Studies on ponds and lakes can have great value in general, but researchers will tell you that ponds are open systems with a myriad number of variables. Mortality rates can be impacted by weather, angling pressure, predator/prey balance, availability of food items and water chemistry. It's a lot more accurate to use scientific sampling measures such as gill nets, trap nets, or electrofishing to see what HAS happened than it is to use a single study to guess what WILL happen.
Crossing two hybrids never yields a pure bred offspring in any percentage. The common term is F2 hybrid. Crossing two green sunfish/bluegill hybrids would result in an entire generation of F2 hybrids, none of which are either completely bluegill or green sunfish. Interestingly enough, a lot of the F2 may "look" like a green sunfish but they really aren't. F1 hybrids possess what is known as hybrid vigor or heterosis. Many F1's are non-viable and die early but you never see these. You only see the ones that survive and these fish have the best growth characteristics, water temperature tolerance, disease resistance and feed conversion. F2's generally have poor survival, poor growth rates and variable morphology or body type. This is probably due to pairing of recessive non-adaptive genes present in the genes of both parents, i.e. two recessive genes popping up in the same individual. F2's of most hybrids are also non-viable so they make for pretty lousy forage production.
