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In the wild

Koi have been accidentally or deliberately released into the wild in every continent except Antarctica. They quickly revert to the natural coloration of an Amur carp within a few generations. In many areas, they are considered an invasive species and a pest. In the states of Queensland and New South Wales in Australia, they are considered noxious fish.

Koi greatly increase the turbidity of the water because they are constantly stirring up the substrate. This makes waterways unattractive, reduces the abundance of aquatic plants, and can render the water unsuitable for swimming or drinking, even by livestock. In some countries, koi have caused so much damage to waterways that vast amounts of money and effort have been spent trying to eradicate them, largely unsuccessfully.

In many areas of North America, koi are introduced into the artificial " water hazards " and ponds on golf courses to keep water-borne insect larvae under control through predation.