People have been urged not to release their unwanted goldfish in local lakes after discovering they can grow as big as a football.
Officials at Burnsville, Minnesota have been startled by a proliferation of the huge aquatic invaders, which bear only a passing resemblance to those bought at pet shops.
The monster goldfish are believed to have been dumped by bored owners when they were little more than tiddlers.
But it turns out that these fish are tough customers. Capable of living up to 25 years, they can survive for months without oxygen.
Minnesota’s fearsome winters – the state regularly sets cold weather records – have little impact on the goldfish as they munch their way through plants until they grow to over a foot in length.
Monster goldfish found in Minnesota lakes
According to the Minnesota Department of Natural Resources, the fish have wrought havoc on the ecological balance of lakes in the state.
An invasive species, the goldfish have harmed native plants and the habitat other fish rely on for their survival.
“A few goldfish might seem to some like a harmless addition to the local water body—but they’re not,” it warned earlier this year.
“Goldfish are in the minnow family and can work their way through city stormwater ponds and into lakes and streams downstream with big impacts, by rapidly reproducing, surviving harsh winters, and feeding in and stirring up the bottom like their close relatives, the common carp.”
Other dumped pet fish to have been found in lakes include koi carp and the somewhat ferocious piranha.
Elsewhere, the Washington Post reported, officials in Carver County, Minnesota removed tens of thousands of goldfish in a single day.
It has since signed an $88,000 (£63,000) contract for a study into how to manage the shoals of monster goldfish.
Similar problems have been reported near Spokane in Washington State and Alberta in Canada.