It is impossible to sex koi smaller than about 25cm(l0in) in length,
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 tuberdes on the head and pectoral fins, principally
along the bones of the fin rays. These breeding tuberdes appear as fine
raised spots and could be mistaken for white spot (Ichthyophthirius. The
tuberdes 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 20C (68F) is ideal, although koi will occasionally
spawn at a temperature of 17C (63F). 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 colour 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 aquarium.
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 10cm(4in) lengths of 10-l5mm (0.4-0.6in)thick nylon-fibre
rope and thread these between the strands of a l20cm(4ft)- piece of the
same sort of rope.
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 household water
storage tank (about 450 litres/100 gallons) is ideal - and should, of course,
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 fine (0.3-0.5mm) stainless steel or plastic mesh. Fine nylon stockings
may provide an adequate alternative, although they are less robust.
Lay the ropes out in the vat with 0.2mg/ litre malachite green solution
to stop fungus (Saprolegnia) from attacking dead eggs and spreading infection
to living eggs. Place three lengths of string across the vat to keep the
ropes (2in) below water level. Do not add any more water at this stage,
but place an airstone 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 20-22C(68-7lF).
Koi fry can develop at temperatures as low as l7C(63F), in which case their
incubation will take five or six days, or as high as 25C(77F), which will
decrease their hatching time. However, there is a greater chance of the
fry being deformed at such extreme temperatures.
The development of the koi egg and fry
When the fry have hatched, they will inH stinctively 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 swimbladder, mouth or vent. They breathe by absorbing
oxygen through the fine blood capilaries 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. Asthe koi grow, feeding on their yolk sac until all the yolk
is utilized, they develope 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 swimbladder. They then
start to swim freely in mid water, usually congregating around the airstones
- 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 (Artemia sauna) larvae are also
a good food source for young koi fry. Start feeding the brineshrimp 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 5-l0mm(0.2-0.4in) in length and will be starting
to take larger quantities of more generously sized foods.
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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 20- 25C(68-77F) to provide faster growth rates, but
be careful; if koi grow too fast, they lose their colour. 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 (6in) or larger. The size of pelleted 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.