"In Australia there is enormous anecdotal and mounting scientific evidence that introduced carp are the cause of permanent turbidity and loss of submergent vegetation in the
Murray-Darling river system, with severe consequences for river ecosystems, water quality and native fish species.
[18] In
Victoria, Australia, Common carp has been declared as noxious fish species therefore there is no restriction on the quantity that a fisher can take.
[19] In
South Australia, it is an offence for this species to be released back to the wild.
[20] An Australian company churns common carp into plant fertilizer.
[21]
Common carp were brought to the
United States in 1831.
[22] In the late 1800s they were distributed widely throughout the country by the government as a foodfish. However, common carp are no longer prized as a foodfish in the United States. As in Australia, their introduction has been shown to have negative environmental consequences
[23] and they are usually considered to be invasive species. Millions of dollars are spent annually by natural resource agencies to control common carp populations in the United States.
[24]"
That quote is from Wikipedia.............but there are hundreds of studies you can find on the Web that support this, in Europe Carp are kept under control because fishing is so popular for them ....thanks Joe.