It is impossible to sex Koi smaller than
about 3” because they are sexually immature. Once the Koi exceed this size, the testes in males and
ovaries in females begin to develop. The ovaries are much larger organs than the testes. Females are
usually easier to spot, as the belly of a mature female Koi is generally plump, whereas males remain
streamlined and more torpedo shaped.
When males are ready for spawning, they develop breeding areas on the head and pectoral fins, principally
along the bones of the fin rays. These breeding areas appear as fine raised spots and could be mistaken
for white Ichthyophthirius “The Ich.” The areas are most profuse on the pectoral fins, where they are
quite rough to the touch and arranged in fairly regular rows. They are used during breeding, when the
male nudges the female with his head and fins to induce her to spawn.
Koi will naturally spawn in the hobbyist pond
in the early summer. A water temperature of 68° is ideal, although Koi will occasionally spawn at a
temperature of 63°. As the water in the pond warms, the Koi will try to spawn 'en masse'. This is known
as flock spawning and, although it can produce very healthy offspring, the quality of the patterns and
color are generally poor.
The female Koi deposits her eggs, approximately
100,000 per kilogram of her body weight, over the pond walls, floor and on any plants. Unfortunately,
it is very difficult to collect eggs deposited in this random manner in order to incubate them in an
environment where they will not be eaten by their parents or attacked by parasites. If left to their
own devices, a few of the eggs will hatch and you can then collect the offspring and raise them in an
You can, however, ensure the survival of a greater
number of eggs by placing artificial spawning ropes in the pond. The Koi will deposit their eggs on
these in preference to using the hard pond wall. To make these spawning ropes, cut 4 foot lengths of
½” thick nylon-fiber rope and thread these between the strands of a 4 foot piece of the same sort of
As the Koi prepare to spawn, you will notice males
chasing a female, nudging her side with their mouths. The female will occasionally stop and suck at
the sides of the pond in an attempt to clean an area on which she can deposit her eggs. This is the
time to gently lower the spawning ropes into the pond. Spread out the coils of rope into a fanlike shape
and anchor them to the side of the pond.
Try to avoid disturbing the fish before and during
spawning, but keep a careful eye on them, as the males may bully some females. If this happens, remove
the female and place her in a separate pond. Koi may prefer to spawn around dawn but they may also spawn
throughout the day, when they have finished spawning, the females hang head down, respiring heavily,
and the other fish will become less excited. Gently remove the spawning ropes and carefully place them
in a vat for incubation. Koi are not good parents and unless you remove the eggs from the pond quickly,
they will begin to eat them. Immature Koi may also eat the eggs, so if you want to rear them, you should
separate the spawning Koi from others in the pond.
Incubating Koi Eggs
The incubator should have a reasonable capacity. A water storage tank about 100 gallons is ideal, and
should be made of non-toxic materials. It will need a valve-operated water inlet point and an outlet
with as large a surface area as possible screened with a very fine plastic mesh. Nylon stockings are
an adequate alternative although they are less robust.
Lay the ropes out in the tank water which has
been treated with malachite green solution to stop fungus from attacking dead eggs and spreading infection
to living eggs. Position the ropes somehow just at 2” below the water level. Do not add any more water
at this stage, but place an air stone on the floor of the vat to gently and continuously aerate the
water, as developing eggs need plenty of oxygen. When you begin to see the growing Koi's eyes in the
egg, after one or two days, run a fine trickle of water into the incubation vat. The day before the
Koi hatches the eyes in the egg will have a shine to them. Soon afterwards, the young Koi will begin
to wriggle then, gradually, over the next few hours, it will break out of its 'shell'. It will take
three to four days for the Koi eggs to develop and the fry to hatch at temperatures of 68°-7l°. Koi
fry can develop at temperatures as low as 63°, in which case their incubation will take five or six
days, or as high as 77°, which will decrease their hatching time. However, there is a greater chance
of the fry being deformed at such extreme temperatures.
Koi Eggs and Fry
When the fry have hatched, they will instinctively seek shelter and hide in any cover they can find.
The spawning ropes are ideal for this. Using a special sticky pad on their heads, the fry attach themselves
to the ropes' fronds, or to the vat wall. At this stage in their development, the young Koi have no
swim bladder, mouth or vent. They breathe by absorbing oxygen through the fine blood capillaries that
surround the yolk sac, which is still attached to the gut. It is essential that there is plenty of oxygen
entering the water at this stage, as a reduction in the quantity of dissolved oxygen in the incubator
could lead to mass mortality.
The Koi fry have only one fin, which encircles
the posterior end of the body. As the Koi grow, feeding on their yolk sac until all the yolk is utilized,
they develop paired fins, a mouth and other organs. After two or three days, the young Koi swim up to
the surface and take two or three gulps of air, which they force into their swim bladder. They then
start to swim freely in mid water, usually congregating around the air stones, a sign that they are
ready to be fed.
Koi fry at the “swim up” stage do not have any developed taste buds and so must detect their food by
sight. Therefore they need to have food all around them. Hard-boiled egg yolk is an ideal food for the
first day or so. This has very little dietary value, but will increase the size of the stomach. Newly
hatched brine shrimp larvae are also a good food source for young Koi fry. Start feeding the brine shrimp
when the Koi are about one week old. After another week or so, the Koi will be ready for a mash diet
(the powder dust that is left after the manufacturing process of fish food). From this time onwards,
feed the Koi food of a size that can be taken by the smallest fish in the vat.
You will need to remove accumulated debris and
waste frequently during this first feeding stage. A siphon made from aeration tubing is ideal for removing
settled and suspended waste from the incubator, and an old toothbrush is useful to clean the outlet
screen. You should also add fresh water regularly to the vat to remove nitrates and ammonia. Tap water
which contains high concentrations of chlorine should be aerated before it is added to the vat, to allow
the chlorine to evaporate. After three or four weeks, the fry will have grown to
¼” to ½”
and will be starting to take larger quantities of more generously sized foods.
At this stage, move the Koi to a growing-on facility, such as an aquarium, tank or pond. Watch the young
Koi carefully, as cannibalism is not uncommon. If you suspect this, remove any larger fry to another
tank. You can heat this to 68°-77° to provide faster growth rates, but be careful; if Koi grow too fast,
they lose their color. One month old Koi need about five percent of their body weight in food and should
be fed little and often. As they grow Koi need less food, about two percent of body weight 6" or larger.
The size of pellet food is critical; not all fish grow at the same rate and it is too easy to look at
the larger Koi and select the pellet size to suit them, forgetting the smaller fish. Unfortunately,
Koi that are nearer to the common carp in appearance grow faster than the prettier Koi. This is because
Nishikigoi are highly inbred, that is they are mated with other Koi to which they are closely related,
such as either parents or siblings, and as with any highly inbred animals, Nishikigoi are not as hardy
as their wild counterparts.