How to Tell If Your Koi Is Pregnant (Signs & Behavior)

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How Do You Tell If Your Koi Is Pregnant? Guide to Common Signs & Behavior.

Koi eggs on a person's hand
Female koi produce large amounts of eggs, sometimes up to hundreds of thousands! Adam / CC BY-SA 2.0

During springtime when koi are more active you might observe some tell-tale signs of koi pregnancy. If all goes well and your pond conditions are favorable, you may soon find tiny clusters of clear, round eggs attached to submerged rocks and plants. If you’re lucky, your pond community will grow to accommodate a few spritely juveniles!

Before listing the signs of koi pregnancy, it is first important to point out that koi don’t actually get “pregnant.” They are oviparous fish that, unlike us, carry unfertilized eggs. For fertilization to occur, there need not be any actual contact between males and females, as this takes place in the water column. The fertilized eggs then develop outside of the mother, exposed to the many dangers of the open pond.

For these reasons, female koi produce large amounts of eggs – sometimes numbering up to hundreds of thousands in a single breeding season! Of course, just a meager fraction of these becomes successfully fertilized and an even smaller amount goes on to survive as fry. Female koi expend a lot of energy simply getting their eggs out there, but it all becomes worth it for those few survivors! Here are a few things to remember and be on the lookout for when your female koi reaches sexual maturity. 

Signs Your Female Koi Are Carrying Eggs

1) Appearance

Three koi fish underwater
As female koi produce eggs, their bodies become increasingly swollen, as seen in this white koi. Gord Webster / CC BY-SA 2.0

To determine if your koi are pregnant, you would first need to find out which ones are females. Females will typically be larger than males and have rounder, more translucent fins. They also tend to have a wider build. As they produce eggs, their bodies will appear increasingly swollen, as their reproductive organs make room for thousands of eggs. These organs, more appropriately referred to as gonads, can even expand to 70% of their total body size!

Check out the video below for a closer look at this appearance and behaviour:


When they are just about ready to spawn, female koi have a “bloated” appearance. They can retain this appearance for a few days until they feel ready to release their eggs. At this point, they must be fed with high-protein food and provided with optimal conditions. If mature males are present, this stage of koi “pregnancy” will last for just a few days to a little more than a week.

Do note that a swollen appearance may not always be a sign of pregnancy, especially when detected outside of spring or summer. A number of serious health problems may cause significant bloating, so it would be wise to familiarize yourself with diseases (e.g. dropsy, intestinal blockage, and swim bladder disorders) before assuming that your female is pregnant. 

2) Pre-spawning Behavior of Female Fish

Female koi will eat vigorously before spawning to gather as much energy as possible. Jack Dorsey / CC BY-SA 2.0

Pregnant females will only lay their eggs if they find the surrounding environmental and hormonal conditions suitable. As they survey their surroundings, they may behave oddly and gravitate towards specific sections of the pond. You’ll find that a pregnant female will often appear to be foraging in these areas, in preparation for egg-laying. Once she has selected a surface on which to drop her eggs, she will begin “cleaning” it. This is done to remove any potentially harmful objects and so that eggs may properly stick to the surface.

Before spawning, female koi will also eat vigorously and may even continue searching for food after feeding sessions. Producing thousands of eggs is no small feat! She must collect as much energy as she can if she wants to sustain both herself and her protein-rich eggs.

3) Behavior of Male Fish

Male koi tend to be ‘clingy’ towards bloated females if they are producing eggs. Andy Rogers / CC BY-SA 2.0

Male behavior notably changes when females are producing eggs. They become more active and try to pair up with pregnant females. If you find that the males in your pond show distinct signs of being “clingy” to bloated females, you can be positive that she is producing eggs!

Check out the video below which shows this behaviour in ponds:


When females finally spawn, males have the tendency to nudge or bump into them. This gesture supposedly helps females drop their eggs, but may also occur during the pre-spawning period or with juvenile females. Presumably, these are simply “practice bumps”! Males also instinctively draw females towards areas of the pond that have denser vegetation. These sites provide conditions that would allow them to spawn safely and increase the chances of egg survival.

4) Age

Young Japanese koi fish
Usually, female koi reach sexual maturity at 3 – 4 years old, but it can sometimes happen before 2 years of age. Martina Rathgens / CC BY 2.0

Of course, only sexually mature females are actually able to produce eggs. Usually, female koi reach the peak of sexual maturity at around 3 – 4 years old. Females that have reached this stage of their lives will normally be longer than 10 inches. If you do see females that appear bloated but have not yet reached this size, they may simply be voracious eaters or may have underlying digestive problems.

In some cases, females reach sexual maturity before 2 years of age. It is not advisable to allow them to breed at this age, however, as egg production can compromise their development. At this young age, their bodies may not be able to allocate sufficient nutrients to their eggs, making their fry less likely to survive.

As captive koi reach the age of 7 years, their sexual vigor begins to slow down. Females at this age will begin to produce fewer eggs, and these will likely be less viable.

How To Encourage “Pregnancy” in Sexually Mature Females?

Egg production generally occurs as all females reach sexual maturity, but successful development is dependent on a few things. For females to carry their eggs “to term”, your pond’s water conditions must be ideal for mating. Late spring or early summer conditions are best for reproduction, as the mild temperatures (65 – 70˚F/18 – 21˚C) increase chances of fry survival. Concentrations of potentially toxic nutrients (such as nitrite or ammonia) should be kept at zero and pH levels should be prevented from fluctuating too often. Moreover, proper oxygenation must be maintained so that females do not grow stressed.

A high protein diet, supplemented with healthy treats, is vital for proper egg development. If insufficient nutrients are provided, it is likely that the majority of eggs will not be viable for fertilization. Moreover, those that do become fertilized may contain substandard amounts of yolk. This would prevent fry from developing into healthy fish.

You will also need to provide ample vegetation or structures that will allow females to safely release their eggs. Submerged aquatic plants will work best. To protect the plants’ root systems, you can grow them out of planting baskets situated at the bottom or along the sides of your pond. Fertilized eggs will likely attach to the leaves or stems of these plants. With luck, these plant structures will soon be called home by a handful of baby koi!

2 thoughts on “How to Tell If Your Koi Is Pregnant (Signs & Behavior)”

  1. I have a pond made out of concrete it’s actually coupons and a canal have four early good size goldfish and they got 10 babies which are about 2 in long the pond is smelly and I can’t afford that aerator you suggested until I moved to North Carolina and building a new pond what can I do to stop the smell I have two box filters which I clean regularly and just aerator little pump or else can I do or should I just keep doing my half water changes

  2. Koi eggs are fertilized outside the body so there is no pregnancy. Female koi with eggs typically have enlarged “abdomen.”


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