How to breed koi. A comprehensive guide

Perhaps after keeping and growing koi successfully, the next challenge is to breed them. Fishkeepers of all disciplines are generally regarded as having achieved a higher level of fishkeeping once they have successfully bred and reared a species, having researched and planned carefully how to do it.

What could be more rewarding than thinking to yourself, “If it hadn’t been for my intervention, manipulation and attention to detail then these new creations would not have entered the world”.

What’s all that fighting in the pond

I cannot remember precisely how old I was when I first saw a pond full of fish, goldfish, shubunkins and orfe, ‘fighting’ in a garden pond. So severe was their apparent anger and aggression towards each other that they were threatening to leave their watery world and enter my terrestrial world, splashing and jumping, almost grounding themselves on planted baskets and crazy paving.

What was not explained to me at the time was that I was not witnessing fish trying to kill each other but quite the reverse, these fish were so distracted and oblivious to my observations because they were so intent on spawning with each other.

Whether that experience was locked into my subconscious mind and guided me since, I will never know, but I have since had the privilege of breeding koi in this country for a living and continue to use similar techniques managing the production of many thousands of ghost koi (hybrid metallic koi and carp cross) at Brooksby College.

To be successful at breeding koi, as with many other fish, requires an eye for detail, some extensive research, practical experience and the ability to think like a fish. I soaked up as much theoretical knowledge and detail as I could at university studying marine biology and several years later was able to adapt and try some of that theory on one of the finest collections of koi broodstock in the UK at Prokoi (Lancs.) with Tony Richards.

One of the most valuable lessons I learned was that preparation was everything when breeding koi. To breed and rear koi successfully in May, required preparation of the broodstock, ponds and other facilities as far back as the previous June. Nothing could be left to chance, especially if your living depended upon it.

How can koi keepers breed koi?

Many koi keepers do not wish their koi to breed, as the spawning event is quite stressful for their fish. Unlike at all other times, koi are not graceful or reserved in their spawning behaviour. If males significantly outnumber ripe females then all of the participating fish but particularly the females can become quite exhausted and physically battered as the males drive and bash at the swollen females to expel their eggs.

This can lead to loss of scales and permanent scarring and I have regular contacts with a number of koi keepers who want their females to be induced and stripped artificially to release their eggs in a controlled manner. Many of these females already exhibit damage from previous spawning activity.

Koi spawn in late spring/early summer in response to environmental stimuli which signal that their environment will provide their fry with the physical conditions and natural food supply so crucial for their good growth and survival. Even though koi are in-bred selected descendants of wild carp species, they have retained this biological clock from their ancestors.

Two key factors that stimulate koi to breed are water temperature and day length (photoperiod). These stimuli work in tandem to influence when koi spawn. Water temperature will fluctuate at comparable dates from year to year, whereas photoperiod is far more consistent. I.e. We know that June the 21st will be the longest day each year but that temperature will fluctuate year on year and are not as predictable as day length. The overall effect of these interactions means that koi will spawn at different times all over the country.

It is the increasing day length that has the greatest effect on maturing the female’s eggs, with water temperature having more of an effect towards the time of spawning. Koi are happiest to spawn at about 20 degrees C or on subsequent cooler mornings once these temperatures have been achieved. The final trigger for spawning may sound obvious, but once the females’ eggs are ripe, the factor that finally causes the eggs to be released and spawning activity stimulated is the presence of males. By keeping the sexes separate, commercial farmers can prevent spontaneous spawning events.

Furnished with the above information, it is possible to manipulate koi broodstock in captivity to stimulate them to breed when required. Broodstock are separated into their sexes and brought inside after winter having experienced a good summer’s feeding to ensure that quality eggs have been produced in the ovaries. The eggs for spawning this summer were laid down last summer after last year’s spawn. If a female is not carrying eggs by the winter then that fish will not spawn this year.

The water temperature is raised gradually, held at 15 degrees c and the fish are subjected to an artificial photoperiod, aimed to mimic mid-June at the desired time of spawning. This enables spawning to be carried out earlier than the fish would naturally do so.

The golden rule in maturing females is to subject them to 1000-degree days. That is, 3 days at 15 degrees C = 45 degree days and so on. A tally of degree-days should be kept to ensure that at least 1000-degree days are achieved prior to spawning. So long as the sexes are kept separate then even if this is exceeded spawning should not take place.

As you may have noticed, the artificial production cycle revolves around the females as these require more manipulation than the males which are producing sperm all the time at these temperatures and are usually ready to go at the drop of a hat!

To prevent damage to the valuable broodstock, the natural spawning activity is avoided by inducing females to release their eggs from the ovaries ready for stripping with a hormone injection. Males are given a similar injection to increase sperm production to an amount that can be collected.

The sperm and eggs from specific koi varieties can then be crossed, with a view to producing significant quantities of similar fish. These eggs are then incubated for approximately 5 days until they hatch, are raised on freshly-hatched brineshrimp in hatchery tanks and then stocked out into daphnia-rich mud production ponds where they are eventually weaned onto a dry diet and culled on size, deformity and pattern.

To the commercial breeder, spawning is one of the most exciting but also quite predictable parts to the production cycle. The most challenging period is after stocking the fry out into the mud ponds, trying to rear sufficient numbers of fry to the crucial 1-2 inch size. Once they have achieved this size and are taking a small pellet, subsequent growth and development is quite predictable.

This is often the experience for the koi keeper who experiences an unplanned spawn. The short notice does not allow sufficient preparation which means that many thousands of eggs will become hundreds of fry and ultimately only a handful of homebred koi will reach fingerling size. For this reason, if the hobbyist is keen to breed from their collection, they must provide the eggs and fry with suitable conditions to hatch and grow out in. Quite paradoxically, the best environment for fry to flourish in is a green, unfiltered but natural pond, full of microscopic zooplankton. It is often resistance against such an ‘unsightly’ pond that results in only a handful of fingerlings being produced in the same pond as the broodstock. Stocking fry into ‘stew ponds’ is the method used by commercial koi farmers grow on their fry in both the UK and Japan.

Other problems that have to be overcome when breeding koi naturally in a pond is a ‘flock spawn’ where spawning partners are not selected and a breeding free-for-all is encountered. There is little or no control over the varieties of koi crossed and this will have a knock-on effect with regards to the offspring.

For example, a commercial breeder who has carefully been able to develop and select his broodstock, creating quite a stable bloodline over many years is more likely to produce a large number of fish true to that variety. This is unlikely to be the case with an unplanned flock spawn involving crosses between say shusui, kohaku and ogon broodstock.

The commercial breeder also has the luxury of being able to produce large quantities of fry, where through broodstock and pond management techniques he is able to produce many thousands of fry from which to select those that are worth growing on. The fish that are culled will generally be mono-colour showing no pattern or poor growth. The chance pond spawning however is likely to produce such low numbers of fingerlings that if any culling was practised, it is likely that no fish would make the grade.

It must be remembered that each year commercial breeders produce many thousands of very poor, low-grade fry similar in appearance to those produced by accident in koi ponds each year. The difference being that the commercial breeders cull such fish, ensuring that only those fish exhibiting good characteristics make it to market. As hobbyists, we are usually thrilled with any extra home grown fish we can produce and are loathed to cull any of them.

Some varieties are easier to produce than others as the odds are generally stacked against producing some of the more complex varieties of kohaku, sanke and showa whereas the metallic varieties are generally more rewarding. A poor metallic koi is more appealing than a poor kohaku that would generally be culled being a mono-colour orange or ‘peach’ coloured fish.

Choosing broodstock

When choosing broodstock, look for the obvious desired characteristics of that variety. Such fish are of course likely to be very expensive so try to go for future potential broodfish at 10 inches +. However, an excellent kohaku will not guarantee excellent offspring. Some of the most consistent koi broodstock are actually poor examples of that variety themselves. However, these fish retain quality genes that may not be expressed in the individual broodstock but when crossed with others, will become expressed in the offspring.

Koi, like all organisms, not only pass on those genes that are visibly expressed, but also genes that are inherited from their parents that are not expressed. Such hidden or recessive genes can become expressed if they are inherited with other compatible and complementary genes. Consequently, a poorly marked broodfish when crossed with other broodfish can produce fish of a higher grade than itself.

The Japanese have recognised this for many years and have developed bloodlines that are now held in such high regard that the breeder’s name or bloodline is synonymous with that variety eg Matsunosuke sanke, Gosuke kohaku. Bloodlines are developed by the inbreeding of desirable sibling fish in an attempt to stabilise the characteristics and thereby improve the number of desirable fish from each spawn. This may be another explanation as to why only poor quality fish are produced after a koi keeper’s high-grade broodstock spawn. They are likely to be from two different bloodlines and their genes are not as complimentary as if they were from the same bloodline. It may be of more significance to a potential breeder if the broodfish are from the same breeder or bloodline than if they are high quality examples of that variety. This is one of the reasons why UK koi have not reached and are not likely to reach the heights of Japanese koi which is a testimony to the uncompromising and passionate approach of the elite koi breeders in Japan.

The more inbred a line becomes, the greater the probability that recessive genes responsible for quality skin patterns will combine and be expressed. However, inbreeding presents a number of associated problems for those beautifully patterned fish, in that they can lack vigour and are more likely to be prone to health and disease problems.

Commercial koi production in the UK?

There are only a handful of commercial koi breeders in the UK and a number of them cater for a wider market by producing a ‘mixed bag’ of koi that have not been crossed with compatible varieties. This has two benefits for the breeder in that the offspring are generally hardier than the pure-bred varieties and it also means that less emphasis is placed on culling. These fish are less appealing than the Japanese stock but they appeal more to the ‘garden centre’ market which may make sound commercial sense as there are many more garden ponds than koi ponds in the UK

On reflection, are we being realistic in our expectations in believing that an accidental spawning in a koi pond can produce true replicas of the same koi in our collections? As primarily koi keepers, the odds are stacked against us, as we do not have the time, facilities, and broodstock with which to succeed to any significant level. However, it is always rewarding to raise a number of home-grown koi which if nothing else, help us to appreciate the heritage, skills and achievements of the Japanese koi breeders.

How to make a fry pond

The simplest way of improving the chances of survival of fry from a pond spawning is to provide a natural fry pond for them.

Ideally a fry pond should be dug out of clay as this provides an excellent environment for culturing fry food naturally. If clay is not available then at second best a liner can be used. Try to provide as large a surface area as possible with a depth of between 2 and 3 feet.

In May, fill the pond up with water, aerate and add a few handfuls of well-rotted manure or topsoil and a sprinkling of inorganic fertiliser. The pond water should turn green in 1-2 weeks and will be ready for fry in 4 weeks.

If the pond is filled too soon then predatory invertebrates such as damsel fly larvae will have colonised leading to poor fry survival rates. If it is filled too late then sufficient livefood to support the fry would not have been produced.

After spawning has taken place naturally (June/July), place eggs in the pond and do not add any dry food for at least the first wee

Equipment you will need

Suitable mature broodstock

Spawning grass/rope onto which the adhesive eggs will be laid

Fry rearing pond

Finely ground high protein diet as a first food for the fry

Bowls and fine mesh hand nets for culling

The Signs

It is difficult to predict when spawning will occur in a pond but watch out for the following signs.

Females begin to swell and regularly feed more eagerly than males

The opercula and head on the males become quite rough like sandpaper. Males start cruising around the pond edges in groups at the surface.

Koi will generally spawn just after or during an isolated warm spell. Spawning activity may be encouraged by dropping the pond level by at least 30% after a warm day and topping it up with fresh, cool water. Spawning will occur in the shallows and around the pond edges so place spawning rope in the corners. Be sure to remove any sharp edges to prevent fish from damaging themselves.

Spawning may only last for 20-30 minutes and can easily be missed. Keep on checking the rope for translucent eggs, as the koi will eagerly eat them. A good indicator that spawning is imminent or has just finished is the presence of a proteinaceous froth or scum on the water surface, with venturis or waterfalls creating bubbles on the pond surface.

Breeding Timetable Northern Hemisphere


Feed quality, high protein diet for egg and sperm development Dig/construct a simple fry pond


Purchase and install spawning mops. Fill fry pond with water and fertiliser


Remain vigilant with broodstock After a warm spell (water temperature 18-20 degrees C) try to stimulate fish to spawn through a 30% water change.

After spawning, place spawning mops into the aerated nursery pond and monitor the hatch. Check that the broodstock are undamaged after the spawning activity.

August Add powdered dry food sparingly and cull if numbers allow Try to minimise stocking density as this has a great influence on fry growth rate.

Breeding Tips


Use a removable, non-abrasive spawning media in pond Prepare a suitable nursery pond for fry well in advance Remove rough/sharp objects from the pond Provide broodstock with clear, open swimming space Monitor and maintain ideal water conditions Monitor water temperatures to predict spawning activity Isolate female(s) after spawning to allow recovery. Perhaps use a floating net cage.


Rely on hatching eggs and raising fry in the koi pond Be disappointed if the eggs don’t hatch Be surprised if all of the fry do not resemble your broodstock Be tempted to overfeed the nursery pond with dry artificial food.

Kill blanketweed and string algae.