Koi

Kohaku koi（圖片來源：Maxikoi，採用 CC BY-SA 3.0 授權）

Koi (Cyprinus rubrofuscus haematopterus) are ornamental strains selectively bred from the East Asian carp (Cyprinus rubrofuscus). Wild carp are widely distributed throughout East Asia, inhabiting large river basins across China, Korea, Russia, Vietnam, and Laos. In the wild, these fish typically display subdued coloration, most often dark brown or black, which allows them to blend into the bottom environments of lakes, rivers, and ponds. They are omnivorous, feeding on aquatic insects, mollusks, aquatic plants, and detritus, and they are known for their remarkable environmental adaptability.

As humans began domesticating and breeding carp, variations in coloration gradually appeared. In Japan's Niigata region during the nineteenth century, farmers began deliberately selecting individuals with red-white patterns, mottled markings, or metallic sheen for breeding. Over time, this practice produced the ornamental fish now known as koi, distinguished by vivid colors and striking patterns. Today, many well-known varieties exist, including Kohaku with red and white patterns, Taisho Sanke with three-color markings, and Showa Sanshoku featuring red and white patches over a black base. Additional forms include the long-finned Butterfly Koi and the sparkling Ginrin varieties characterized by reflective scales.

Koi are exceptionally long-lived fish. Under proper care they can survive for more than fifty years, and historical records indicate that some individuals have surpassed a century in age. They are also large fish, with adults commonly exceeding 60 centimeters in length and occasionally reaching over one meter. Koi are generally gentle in temperament and readily become accustomed to human presence. During feeding, they often gather actively at the surface and may even learn to take food directly from their keeper's hand. In both Japanese and Chinese culture, koi symbolize good fortune, perseverance, and prosperity, which has contributed greatly to their popularity.

Taisho Sanke koi（圖片來源：Paulman，採用 CC BY-SA 3.0 授權）

Hi Utsuri koi（圖片來源：Benutzer:Fehlwort，採用 CC BY-SA 3.0 授權）

Housing Requirements in Captivity

Because koi grow large and live for decades, they are best maintained in specially designed outdoor ponds rather than conventional aquariums. When constructing a pond, the expected adult size of the fish should be considered. Each koi should ideally have at least 250 liters of water available, and even a small group often requires a pond exceeding one thousand liters in volume to remain healthy. Pond depth should reach at least 1.5 meters, and in colder regions depths greater than 2 meters are recommended to prevent winter ice formation from threatening survival.

Although koi are highly adaptable, their optimal water temperature ranges from 15 to 25°C. Prolonged temperatures below 10°C can weaken immune function, while temperatures above 30°C may disrupt metabolism and increase susceptibility to disease. Water pH should generally remain between 7.0 and 8.6, and effective circulation and filtration systems are essential. Because koi produce substantial metabolic waste, including ammonia and organic matter, ponds must be equipped with strong mechanical and biological filtration. Ideally, the entire pond volume should pass through the filtration system within two hours.

Predator prevention is another critical aspect of pond management. The bright coloration of koi makes them highly visible to animals such as herons, ospreys, raccoons, and feral cats. Pond design can reduce predation risk by incorporating steep vertical edges, installing protective netting, or planting vegetation that provides shaded refuge areas. The pond should also avoid constant exposure to intense sunlight, which may cause excessive water temperature fluctuations and promote algal overgrowth.

Feeding Conditions

Koi are omnivorous fish and benefit from a varied diet in captivity. Commercial koi feeds are commonly available as floating pellets, which encourage fish to feed at the water surface and allow keepers to observe their health and behavior. However, since carp naturally forage near the bottom, sinking feeds may also be included to support natural feeding behavior.

In addition to formulated feed, koi readily consume vegetables and fruits. Shelled peas, spinach leaves, lettuce, and sliced fruits can all serve as nutritional supplements. If insects or frozen foods such as crickets, bloodworms, or shrimp are offered, their sources should be clean and the food should be thoroughly thawed to prevent contamination by bacteria or parasites. Each feeding should be limited to the amount the fish can consume within about five minutes in order to prevent uneaten food from decomposing and degrading water quality.

Feeding frequency should be adjusted according to water temperature. When water temperatures range from 20 to 25°C, koi can be fed twice daily. If temperatures fall below 15°C, feeding should be reduced to once per day. When water temperatures drop below 10°C, digestion in koi slows dramatically and feeding is generally suspended, allowing the fish to survive winter largely on minimal natural food sources such as algae.

Breeding Conditions

Koi breeding typically occurs during spring and early summer when water temperatures stabilize above 20°C. During spawning, males pursue females and nudge them with their bodies to stimulate egg release. A single female can produce several hundred thousand eggs. The eggs are adhesive and attach to aquatic plants or artificial spawning materials. Fertilized eggs usually hatch into larvae within three to five days, though survival rates are extremely low, as many fry are eaten by other fish, including their own parents.

For most hobbyists, allowing koi to reproduce naturally may be interesting but is difficult to manage. Professional breeders usually prepare separate spawning ponds and remove the parent fish after egg deposition to prevent egg predation. Newly hatched fry require extremely fine food, such as water fleas, diluted egg yolk suspensions, or powdered fry feed, and they gradually transition to pellet diets as they grow.

Selective breeding for coloration presents an additional challenge. Most offspring develop dark, non-ornamental coloration, while only a small proportion possess desirable patterns. For this reason, professional breeders conduct repeated culling at different growth stages, retaining only individuals with superior color and pattern characteristics.

If koi spawn naturally in a home pond, casual observation is generally sufficient. Attempting to raise large numbers of fry is not recommended, as overcrowding can rapidly degrade water quality and lead to disease outbreaks.

Preventing Invasive Spread

Koi are ornamental fish produced through human selection from wild carp and do not naturally occur in the wild. If they are intentionally released or abandoned into natural waters, they may create significant ecological problems. Koi can interbreed with wild carp, resulting in genetic mixing that may threaten the survival and ecological stability of wild populations. Furthermore, wild carp are native primarily to eastern Asia. When introduced into non-native regions, they can damage local ecosystems. For example, invasive Asian carp populations have already become a major environmental issue in parts of the United States and Australia, where uncontrolled spread can lead to severe ecological consequences.

For this reason, fish keepers should strictly follow the principle of not abandoning or releasing captive animals into the wild. If a keeper can no longer care for their koi, they should seek alternative caretakers or contact facilities that may accept them, such as koi farms or specialized fish keepers. Ensuring that pets are responsibly rehomed is an essential aspect of responsible animal care.

Author: Shui-Ye You