How long are Koi fish pregnant? Live Spawning or egg Layers

If you notice that a koi is expanding rapidly, you may be bracing yourself for the flip-flap of little fins sometime soon! The sudden bloating of a laden female koi certainly does give the impression that baby fish are well on the way! Despite this fecundity being known as ‘pregnant koi’, all is not what it seems.

We are sorry to disappoint you but as remarkable as a truly pregnant koi carp would be, these fish are actually not viviparous. They do not give birth to live young but rather reproduce by spawning. It is far more likely that the rotund proportions you are observing are those of a female who is getting ready to release up to a million eggs, which is, in our opinion, just as exciting!

So, how long is a Koi ‘pregnancy’?

The duration of the spawning period can range from a few days up to a week or more while the lady in question awaits the right environmental and hormonal conditions to release eggs. Brace yourself for hours of following her around the pond! The spawning period can cause havoc in lots of ways especially as males vie for her attention.

Let’s take a look at how these magnificent fish reproduce and some timely tips for handling any gravid ladies that may be in your pond!

A big tummy is a good way of spotting the females in your pond

A spawning female may have been the first or only indication of the sex of your koi carp. After all, if you are not going out of your way to know their sex, koi are quite morphologically similar. Sexing koi is easiest when they are over 12 inches in length and mature.

There aren’t many clues but the fin shape and size are often a good pointer. In females, fins are large, round, and translucent in comparison to the colored and angular fins of a male. For a definitive check, the underside of the fish can be inspected.

A female has two cents and a male, one. Also, as a consequence of breeding, these ladies tend to grow larger and are typically built wider than the males. This will mean that a spawning female is going to be hard to miss as she is likely to be highly swollen and blimp-like!

garden pond
Providing the best conditions possible for you Koi will allow them to breed more frequently

It is important that you don’t overlook other causes of a swollen koi and miss something urgent

Before you start thinking up names for your new fry, make sure you exclude other more serious causes of a swollen koi. Unusual enlargement or swelling can be a harbinger of serious health problems. It is prudent to exclude these key diseases of koi and goldfish that will warrant immediate intervention.


In this painful condition, the belly area of the koi is swollen all over or bulging to one side, with or without the scales raised. It is usually caused by a bacterial infection, tumor growth, or kidney failure. A ripe female laden with eggs can also succumb to this condition, so you should watch new mamas carefully. Poor water conditions are a known precipitant, and affected fish will need to be quarantined and treated quickly.

2.Swim bladder

Swim bladder disease is another nasty cause of swelling in koi. Again water quality is the most common cause of this uncomfortable swelling which is accompanied by buoyancy problems. Overfeeding can also contribute to problems with the delicate swim bladder.

3. Intestinal blockage

This problem is often only diagnosed post mortem, so be careful to avoid it. It can be induced by overfeeding and often affected fish will be gasping, struggling to swim, or trailing feces.

The reproductive organs of a female koi are built for rapid expansion!

Female koi have paired gonads that are located way down in the pelvis of the fish, between the intestines and swim bladder. Pink smooth ovaries may be separate or fused and have their own dedicated opening near the anal vent.

When breeding is around the corner, both male and female koi gonads will massively enlarge with female organs swelling far more than males. Female gonads undergo astounding growth as they become absolutely laden with eggs (up to 100,000 per kilo of body weight) and can expand to up to 70 percent of body size!

So, where do baby koi carp come from?

In this case, you will be glad it’s not the stork! With a large mature female packing in over 400,000 tiny pin-head sized eggs, koi carp spawning is something you definitely don’t want to miss. Koi will spawn up to twice per year, and usually, the first spawning is the larger of the two.

For spawning to be successful, females are usually around 3 years old and males 2 years. Conditions like day length, water chemistry, and temperature need to be just right. For spawning, koi carp usually requires a temperature of between 60 and 70 degrees Fahrenheit (15 and 21 degrees Celsius).

In the days that proceed with the egg scattering activity, your female koi will not only swell up but is likely to attract the ardent interest of the pond’s males who may be more than a little vigorous in their pursuit and may nudge the female’s swollen belly to stimulate egg release.

This close shadowing of the female is essential as males fertilize the eggs as soon as they are released. The sticky eggs will cling to any vegetation or rocks in your pond and if successfully fertilized will start to develop.

Brace yourself as the young koi fry hatch around 4 days after fertilization. They attach themselves to a sheltered spot and start to feed themselves off their yolk sac.

Make your broody koi comfortable

When packed full of ripe and ready eggs, comfort is everything! Make sure that you attend first to water chemistry and cleanliness.

  • Keep oxygen levels optimal

Good aeration is vital to support the rapid physiological and metabolic changes your female will be undergoing. If you do not have a pond pump to support aeration, it is well worth getting one. Dissolved oxygen should be at least 5mg of oxygen per liter. (Looking for a pond pump for your spawning koi? Read our helpful article on choosing a great pond pump and filter for small ponds).

  • Keep pH swings to a minimum

Remember that pH is on a logarithmic scale so even a small change can feel significant to your pregnant koi. Test your water and aim for a pH range of 6.8 and 7.9.

  • Water hardness

Keep harness between 60 and 150 ppm

  • Nitrates are a no-no

You should have an absolute moratorium on ammonia, nitrates, and nitrites which are the last thing a tender and bloated lady koi wants to deal with when bringing forth her eggs. Keep a tight rein on ammonia levels and the test is you see pH levels start to creep up.

Work with your koi’s nesting instincts and provide lots of shallow water vegetation that she will love to release her eggs in. Aquatic plants like Hornwort, water lettuce, and water hyacinth provide ideal surfaces and crevice for the sticky fertilized eggs and subsequent fry.

Fancy a bit of a koi breeding?

Perhaps the first encounter with spawning has given you the breeding bug. Yes, there is potentially money to be made but you will have to dedicate time and commitment to providing the right conditions and pond space for successful mating. Surprise spawning is much less stressful than active breeding for both you and your koi.

Once the optimal conditions are in place you could be waiting days for mating to occur and when it does it can be a little hard going on the fish.

The water needs to be clean, clear, balanced, and predator-free. One male and one female of suitable maturity will suffice. You will also need to feed your mating pair regularly with a protein-rich feed to support them through the mating process.

Breeders also pay attention to providing a suitable fry mat or ropes that can hold the deposited eggs.  Once the eggs are released and fertilized, the adult koi will need to be removed from the pond to prevent them from predating the eggs and young fry.

Spawning leaves a frothy scum on the surface of the water, so get your filter in gear and be prepared for water changes. Usually, experts leave nothing to chance and inject both male and female koi with hormones to induce expulsion of gametes in a timely and controlled manner keeping all associated drama to a minimum.

This is called stripping of eggs and milt.

Rounding up

In the absence of untoward symptoms, a laden female koi is a sign of a mature healthy fish. Surprise spawning is unlikely to give rise to a pond full of fish as the eggs and fry are likely to be predated. However,  keeping a close eye on a gravid female and maintaining tip-top tank conditions will get them through their ‘gestation’ intact.