It is very difficult to breed good quality Nishikigoi. Professional Koi breeders even find it hard. Hobbyists still try Koi breeding to see the results they get.
The most important thing is to choose good quality parents with excellent blood lines. This is critical. Unknown blood lines and background may cause an undesirable variety of babies. For instance, from a Kohaku parent there might be many different types of Koi such as Taisho-Sanshoku or Showa-Sanshoku, even Ogons and Asagis. Since there is no such thing as a pure blood line, there is always a chance the babies could be drastically different from the parents.
If planning to spawn Kohaku, Taisho-Sanshoku or some other popular breed of fish, make sure the parent Koi have snow white areas, bright colors, even colors and sharp edges on the patterns. Balance and formation of pattern on the parent Koi is not an important factor. The resulting patterns on babies is a matter of chance and cannot be panned.
The age of the female should be three to ten years. If the female is too small, her eggs will be small and cause premature birth and most of the babies will die. For the most effective spawning, the male should be three years or older, if any younger he is probably unable to produce sperm effectively.
Some well known breeders in Japan will breed with only one male and one female, however, this can develop into a lengthy process since it is difficult to select both fish in the peak of spawning condition. If the breeding is only a part of your hobby, one female to three males is suggested. If the mating is a failure the first time, the male partners should be changed. Also, if the female fails to produce eggs, there is a possibility she has a stomach tumor and should be exchanged for another female with eggs. The major classifications of Koi should not be mixed forbreeding. For instance, if Kohaku breeding is being tried both male and female parents should be Kohaku.
After selecting the parents a decision must be made concerning the container for the spawning. The should be sturdy and smooth as, in the process of spawning, the Koi will move around a lot and often rub up against the sides of the tank. If it is rough, they will injure themselves. The best type container is a tank 6 to 8 feet in diameter and three to four feet deep. A large show tank is a good choice. Make sure adequate aeration is available as the fish will be extremely active and use large amounts of oxygen.
In Niigata spawning does not start until late May or early June because of the cold temperatures. In California and the southern areas of Japan the spawning season starts much earlier, around the beginning of March and early April.
To induce the female to lay eggs there must be a soft and stringy media on which she can deposit them. Typically, items like willow tree branches, water hyacinth, a clean mop head or long stringy algae are used. Most professional Koi breeders use a specially made fiber but, whatever the material, it must be free from disease, parasites, and chemicals.
Another method of spawning applies anificial techniques. This is done by squeezing the eggs out of the female and spraying the male sperm on the eggs. The eggs are then mixed with the sperm and placed into a special hatching tank This is not recommended for amateurs as it can cause injuries to the Koi.
In order for successful spawning to take place the daytime temperature must be above 75 degrees. Many times a female will lay eggs because the pond is shallow and cannot maintain an even temperature. The change in temperature between night and day triggers the fish's natural response to spawn. However, if the pond is deep and the temperature stays even the fish will not spawn unless induced.
Setup the spawning tank early in the morning so the water has a chance to warm up before cooling down at night. A day must be picked when it's obvious the temperature will be over 75 degrees and will stay this way for a couple days. Make sure the spawning materials and aeration systems are ready and installed in the tank. The early afternoon is the best time to release the spawning pair into the tank as they need to adjust to the water temperature. To do this properly, put the fish in a plastic bag with water and oxygen and float it in the tank for at least fifteen minutes then release the fish. Because fish tend to jump during spawning, the tank shoud be covered; such as with tautly secured shade cloth, neiling or even a heavy board. The spawning will start at about 4:00 a.m. next morning and continue for two to three hours.
An additional tank, with water at exactly the same temperature as the main tank, should be ready for the fertilized eggs. (A variation of just a few degrees is sufficient to kill the eggs). These should be removed from the spawning tank as soon as the parents have finished their activity. The water in the spawning tank will be white, smelly and soon become rotten because only a small portion of the sperm, sprayed by the male, is adhering to the eggs, the rest is distributed in the water. The parents must also be removed and, if possible, the female placed in a separate tank to allow her time to recover. Finally remove any eggs remaining and place them with the others in the rearing tank.
The hatching tank should be placed in an area that does not show a temperature fluctuation of more than five degrees day or night. The eggs will die if the temperature changes even 10 degrees in a twenty four hour period. The average sized female will lay between 200,000 to 400,000 eggs. An attempt should not be made to save all these eggs because the hatching tank space is limited. They will not all hatch if the tank is overstocked.
Within four to seven days the eggs will start to hatch. This will vary dependent on water temperature. Warmer water will accelerate the cyde and soon the baby Koi will start to swim. They can survive for a few days on their yolk sack but after that is depleted, they will have to be fed. The best food for them is microscopic animals, but if these are not available they can survive on very small particles of dried food such as powdered milk They will eat continuously throughout the day, and must be fed every hour. Be cautious to feed only tiny amounts of food because any leftovers will become rotten and kill them all. In about two weeks, they should be moved to a larger pond for faster growth, still feeding all the time.
The combination of feeding and culling is the most important feature in Koi breeding. Within the first month, the first culling should take place. Eliminate those with deformed bodies, single colors (if raising Koi with pattern), and those that are unusually large. This type of Koi is called "Tobi" and they usually do not have good patterns plus, they eat the smaller ones and these smaller fish, very often, have the better patterns. Professional Koi breeders usually save only about 10% of their hatching. After four months most professional breeders will have grown their Koi to about 5 to 6 inches.