Most grass carps that are introduced nowadays to combate the weed problems of a lake are unable to breed. And I've personally been to lakes that post that it is unlawful to keep a grass carp in your creel. This has been in Missouri and in Kentucky. These lakes were fairly small and self contained and very heavily weeded. The other fish in the lake seemed to be doing quite well so if an un-native species is introduced nowadays with the proper percautions and foward thinking then it shouldnt damange any waterways.