Hazards: Koi need not spawn in order to remain healthy. On the contrary, the process has accompanying dangers to health, color, and patterning of fish which are involved. Under normal pond conditions, indiscriminate and spontaneous spawning may occur, unobserved by the pond owner, and the resulting eggs either are eaten by the adults or disappear down the overflow, particularly when the pond is free of vegetation. When spawning is a deliberately encouraged and planned event, many more considerations are involved.

Age as a Factor: A procedure which works for one person usually can be found to have an alternative used by another successful breeder. However, one consideration must be recognized by all breeders: age of the fish. A young koi may be fertile, but the hatch which it produces of young fish is not strong. On the contrary, although a female may have a span of fertility over 15 years, as she ages, her eggs develop a tougher covering which sperm have difficulty in penetrating, so the hatch may be unpredictable an quixotic.

Ratio: The number of males to female in mating is controversial. Certainly, a 1:1 relationship has a more predictable genetic outcome than when several males are used. Equally important as a consideration is the possibility that a number of aggressive males may subject the female to injury in their spawning excitement. Under NO circumstance should the ratio ever exceed 2:1. Taken for granted is that fish being mated will be of relatively similar size.

Temperature/Season: April to July are the usual months when fish tend to spawn, at a time when the temperature range is fairly consistent and the difference between day-time and night-time low probably does not exceed 30 degrees F. Another reason for preferring this time of year is that live food is available and a stronger hatch is probable.

Preparations: Dr. Caswell has found a seeming coincidence between actuality and the Japanese belief that the best spawning time is at the full of the moon. As a consequence, she starts preparing a few days ahead of the anticipated full moon, just in case... Mated koi are separated from others into their own tank or pond. Isolating specimens actually is helpful if done about a month ahead of the possible spawning. A location should be chosen where the fish readily can be picked up and where they can be calm. Hand feeding helps assure them as well.

Nesting Materials: Koi actually do not form nests, but that terminology is used to describe any medium which fans out under water and which does not foul the environment. A synthetic, brushlike plastic media is available, but Dr. Caswell does not use it. Instead, she prefers natural substances: willow cuttings with long stringy roots, tips only of the small type podocarpus, with the bunched branches tied outside the pond. Although she also does not recommend juniper, which is somewhat oily and may need advance cleansing, some breeders do use it. Any medium used should be submerged totally or at least to a large proportion of its are subject to breakage into small parts, which can be a nuisance. If they are used, the plants should be treated with Dipterex (Dylox), then thoroughly rinsed and cleaned to rid flies, which themselves are harmless, but their larvae hatch into the parasitic “water tigers” which attack the very tiny koi.

Setting the Stage: Female koi become large and bloated with eggs and usually are readily identified. Their abdomen is heavy and soft but resilient. When they are ready to lay the eggs, the male is presumed also to be ready. Because the heaviness of the female partially is buoyed up by the water support, she never should be out of water at any time when netted or moved by hand. Male or female fish being mated must be handled with utmost carefulness.

Some breeders put both male and female into their separate pond at the same time. Others put the female in early, to get her acclimated, then add the male later in the afternoon or early evening.

.pH is not too much of a factor, though a neutral pH 7 is preferred. However, if fish have become accustomed to a higher value over a period of perhaps a year, no problem should exist in their adjustment. Of course, pH does affect color: Japanese acidic water conditions help provide them with a good red but poor black; U.S. alkaline water gives a good black but an uncertain red.

Housing: A rounded pool of at least 4' x 8' size or a Doughboy swimming pool of at least 1,000 gallons should be about half-filled with fresh but unchlorinated water to a depth of about 12 to 18 inches, sufficient but not enough to encourage jumping. Probably the pond should be prepared by adding a quart of Clorox or equivalent, draining and refilling, and then letting it stand for three days before adding fish. Usually a strong, gushing current or a splashing waterfall is desirable for providing excellent aeration, but, for spawning, these are less conducive for the tranquil atmosphere which is needed. Under whatever conditions of a still pond or collapsible swimming pool, good aeration is vital if the fish are to have good living conditions. Protection from within and without is provided. Some use fishnet, while others construct a chicken wire tent/fence. For the latter, metal posts about 4" long are driven into the ground around the circumference, from 5-7 in number, to support small gauge wire to about 2 ft. above the pond. Bamboo poles placed horizontally give top support, with ties between and at the vertical support posts to cinch up the chicken wire. Shading can be from black flameproof polypropylene shade cloth of 55-72% density, fastened to the supporting wire by metal spring clips. While most of this preparation is to keep adult koi from jumping out, it also deters kingfishers, herons, raccoons, and child rock/debris throwers!

Timing: Spawning usually occurs in early morning. If no action has occurred by 9 AM, probably nothing will happen that day. If so, fish can be fed lightly, be given attention, and can be made happy and comfortable. Particular attention should be given to possibly a fine, gentle water spray or the canvas/polypropylene covering to regulate the crucial temperature, because the koi may get over-heated in the limited volume of water. Koi may refuse to spawn if the water is too warm or if the water is very hot. After three days, if there has been no action, they should be returned to the main pond because of the mentioned danger of over-heating.

Mating: When the female is interested in spawning, she makes “household” moves with the plant/plastic medium, as though building a nest. The male is attracted to the “spawning position,” in a type of courting or foreplay. If there are single male tries to force her against the side of the pond. The males thrash and bump the female, literally forcing out thousands of her eggs while simultaneously excreting sperm. After the first discharge of eggs, if there is but one male, it positions itself on the opposite side and helps the female get rid of the remaining eggs by a second session. The turbulence of the action scatters eggs all over the pond or pool. They are very sticky and adhere to anything they touch: nesting material, pond walls, bottom. Left alone, thousands upon thousands will eventually hatch. As a result of the frenzied and violent action, the female may be injured, occasionally getting a split fin or tail. She may also retain some of the egg residue, which can be very dangerous. Additionally, the insensitive male may continue driving her, adding to the problem of her beaten condition. Therefore, the breeder should observe the fish at 15 minute intervals during the 4-61/2 hours consumed by the process.

Aftermath: Upon the completion of spawning male and female seem interested only in eating the eggs. If the male continues to be aggressive, he should be removed, carefully. He now is harder to handle and is vulnerable because his usual protective, slime-layer body covering has been worn off, so that his normal slippery feeling gives way to a sandpaper-like touch. He also probably will fight any netting. At this time, he can be prey to fungus or disease, and best can be protected by returning him to high quality, regular pond water, possibly supplemented by Bam-O-Flavine or the equivalent. The female should be isolated in a pond by herself, with other females, or with young koi for at least 24 hours, so she can recuperate. She will lose the spawning odor and have a chance to rest in this length of time.