# Koi — Facts, Threats & Conservation

*Cyprinus carpio haematopterus — ornamental domestic carp (Cyprinus carpio)*

> Koi are ornamental domestic carp bred for pond display — wild Cyprinus carpio is Vulnerable; koi need large filtered ponds, quarantine and never release into wild waterways.

**IUCN status:** Wild common carp Vulnerable in native range; koi are domestic ornamental lines  ·  **WARN range:** Worldwide — ornamental ponds; wild carp native to Europe and Asia

## Quick facts
| Fact | Value |
| --- | --- |
| Wild carp status | Vulnerable (IUCN) in native range |
| Lifespan | 25–50+ years in good ponds |
| Pond minimum | 1,000+ litres per adult — larger preferred |
| Origin of nishikigoi | Niigata prefecture, Japan — 19th century |
| Key disease | Koi herpesvirus — quarantine essential |
| CITES | Not listed — domestic ornamental trade |

## Scientific classification
- **Kingdom:** Animalia
- **Phylum:** Chordata
- **Class:** Actinopterygii
- **Order:** Cypriniformes
- **Family:** Cyprinidae
- **Species:** Cyprinus carpio (Linnaeus, 1758)
- **Variety:** Ornamental koi (nishikigoi)

## Conservation status
- **Status:** Wild Cyprinus carpio Vulnerable in native range. Koi are domestic ornamental varieties; feral releases invasive globally.
- **Population:** Wild native carp declining; billions of domestic carp in aquaculture and ponds
- **Trend:** Wild native populations decreasing; feral populations increasing where introduced
- **Assessed:** 2008 (Cyprinus carpio native range)
- **CITES:** Not listed

## Key facts: Koi
- Koi are domestic ornamental carp — wild Cyprinus carpio is Vulnerable in native range.
- Adult koi need large filtered ponds — not indoor aquariums long-term.
- Koi herpesvirus (KHV) outbreaks kill entire collections without quarantine discipline.
- Released carp become invasive pests in native wetlands worldwide.
- Japanese breeders preserve pattern genetics — each named variety has show standards.
- Pond filtration, depth and predator protection are welfare essentials year-round.

## From food fish to nishikigoi art
Common carp were domesticated for food in China and Europe millennia ago. In nineteenth-century Japan, colour mutations in Niigata prefecture rice-fish ponds became nishikigoi — brocaded carp — bred for red, white and black patterns. Kohaku (white with red), Taisho Sanke and Showa Sanshoku form the gosanke trio prized at shows.

Modern koi export is a global industry — Israeli, Indonesian and British breeders produce show fish worth thousands of pounds. Pattern perfection is art; welfare still depends on water volume, oxygen and disease management regardless of price.

## Wild carp conservation versus ornamental lines
Cyprinus carpio is Vulnerable in its native European and Asian range — dams block spawning migration, wetlands drain and genetic pollution from released feral carp homogenises wild lineages. Koi are not wild conservation targets, but they contribute to invasive spread when pond owners release unwanted fish.

Feral carp in Australia and North America uproot vegetation, increase turbidity and reduce native fish spawning success. Koi keepers must treat fish as lifetime commitments — rehome through koi clubs rather than release.

## Pond design, filtration and stocking density
Adult koi exceed 60 cm and produce heavy bioload — minimum pond volumes of 1,000 litres per fish are conservative; dedicated enthusiasts build 10,000-litre-plus systems with bottom drains, UV sterilisers and biological filtration. Depth below 1 metre protects from herons and extreme temperature swings.

Overstocking causes ammonia spikes, fin erosion and stunting. New fish require four-week quarantine in separate tanks to prevent koi herpesvirus introduction — KHV kills at water temperatures above 18 °C and has no cure. Test kits for ammonia, nitrite, nitrate and pH are mandatory, not optional.

## Health, seasons and predator protection
Koi slow feeding below 10 °C in temperate climates — wheatgerm diets suit autumn; avoid overfeeding in winter when metabolism drops. Spring brings parasite flushes — inspect for flashing, clamped fins and ulcers. Herons, otters and cats kill pond fish — netting, deep water and motion deterrents reduce losses.

Handling stresses fish — use koi socks and bowls for vet exams only. Anaesthesia by trained vets enables safe examination; never grab scaled skin. Long-lived koi outlast many owners — succession planning prevents neglect when ponds are sold with houses.

## Aquarium welfare and related WARN guides
Koi are pond fish — keeping juveniles temporarily in aquaria is acceptable only with filtration sized for rapid growth. Readers interested in smaller carp relatives should explore WARN's goldfish guide for tank-suitable varieties and our betta guide for labyrinth fish welfare in heated indoor tanks.

All ornamental fish share one rule: never release into wild water. WARN links pond stewardship to freshwater habitat protection — wild carp recovery and invasive carp control are opposite sides of the same conservation coin.

## What WARN does
WARN publishes free freshwater welfare and habitat education. Wild carp decline and invasive carp spread both reflect human movement of domestic fish — honest koi guides reduce releases that harm native ecosystems in partner countries.

If this guide helps you understand wildlife and the pressures it faces, a gift to WARN supports habitat protection and free public education in our partner countries.

## Frequently asked questions: Koi
### Can koi live in a fish tank?
Juvenile koi can grow in large aquaria temporarily, but adults exceed 60 cm and need pond volumes of thousands of litres with professional filtration. Long-term tank keeping is inappropriate.

### How long do koi live?
Well-maintained koi often live 25 to 50 years or more. Lifespan shortens with overstocking, poor filtration and disease outbreaks such as koi herpesvirus.

### Are wild carp endangered?
Common carp (Cyprinus carpio) is Vulnerable in its native range due to habitat loss and hybridisation with feral domestic carp. Koi are ornamental domestic lines, not wild conservation targets.

### Can I release koi into a local lake?
Never. Released carp and koi become invasive, damaging native wetlands and outcompeting local fish. Rehome through koi clubs or pond societies.

### What is koi herpesvirus?
KHV is a deadly viral disease of carp spreading through unquarantined new fish. It kills at warm temperatures with no cure — strict quarantine prevents outbreaks.

### How big should a koi pond be?
Plan at least 1,000 litres per adult koi with deep water, filtration rated for total bioload and predator protection. Larger ponds are easier to maintain and reduce stress.

## Sources
- [IUCN Red List — Cyprinus carpio](https://www.iucnredlist.org/species/6181/12386523)
- [OATA — pond fish welfare](https://www.ornamentalfish.org/)
- [ZNA — Zukunft Nishikigoi (koi standards)](https://www.zna.org/)

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Full guide: https://worldanimalrescuenetwork.org/wildlife-guides/koi