Over here in australia they have wrecked our major water systems. They were in plague proportions about 10 years ago but have been reduced through the protection of the silver bream which is supposedly eating the carp eggs. They are ok to catch but it is illegal (thats right illegal) to throw one back once you catch it, you must kill it in whatever way suits. They have wrecked the murray river from which my hometown is on, turning the waters muddy, killing the ecosystem and reducing the numbers of native fish.
(stupid gold fish)