Koi (鯉, English: /ˈkɔɪ/, Japanese: ) or more specifically nishikigoi (錦鯉, Japanese:, literally "brocaded carp") are colored varieties of the common carp (Cyprinus carpio) that are kept for decorative purposes in outdoor koi ponds or water gardens.
Koi is an informal name for the colored variants of C. carpio kept for ornamental purposes. There are many varieties of ornamental koi, originating from breeding that began in Niigata, Japan in the early 19th century.
Several varieties are recognized by the Japanese, distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, orange, yellow, blue, brown and cream, besides metallic shades like gold and silver-white ('platinum') scales. The most popular category of koi is the Gosanke, which is made up of the Kōhaku, Taishō Sanshoku and Shōwa Sanshoku varieties.
The words "koi" and "nishikigoi" come from the Japanese words 鯉 (carp), and 錦鯉 (brocaded carp), respectively. In Japanese, "koi" is a homophone for 恋, another word that means "affection" or "love", so koi are symbols of love and friendship in Japan.
Colored ornamental carp were originally called Irokoi (色鯉) meaning colored carp, Hanakoi (花鯉) meaning floral carp, and Moyōkoi (模様鯉) meaning patterned carp. There are various theories as to how these words came to be disused, in favor of Nishikigoi (錦鯉), which is used today. One theory holds that, during World War II, the words Irokoi and Hanakoi (which can have sexual meanings) were changed to Nishikigoi because they were not suitable for the social situation of war. Another theory is that Nishikigoi, which was the original name for the popular Taishō Sanshoku variety, gradually became the term used for all ornamental koi.
In Japan, the koi is a symbol of luck, prosperity, and good fortune, and also of perseverance in the face of adversity. Ornamental koi are symbolic of Japanese culture and are closely associated with the country's national identity. The custom of koinobori (carp streamers), which began in the Edo period (1603-1867), is still practiced today and displayed in gardens on Children's Day, May 5.
In Chinese culture, the koi represents fame, family harmony, and wealth. It is a feng shui favorite, symbolizing abundance as well as perseverance and strength, and has a mythical potential to transform into a dragon. Since the late 20th century, the keeping of koi in outdoor water gardens has become popular among the more affluent Chinese. Koi ponds are found in Chinese communities around the world, and the number of people who keep koi imported from Niigata, has been increasing. In addition, there are increasing numbers of Japanese koi bred in China that are sold domestically and exported to foreign countries.
Koi are also popular in many countries in the equatorial region, where outdoor water gardens are popular. In Sri Lanka, interior courtyards most often have one or several fish ponds dedicated to koi.
The Amur carp is a hardy fish, and koi retain that durability. Koi are coldwater fish, but benefit from being kept in the 15–25 °C (59–77 °F) range, and do not react well to long, cold, winter temperatures; their immune systems are very weak below 10 °C (50 °F). Koi ponds usually have a metre or more of depth in areas of the world that become warm during the summer, whereas in areas that have harsher winters, ponds generally have a minimum of 1.5 m (5 ft). Specific pond construction has been evolved by koi keepers intent on raising show-quality koi.
The bright colors of koi put them at a severe disadvantage against predators; a white-skinned Kōhaku is highly noticeable against the dark green of a pond. Herons, kingfishers, otters, raccoons, skunk, mink, cats, foxes, and badgers are all capable of spotting out koi and eating them. A well-designed outdoor pond has areas too deep for herons to stand, overhangs high enough above the water that mammals cannot reach in, and shade trees overhead to block the view of aerial passers-by. It may prove necessary to string nets or wires above the surface. A pond usually includes a pump and a filtration system to keep the water clear.
Koi are an omnivorous fish. They eat a wide variety of foods, including peas, lettuce, and watermelon. Koi food is designed not only to be nutritionally balanced, but also to float so as to encourage them to come to the surface. When they are eating, koi can be checked for parasites and ulcers. Naturally, koi are bottom feeders with a mouth configuration adapted for that. Some koi have a tendency to eat mostly from the bottom, so food producers create a mixed sinking and floating combination food. Koi recognize the persons feeding them and gather around them at feeding times. They can be trained to take food from one's hand. In the winter, their digestive systems slow nearly to a halt, and they eat very little, perhaps no more than nibbles of algae from the bottom. Feeding is not recommended when the water temperature drops below 10 °C (50 °F). Care should be taken by hobbyists that proper oxygenation, pH stabilization, and off-gassing occur over the winter in small ponds. Their appetites do not come back until the water becomes warm in the spring.
Koi have been reported to achieve ages of 100–200 years. One famous scarlet koi named "Hanako" was owned by several individuals, the last of whom was Komei Koshihara. In July 1974, a study of the growth rings of one of the koi's scales reported that Hanako was 226 years old. Some sources give an accepted age for the species at little more than 50 years.
When koi naturally breed on their own they tend to spawn in the spring and summer seasons. The male will start following the female, swimming right behind her and nudging her. After the female koi releases her eggs they sink to the bottom of the pond and stay there. A sticky outer shell around the egg helps keep it in place so it does not float around. Although the female can produce many spawns, many of the fry do not survive due to being eaten by others.
Like most fish, koi reproduce through spawning in which a female lays a vast number of eggs and one or more males fertilize them. Nurturing the resulting offspring (referred to as "fry") is a tricky and tedious job, usually done only by professionals. Although a koi breeder may carefully select the parents they wish based on their desired characteristics, the resulting fry nonetheless exhibit a wide range of color and quality.
Koi produce thousands of offspring from a single spawning. However, unlike cattle, purebred dogs, or more relevantly, goldfish, the large majority of these offspring, even from the best champion-grade koi, are not acceptable as nishikigoi (they have no interesting colors) or may even be genetically defective. These unacceptable offspring are culled at various stages of development based on the breeder's expert eye and closely guarded trade techniques. Culled fry are usually destroyed or used as feeder fish (mostly used for feeding arowana due to the belief that it will enhance its color), while older culls, within their first year between 3 and 6 inches long (also called tosai), are often sold as lower-grade, pond-quality koi.
The semi-randomized result of the koi's reproductive process has both advantages and disadvantages for the breeder. While it requires diligent oversight to narrow down the favorable result that the breeder wants, it also makes possible the development of new varieties of koi within relatively few generations.
