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”.

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?

Breeding koi is something many keepers are curious about, but it isn’t always as straightforward — or as gentle — as people imagine. In fact, some koi owners actively try to prevent spawning because the process can be extremely stressful for the fish involved.

When koi spawn, all grace disappears. Males will relentlessly chase and push swollen females in an effort to stimulate egg release. If the ratio is unbalanced and too many males pursue one female, she can become exhausted, lose scales, and even suffer permanent scarring. It’s not uncommon for experienced koi keepers to seek controlled egg stripping after seeing repeated damage from natural spawning events.

So if breeding koi is your goal, it needs to be managed properly.

What Triggers Koi to Spawn?

Koi spawning is governed by nature. Despite generations of selective breeding, koi are still descendants of wild carp and retain their biological instincts.

Two environmental signals control spawning:

Water temperature
Day length (photoperiod)

As spring progresses and daylight hours increase, female koi begin maturing their eggs. Temperature plays the supporting role. Spawning usually occurs once water temperatures reach around 20°C, often on a slightly cooler morning following a warm spell.

Temperature varies year to year, but day length is predictable. That’s why koi may spawn at slightly different times across the country depending on regional climate.

The final trigger is simple — the presence of males once females are fully ripe.

The Importance of Degree Days

Professional koi breeders rely on what’s known as “degree days” to control spawning.

The principle is straightforward. You multiply the number of days by the water temperature. For example:

3 days at 15°C equals 45 degree days.

To properly mature female koi, breeders aim for approximately 1,000 degree days before spawning. Once this threshold is reached, and provided males are introduced, spawning is very likely.

Commercial breeders keep males and females separate during conditioning. This prevents uncontrolled spawning and allows breeding to happen on schedule.

Preparing Broodstock Correctly

Successful koi breeding begins the previous summer.

Eggs that will be spawned this year were actually formed in the female’s ovaries the year before. If a female enters winter without developing eggs, she simply won’t spawn the following season.

Strong summer feeding, correct nutrition, and good body condition are essential. Females require careful management, whereas males generally produce sperm readily once temperatures rise.

In truth, breeding revolves around the females. They require the planning.

Controlled Spawning vs Natural Spawning

In a garden pond, spawning usually happens as a “flock spawn” — a chaotic free-for-all where multiple males fertilise eggs from multiple females. There is no control over pairings, and therefore no control over the resulting varieties.

This dramatically reduces your chances of producing predictable colour patterns or recognised varieties.

Commercial breeders avoid this by inducing spawning artificially. Females are given a hormone injection to release eggs, which are then stripped by hand. Males are treated similarly, and fertilisation is carried out in a controlled environment.

This approach prevents physical damage and allows precise crosses between selected varieties.

Raising Koi Fry – The Real Challenge

Spawning is exciting, but rearing fry is where most hobbyists struggle.

Newly hatched fry require microscopic live food and nutrient-rich conditions. Paradoxically, the best environment for koi fry is often a green, unfiltered mud pond rich in zooplankton.

This is why commercial breeders in the UK and Japan grow fry in dedicated mud ponds before gradually weaning them onto dry food.

In an unplanned pond spawn, thousands of eggs may become hundreds of fry, and ultimately only a handful will reach fingerling size. Without preparation, survival rates are low.

The Problem with Flock Spawning

Uncontrolled breeding produces unpredictable results. Crossing different varieties in a mixed pond rarely produces consistent offspring.

While professional breeders carefully refine bloodlines over many generations, a typical garden pond spawn is genetically random. That’s perfectly acceptable if your goal is simply to enjoy home-bred fish — but expectations need to be realistic.

Should You Breed Your Own Koi?

Breeding koi can be hugely rewarding, but it requires planning, preparation and realistic expectations.

It’s not simply a case of letting nature take its course.

If you want to produce healthy fry:

• Manage temperature carefully
• Separate sexes until ready
• Provide spawning media
• Prepare a separate fry rearing environment
• Accept that quality will vary

Most importantly, understand that breeding koi successfully is as much about preparation and management as it is about the spawning event itself.

Done properly, it can be one of the most fascinating aspects of koi keeping.

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.

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.

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.

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 week.

1 Suitable mature broodstock

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

3 Fry rearing pond

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

5 Bowls and fine mesh hand nets for culling

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

1 Females begin to swell and regularly feed more eagerly than males

2 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.

March/April Feed quality, high protein diet for egg and sperm development

Dig/construct a simple fry pond

May Purchase and install spawning mops

Fill fry pond with water and fertiliser

June/July 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.