They do make great fertilizer, and hog food. If you have the time and taste for them, they are edible smoked (don't care much for the taste myself, and the forked bones are a hazzard if you were to swallow one).
The best recipie I've had was the same one used to can coho salmon--most of the bones are removed, and the smaller ones become edible in the canning process. Looks and tastes like pink salmon, but it's a ton of work and you have to really like canned salmon--one recipie makes a dang bunch.
There's a line everyone has to draw for themselves. More power to everyone that has the time, knowledge, and inclination to use every part of every critter they kill. If you want to use the bladder of a deer for a water bag, more power to you. If you can find a use for the bugs and mice you kill in the house, you have my admiration.
Carp are, in most areas at least, a pest. We can't always utilize them as food. Sure, deer are considered pests in some areas also, and in some places they go to waste, but they have to be thinned out for the good of the herd and the area in general. It makes more sense to me to have some of them go to waste rather than let them be and ruin the entire area. I feel the same way about carp. Fertilize the garden (some Indians used them for fertilizer), feed the hogs if you have them, or let them feed nature. As was already stated, they are non-native and will destroy the habitat of native fish and plants. Ask some Aussies about what non-native critters can do to the ecosystem. Talk to some of the plantation owners there in S.C. about feral hogs. You will get the picture.
Now, with that said, does anyone have a recipie for armadillo? I don't think they are even good for fertilizer.........
Chad