Koi are ornamental varieties of the domesticated common carp that originated in Japan, kept for decoration in outdoor ponds with clean, oxygenated water[6]. When koi are accidentally or deliberately released into the wild, they quickly revert to the natural coloration of an Amur-type carp within a few generations and are considered an invasive species and pest in many areas[7][2].The common carp species itself, of which koi are a domesticated form, is widespread in eutrophic lakes and rivers across Europe and Asia in its native range, but is included on a list of the world’s 100 worst invasive species where it has been introduced elsewhere[1]. It has been introduced to most continents and some 59 countries[1].
Why released koi become invasive
Released koi revert toward the coloration of a wild Amur carp within a few generations, blending into feral carp populations[7]. Koi and carp feed by grubbing through bottom sediments, which greatly increases the turbidity of the water they inhabit[2][1]. This stirring of the substrate makes waterways unattractive, reduces the abundance of aquatic plants, and can render water unsuitable for swimming, drinking, or livestock use[2]. In feeding, carp may destroy, uproot, disturb and eat submerged vegetation, damaging food sources and habitats used by native fish and waterfowl[1].Because koi are an artificially bred form, their release can also cause genetic pollution through breeding with native or feral carp populations[2]. In some countries, koi and carp have caused enough damage to waterways that substantial money and effort has been spent trying to eradicate them, largely without success[2].
Australia
In Australia, koi are classed as noxious fish in the states of Queensland and New South Wales[2][8]. More broadly, the common carp was introduced to Australia over 150 years ago but was not recognized as a pest species until the ‘Boolarra’ strain escaped in the 1960s, after which it spread throughout the Murray–Darling Basin, aided by major flooding in 1974[1]. Carp have since established in every Australian state and territory except the Northern Territory[1]. In the Murray–Darling Basin they constitute 80 to 90 percent of fish biomass[5].Victoria declared the common carp a noxious fish species in 1962, with no limit on how many a fisherman may take[1]. In South Australia, it is an offense to release this species back into the wild[1]. The Australian government has investigated using the carp-specific koi herpesvirus (Cyprinid herpesvirus 3) as a biological control agent through the National Carp Control Plan, funded at A$15.2 million in 2016, though a 2020 assessment found this approach unlikely to work because of the fish’s high fecundity, and its environmental safety and efficacy have also been questioned[5][1]. Carp is described as a major pest species impacting freshwater environments, amenity and agriculture in Australia, and is listed among invasive freshwater fish there alongside species such as redfin perch and mosquitofish[5][9].
New Zealand
New Zealand's Department of Conservation lists koi carp among animal pests threatening the country's native fish, insects, birds, lizards and frogs, which evolved in isolation for millions of years and are considered at risk from pests and other threats[3]. New Zealand's biodiversity strategy frames such management within the Te Mana o te Taiao – Aotearoa New Zealand Biodiversity Strategy 2020[3].New Zealand also restricts ornamental aquatic species more broadly: for example, water hyacinth, sometimes sold as an aquarium or pond plant, is listed on New Zealand's National Pest Plant Accord, which prevents it from being propagated, distributed, or sold[10], illustrating the country's general stance on preventing escapes of ornamental aquatic species into waterways.
United States: state-level restrictions
Regulation of koi and other ornamental fish varies by US state. In Alaska, it is illegal to release ornamental fish, fish waste, or wastewater from ornamental fish into state waters, including by pouring them down a storm drain, and cultivation of ornamental species is regulated under state law[4]. Alaska notes that even species not expected to establish reproductive populations can sometimes survive in its waters, citing non-fish examples such as invasive waterweed (Elodea spp.) found growing even under ice[4].Maine classifies captive fish and wildlife into unrestricted, restricted, and prohibited categories; species may be placed in a restricted category if they have a geographic distribution and life requisites that would allow them to survive in Maine if accidentally or intentionally introduced into the wild[11]. Unclassified species may not be possessed until categorized[11].Florida's aquaculture regulations require facilities that hold non-native species, including ornamental fish, to prevent escape into waters of the state, with Best Management Practices covering holding, transport, and culture systems[12]. Restricted non-native species require written authorization from state authorities, renewed annually, and importers may need authorization from wildlife agencies before bringing non-native aquatic species into the state[12]. In the United States generally, common carp were brought in 1831 and later distributed widely as a food fish in the late 19th century, but are now rarely eaten and are generally considered pests[1].
Ornamental and pest-control use in man-made waters
Despite restrictions in open waterways, koi are introduced in many areas of North America into artificial ‘water hazards’ and ponds on golf courses, where they are used to help control water-borne insect larvae through predation[7]. This contained use is distinct from release into natural or connected waterways, which is the activity most commonly restricted by invasive species laws[2].
Context: other invasive aquatic species
Koi and carp are part of a broader pattern of problematic introductions tied to the aquarium and water-garden trade. Many invasive aquatic plants, including water hyacinth and New Zealand stonecrop, were originally sold as oxygenating or decorative plants for aquaria and garden ponds before being disposed of into the environment[13]. Similarly, non-native aquatic plants and animals cultured by aquaculture facilities are an environmental and legal concern if they escape, prompting regulations such as Florida's containment requirements[12]. Other notorious invasive species discussed alongside carp in Australia include the introduced red fox, also listed among the world's 100 worst invasive species[14].
Common questions
Are koi considered invasive everywhere?
No. Koi are kept as ornamental pond fish worldwide, but they are classed as invasive or noxious specifically where they have escaped or been released into open waterways, such as parts of Australia and New Zealand[2][3].
Why are released koi harmful to waterways?
Released koi revert toward wild carp coloration and behavior, and their bottom-feeding habit increases water turbidity, reduces aquatic plants, and can make water unsuitable for swimming, drinking, or livestock use[7][1].
Is it legal to release koi into ponds or rivers in the US?
It depends on the state. Alaska makes it illegal to release ornamental fish into state waters[4], while Florida requires aquaculture facilities to prevent any escape of non-native species such as koi[12], and Maine restricts possession of species capable of surviving if released into the wild[11].
Are koi the same as the common carp considered a pest in Australia?
Koi are a domesticated ornamental form of the common carp, and when released they interbreed with wild and feral carp, contributing to the carp populations classed as noxious fish in states such as Queensland and New South Wales[2][1].
Has any country tried to eradicate feral koi or carp?
Yes; some countries have spent large amounts of money and effort trying to eradicate koi or carp from waterways, largely without success, and Australia has explored biological control using a carp-specific herpesvirus through its National Carp Control Plan[2][5].
^ Fish and Wildlife in Captivity: Fish & Wildlife: Maine Dept of Inland Fisheries and Wildlife. Maine Department of Inland Fisheries and Wildlife. maine.gov/ifw/fish-wildlife/captivity/. Retrieved November 2026.
^ UF/IFAS EDIS FA195: Preventing Escape of Non-Native Species from Aquaculture Facilities in Florida, Part 1. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA195. Retrieved November 2026.
