here's a response I got from our local DNR guy;

Hi Steve:
While I would like to be there to cast a spear through the heart of the last carp in North America, we can't endorse the belly-slitting you refer to. There would be a few problems:
1.) The practice of throwing dead fish, or fish parts into the water constitutes litter. It is sometimes referred to as offaling the water. Wardens will write tickets for this. We have many locations where it is a persistent problem. When the dead carp are returned to the water, they have a tendency to drift with the currents, either wind, waves or stream flow to a location where they pile up and create an obnoxious smell. We also don't want anglers to throw the carcasses on the bank unless it is in an isolated area, and the landowner has granted permission for it. We don't want them left on the banks in parks, public fishing or hunting areas, and we don't want them thrown into dumpsters or garbage cans. In some cities, such as Jefferson the problem of disposing carp in dumpsters has prompted us to work with the city to post signs letting people know that it is legal to release any fish they catch.

2.) Even though carp are a nuisance in most of our waters, they are a resource................so (probably no one has ever received a ticket for it) slitting bellies would still be "wasting a resource."

3.) In waters where we have commercial contracts, the effect of anglers in controlling carp by catch-kill-and release would be insignificant. Even organized archery tournaments will usually only yield about 10,000 pounds of carp. Our commercial contractors are removing over 1 million pounds per year on the Lake Koshkonong/Rock River system.

Our recommendation is the same with any other litter - take it home with you. You can cook them, smoke them, bury them in your garden. You can bag them and put them out with the trash. People with chickens or pigs may be able to handle a few.

Aside from that, the world is changing. To save money, more and more people are fishing carp for food. And as a recreational resource, tournament fishing for carp (catch and release) is on the increase.

Another growing concern comes from the anti-hunting/anti-fishing crowd that paints anglers and hunters as a blood thirsty lot. Having piles of dead carp wash up on the shore only adds fuel to their fire.

Hope this helps

Don Bush
Regional Fisheries Team Leader - East Fisheries Team
DNR Service Center