Spawning of Koi

by Dr. Bertrelle Caswell, Guest Speaker
Ventura County Koi Society
July 14, 1979

Introduction: and text by Bob Barry, 1979 Secretary, Ventura County Koi Society

Dr. Bertrelle Caswell has been involved in the koi hobby since about 1967, and has been breeding fish since about 1971. Her pond at home gradually has been increased in volume to its present 30,000 gallons as it has grown along with her interest in koi.

In 1974, she was given the honor of writing an introduction to the well-known book, Nishikigoi by Takehiko Tamaki. She has served as Chairman of the Associated Koi Clubs of America, and KOI USA magazine editor-publisher.

While Dr. Caswell is a professional fish-breeder, she also recognized that many of her audience were back-yard practitioners who had no intention of selling the products of their novice efforts at rearing baby koi. In the presentation which followed, she tried to give a combination of basic information together with a realistic picture of the costs and hazards of serious, large-scale koi-breeding:


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.


Commercial breeding probably involves planning a year in advance to assure that chosen mates have the best water quality, nutrition, equipment, and personal attention. Varieties which maintain color and come from the finest bloodline must be selected. Sturdy fish, four to five years of age (20" to 24" long) which seem to react well under stress are preferred. The same mutation is controlled by breeding only those of similar scale pattern and classification.

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 and quixotic.


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.


In Southern California, especially, these factors are crucial concerns. 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 high and night-time low probably does not exceed 30 degrees F.

Koi have been successfully bred as early as February or March and sometimes as late as August in the late summer. On one occasion, Dr. Caswell had fish spawn in 110°F weather at 7:30 p.m. With her giving them constant attention, they completed the process at about 2:30 a.m., giving her a warm, memorable night.

Hatching in May or June is preferred, because the babies have some time for growth and become acclimated before a possible harsh winter may affect them. Another reason for preferring this time of year is that live food is available and a stronger hatch is probable.


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 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, brush-like 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 volume.

Water hyacinths can be an excellent material, but 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 them of possible parasites. Hyacinths also attract dragon 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.


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 re-filling, and then letting it stand for three days before adding fish.

diagram 1 Aeration is assured by four Silent Giant air pumps, each with air stones weighted down with ceramic knobs on suitable airlines emanating from a three-gang valve. (See fig. 1 & 2) If kept in the shade, the air pumps operate more efficiently. 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 collapslble swimming pool, good aeration is vital if the fish are to have good living conditions.

Protection from within and without is provided by improvised tenting. (See diagrams.) Some use fish-net, 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!


Spawning usually occurs in early morning, though exceptions have been noted on previous page. If no action has occurred by 9 a.m., 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 weather 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.


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 two males, they sandwich the female between them. A 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 4-6 1/2 hours consumed by the process.


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. Otherwise, she may be subject to further harassment by males not previously involved. Watch jumping; for some reason, recently-spawned females have a tendency to jump, even when alone.

The breeding pond will have taken on a cloudy appearance and a distinctive, unpleasant odor. The "tapioca-like," light-greenish, glazed eggs are quite visible to the eye. Before nightfall, a partial exchange of perhaps half of the water should be processed to clean the pond and help remove the cloudiness. Caution must be observed to keep the eggs moist at all times. Gradually the objectionable odor will dissipate. Because the hatch now prevents normal filtration, some arrangement needs to be devised to maintain the same depth-level, while pumping out the old water and spraying in fresh, new water.


diagram 2 Japanese breeders discard about half of the hatch on the premise that there are just too many to care for. This decision is a matter of individual choice, in anticipation that about 50,000 fish are in the usual hatch.

By the third day, fertile eggs are clear, while the infertile remain cloudy/chalky/opaque and eventually develop a fine, hairy growth. Two black dots, the future eyes, are visible. In 4-5 days, a multitude of jerking "commas" report live individuals. Unless there are very cold nights, all fertile eggs will have been hatched by the sixth day. Now is the time when they are vulnerable to the dragon-fly larvae that hatch.


Feeding now presents a problem, since it must be accomplished without filtration and without polluting the pond. For the first 24 hours, the egg sac supplies sufficient food. Following that period, for the first three months, a feeding is required five times daily!! Hopefully, green algae will develop to provide supplements of natural food.

For Dr. Caswell, feeding follows a prescribed pattern which she estimates costs about $350 during the first month alone:

First Day: 2-3 tubes of Liquifry Red.

Second Day: 5-6 tubes of Liquifry Red plus 2-3 tubes of Liquifry Green and rotifers (microscopic water animals) or brine shrimp if they can be grown (directions from Scripps Institute, La Jolla).

Fourth and Fifth Days: Increased amounts of Red and Green Liquifry, Tetramin Red and Green powder, and 1/2 tsp. Gerber's egg yolk (gradually increased to 2 tsp. by end of first month).

Second Week: Live/frozen Daphnia(fresh water fleas), Tetramin flakes alternated with powdered regular koi food, tubefex worms (but only if verified as being clean).

First Month: Fry should be 3/4" to 1" long.

Second Month: 1"-2" long, eating 70% koi food, frozen Daphnia possibly supplemented with small/chopped Euphasiids(frozen).

Fourth Month: 3 meals a day by September. Largest now will be 3"-4" long, but some may still be as small as 1" long.

(KOI USA, vol. 1, #2 also has an article on nutrition of baby koi.)


During the first two months, a water-exchange should be processed every other day. By putting a nylon stocking over a funnel or by using a special brass end-piece with very tiny holes, a gravity drainage system can be devised to remove water while spraying in fresh water over a three-hour period.

diagram 3 As mortality occurs within the crowded conditions, an unnerving but naturally-expected selectivity, a second doughboy and equipment must be available. Two submergable Cal pumps, each moving about 1,000 gallons per hour, are used with the Caswell-design "barrel filtration device" (see fig. #3,4, & 5). The device cost about $50. to make. (For a more modern and improved barrel filter design - Google search; "birdman sand gravel filter".
The biological filter takes about three months to operate with maximum efficiency. If the filter works so well that no algae are present, feeding may be supplemented once a week by blending together a few lettuce leaves in water.

Culling of young fish should occur at about 6 weeks, as they are examined and moved in small lots. While there may yet be no pattern differentiation, the breeder should use a flat-bottom hand-net to observe the babies, discarding those with obvious physical deformities such as missing fin, single eye, crooked back, deformed mouth, and the like. If you cannot bring yourself to culling, you should avoid spawning, Dr. Caswell warns. Even so, probably some 20,000 baby koi are left after culling, donating, selling to pet shops, and other distribution from the hatch. Since that many cannot be housed in the usual pond, disposing of this number creates a challenge.

If rearing Sanke, Dr. Caswell discards all-blacks and grays and the all red "Aka muji." Remaining are three-colored reds with black dots on white. If originating from Kohaku, a single "Aka muji" might be saved but one is sufficient. Likewise, only one or two "Shiro muji" (all whites) are desired. Patterns cannot reliably be distinguished in breeding of Utsuri or Showa until koi are about two years old. Ogons without metallic head also are among those discarded. And so on, until the best are selected, only to pass them on to dealers except for the particular pet which cannot be let go.

A Koi Breeders' Association centered in Japan is apparently composed entirely of professionals. For the amateur at this time, no such U.S. group exists, probably because breeding has not attained the popularity and spread that is true of cultivating the grown fish.

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