Elmer,
As a former Fisheries Biologist, I can attest to the problem of lose and reproducing non-native fish like grass carp. Early on folks figured out that not only would it be bad to have these things reproduce everywhere but some said that the only thing they needed to migrate was "wet grass". They surely can get around and through some pretty skinny water as well.
Early stockings resulted in escapees when ponds over flowed during spring rains. Most states quickly tried to prevent their inevitable spread by passing laws that said only triploid fish who could breed but not produce offspring, would be stocked. Too little, too late, those early stockings had already done their damage.
As I said in an earlier post, once the fish get big they really don't do the job they are stocked for, which is controlling aquatic weed growth. Unfortunately, most folks are misinformed and stock them to get rid of what many call "moss". “Moss” is actually a type of filamentous algae and the grass carp won't eat it. If it doesn't have leaves it’s not on the grass carp's menu.
Now, as a consulting biologist/ecologist, I NEVER recommend putting the dang things in a pond. They just cause more trouble than they are worth. It’s better to address the cause of the nuisance plant growth than use a non-native species as a band-aid, and a poor one at that (just my opinion, no charge).
I guess they do make for great bow-fishing though. Kill 'em all and make room for native populations, I say.
OkKeith