How to Know if a Koi Fish is Pregnant

koi fish swimming in water

A common question most pond-keepers wonder is, “do koi fish get pregnant?”.   First, we have to determine how koi breed and their internal anatomy

Koi are oviparous breeders.  Meaning they will lay eggs with no embryonic development inside the mother. So they don’t really “get pregnant,” as we think of typically in humans.  View the photo below to see what koi fish eggs look like: 

holding koi fish eggs

Adam / CC BY-SA 2.0

When the eggs are shed from the female, they will fall to the bottom of the pond and stick to objects like plants, with the help of a sticky secretion on the egg’s outer layer.  

This is when the survival of the fittest starts.  Only the strongest will survive if you leave the egg’s in their place.  Although, you can move them to a spawning media container to help save the eggs.  If you do this, it is wise to treat the eggs with malachite green in a ratio of three parts per million.  This will sterilize the eggs for 10-15 minutes.  But, some koi breeders will avoid doing this in fear that the eggs will soak the chemical in—Which could lead to birth defects.  

Now let’s shift gears and figure out how these eggs were created and released in the first place.

Sexing Koi

To identify if your koi are male or female, look for a few main features.

Females tend to be bigger than males and possess larger, more transparent fins.

The pectoral fins of a male koi close to his head appear solid and pointed. Furthermore, the initial pectoral fin’s color could be more significant when it is compared to the female counterpart, which is likely to have more round fins. Ventrical fins on males are also more pointed and more prolonged.

Females also have a thicker body. When producing eggs, they appear to grow “plumper in the belly” because their reproductive organs (ovaries) need to make room for thousands of eggs. These organs, often known as gonads, could increase by 70% of their body’s total size (whoa!). And when most people think their koi fish is “pregnant.”  This means she is just ready to spawn.

When males are ready to spawn, you will see small white, raised spots that are called tubercles appearing on their heads, pectoral fins, and gill plates.  Which almost looks like grains of salt.  

The male uses these rough spots when chasing the female around, nudging her to spawn.  

This video will give you a visual of what to look for:

Generally, if a koi is less than ten inches long, it is assumed to be sexually immature. 

If you have a hard time with this, don’t worry, even professional koi breeders sometimes find it difficult to sex koi.  Most often on smaller fish.  

You can also watch their behavior. When they’re not mating, they behave similarly. However, the female will begin nosing around looking for possible egg-deposition sites during mating season. While the male will pursue the female, urging her (sometimes with enthusiasm) to let her eggs go to fertilize them.

Here is a video showing some typical behavior:

 

If you want to prevent your koi from getting pregnant/spawning, you will want to separate the males and females in the early spring, ahead of the spawning season.

The Time of Year Koi Lay Eggs

If the conditions are right (water temperatures need to be above 62F), koi will naturally spawn in late spring or early summer.  However, some specialty breeders in Japan will breed nishikigoi more often and with more consistent results. 

Spawning

The mating ritual is called the “spawning dance.” It usually occurs in the early morning hours after a warm night. The sound could break a quiet morning of thrashing at the water surface.  This means the dance has begun and can last up to four hours.  Here are the steps of the dance:

  1. Female prepares to release eggs with a signal pheromone, which excites the male.
  2. Male wrestles with the female, nipping, head butting, and pushing her.
  3. The Female spews eggs from her vent.
  4. Eggs are coated with the sticky layer, so they stick to plants.
  5. The male will also release his sperm (milt). 
  6. The milt gets into the egg through a tiny pore called a micropyle.  
  7. Fertilization is complete, and the micropyle closes. 
  8. The dance is paused, and both parents will eat some of the eggs.
  9. This cycle repeats until the female releases all the eggs.
  10. The female will be exhausted, and the male sometimes turns from pushing the female to eating more eggs.

When finished, the pond will become almost motionless.  

You can save some of the eggs at this stage by removing the fish or eggs from another area.

Koi Babies (Fry)

The eggs will hatch in about a week if you have a successful koi pregnancy/spawning. 

Fry usually spend their first couple of days clinging to the pond or plants’ sides and absorbing their egg sacs for nutrients.  Then the fry will need a high protein diet of brine shrimp, flakes, or daphnia.

With the cost of koi fish nowadays, this could be an excellent way to let nature give you free koi. But, be careful.  Fry are vulnerable to other fish, insects, or even being eaten by their parents.

Conclusion

So there you have it.  Now you know the answer when someone asks if koi get pregnant, what to look for during spawning, and how to save eggs if you choose to.