Many species hybridize. The use of hybrids appears to more common. My question is how fertile are these hybrids and how much danger do they pose to polluting the gene pool of the parent species? I'm sure there are more combinations but these are some that I can think of right now.

Hybrid bluegills of all species
Striped/white bass (wipers)
Muskie/northern pike (tiger muskies)
Walleye/sauger (saugeye)
Blue/channel catfish
Brook/lake trout (splake)

I may not have been clear on my question. Certainly hybrids of the sunfish family (bluegills, redear sunfish, green sunfish, pumpkinseeds etc.) have established wild populations. The potential for back crossing with the parent species always exists. How much of this has actually happened? Are genes of striped bass showing up in wild populations of white bass? Are genes of walleyes showing up in saugers? Or the reverse? Is any of this actually happening in the wild?