not catch any more or at least would very rarely catch one (they are much easier to fish out because of their propensity to bite a hook compared to BG) I decided I really did like hybrids. I'm a pan fish guy, not a big bass guy. So I started stocking my main pond with home raised hybrids from my forage and sediment ponds. Both CNBG/GSF and RES/GSF.
In fact I had even thought about specifically stocking a few GSF females to get some GRES (RESxGSF hybrids). Well as luck or fate would have it, it happened by accident instead of intent.
I get LOTS of naturally occurring hybrids out of my two small ponds. My thinking is that the reason I get so many hybrids is two fold. One is I fish out and remove as many GSF as I can (both trap and hook). So the GSF population stays relatively low compared to the other sunfish. Second both of these ponds have very low visibility most of the year. The forage pond because of a very heavy planktonic algae bloom and the sediment pond mostly because of the high rate of runoff from agriculture ground. I surmise it is the combination of the GSF having limited mating options and the turbid water that give me so many hybrids.
All the hybrids get moved to my main pond. We have caught some up in the 9" range recently and filleted them. I really like the fish. And when the pure BG get persnickety about getting caught I can nearly always depend on a hybrid accommodating my hook.
