Breeding Koi – Indepth

A Shiro Utsuri bred by Cuttlebrook Koi Farm, and owned by Martin Barnett,  was awarded the major prize of Baby Champion at the BKKS National in 2010.  Rather fittingly, we had also been asked to write an article on Koi breeding for the show magazine that year too, so a pretty good show for us all round really!  The article is reproduced below for you to enjoy.

Breeding Koi – Indepth

A Shiro Utsuri bred by Cuttlebrook Koi Farm, and owned by Martin Barnett,  was awarded the major prize of Baby Champion at the BKKS National in 2010.  Rather fittingly, we had also been asked to write an article on Koi breeding for the show magazine that year too, so a pretty good show for us all round really!  The article is reproduced below for you to enjoy.

Breeding Koi Indepth

On the face of it, breeding Koi seems like a simple exercise – you put a male Koi and a female Koi together, they spawn and you then get hundreds of thousands of eggs which hatch out into hundreds of thousands of Koi and this time next year, you’re a millionaire!  Clearly, if it were that easy, more people would be doing it. So what do you have to take into consideration when breeding Koi?

If you want to breed specific varieties of Koi you need to select specific varieties of parents, if you don’t do that you will end up with a whole lot of generally unattractive Koi of indeterminate variety.  Female Koi can produce between 100,000 – 120,000 eggs per kilo of body weight but there is no guarantee that all or any of those eggs will be viable so to increase the chances of a good fertilisation rate usually two males and one female will be used when spawning.  Using a clean tank filled to a depth of around ten inches with fresh water, and with spawning ropes draped across it, will provide the right environment to trigger the parent fish to spawn but once spawning has finished, the parents must be removed as they will do their best to eat as many eggs as they can!

Parent Koi in spawning tank

Fish eggs are highly susceptible to fungus but a standard dose of malachite green in the spawning tank, after the parents have been removed, will ensure that they stay as fungus free as possible and will give them the best chance of developing.

The tag on the left shows the eggs just after they were laid and  in the picutre of  the tag on the right, taken a couple of days later, you can see clear eggs with black dots in them – these are the eyes of the developing fish inside the egg. The white eggs are not viable and will not hatch.

Gentle aeration and a steady trickle of fresh clean water will provide a comfortable environment for the newly hatched Koi fry, although for the first few days after hatching, no food is needed as the fry hatch without a mouth and continue to develop by absorbing the remains of their egg sack.  For the first few days of life they use a sticky pad on the top of their heads to stick themselves to anything they can find – spawning ropes, the sides or floor of the tank whilst they undergo this short transitional stage. The rate of development depends upon the temperature of the water and the warmer the water, the faster they develop.  Usually after about three days from hatching, the fry develop a mouth and swim to the surface to take their first gulp of air which they force into their swim bladder and which provides them with the buoyancy that will enable them to move through the water hunting for food and evading potential predators.  It is at this point that they are ready to be moved into a mud pond.

Koi fry in the spawning tank after taking their first gulp of air


Historically, ornamental Koi were a by-product of the Carp that were bred for eating by the rice farmers in the Niigata region in Japan.  They were an ideal form of polyculture alongside the rice because they could be grown in the mud pond water reservoirs that were used to feed the rice fields.  It’s this availability of mud pond space that holds the key to the success of the regions farmers as breeders’ of Nishikigoi.  So why is this mud pond space so important?  Well, in order to produce good quality Koi, you need to ensure that as many fry as possible survive to a point where their colours can be seen and their potential assessed. The mud pond gives you the space to do this and, if managed properly, also provides an environment where the natural flora and fauna that are essential to a young Koi’s development can also thrive.  It is important that a mud pond used for growing Koi fry is of a size and depth that can be easily harvested as if it isn’t, proper selection for quality cannot take place.  Mud ponds have to be managed very carefully to make sure that they provide the ideal environment for Koi year after year and if they aren’t, detritus and bacteria can build up to a degree which becomes harmful to the fish and after a few years of use the productivity of a pond can deteriorate to a degree which makes it unsuitable for raising Koi fry.

We drain and dry our mud ponds each year which helps to provide a surface we can walk on without sinking into when we harvest and which stops any sludge building up on the bottom of the pond. Once the pond is filled up with water, it will undergo a cycle of algae growth and bloom whilst the water finds its natural balance.  Sometimes some management is needed to help things along but generally each pond will cycle in its own way and should give the perfect environment for the rotifers and daphnia to grow and which provide the first weeks food for the young fry when they are transferred to the mud pond.

After this first week, the fry are ready to be fed with a high protein food which helps to supplement their natural diet and which will enable them to grow at their optimum rate.  After around five weeks, they will have reached approximately one inch in length and will be ready for harvesting and selection.

Tens of thousands of fry at first harvest
The selection process

Selection is the next key stage in producing Koi as without this the quality of the Koi produced would be very poor.  We harvest between 30,000 and 50,000 fry from each mud pond at first selection and we sit and look at each and every fry to assess whether it has the potential to develop as a good representation of its variety.  Only the best are kept and in doing so we help to ensure that those we keep do not become overcrowded and outcompeted for food by faster growing but less attractive Koi.  We usually keep around ten percent or less of the Koi harvested at this stage which means that the vast majority are rejected as we do not consider them to be of a good enough standard to grow on further.  The fry we choose to keep are then returned to the same mud pond and grown on for another 4-5 weeks before being harvested again and the numbers reduced to about one third.

Sanke fry after second selection at around nine weeks


This selection process goes on throughout the life of the Koi that we breed and grow on our farm and it is this process that ensures that the Koi that we produce are of the highest quality.

At 32 months old, this Shiro Utsuri has been through seven selection processes – each time we carry out selection we assess the body shape, skin quality, balance of pattern, contrast and the long term potential of each Koi.  Some Koi are kept to develop further and others are sold and continue to improve in the care of their new owners.  Many of these Koi make it onto the show circuit and visitors to this year’s BKKS National Koi Show will be able to see several examples of Koi bred at Cuttlebrook Koi Farm in the show vats.

So, the reason why breeding Koi is not as simple as it seems at first glance is that the spawning process itself is just the first step in what is quite a complex process.  The science behind mud pond design and management are key features to the success of any Koi farm and the selection process is something of an art form which has to be mastered over time.  When you look at the fish that you see in the show vats at this year’s National Koi Show, please take a few moments to consider the years of experience and care that have gone into producing these precious few Nishikigoi.

A Range Of Varieties

We breed a number of varieties of Koi each year and use our many years of experience to select the very best.

Hundreds Of Awards

We breed the finest Koi (Nishikigoi) in the UK and have won hundreds of awards for their owners over the years.

Online Or In-Person

You can buy the Koi we breed direct from us by visiting our farm in Oxfordshire, or you can order online.
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