What Do Koi Fish Eggs Look Like? (Pictures & Facts)

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Koi Egg Guide – What Do They Look Like? How Long Until They Hatch?

Koi eggs on a person's hand
Fertilized koi eggs are clear & have a light yellow or brown color. Adam / CC BY-SA 2.0

If you’re interested in enlarging your koi population without purchasing new fish, you should try your luck at breeding them! Witnessing the entire fish lifecycle is a fascinating and unforgettable experience for all koi pond owners, seasoned or beginner. Like all fish, even the largest of koi start off as tiny fertilized eggs.

When these eggs are lucky enough to survive until the hatching period, they support the very first developmental stages of koi and provide the embryos with all the nourishment they need. Although this stage of the life cycle is fleeting, it is perhaps the most important. Anything that goes wrong during the egg stage of a koi’s life will influence its odds of survival if or when it hatches.

Keep in mind that the quality and viability of koi eggs are largely influenced by the health of your sexually mature fish. In truth, the process of egg production begins long before your female fish prepares to spawn. This goes to show that the quality of care for parent fish is just as important as the care you must eventually provide to eggs and juveniles.

How Many Eggs Do Koi Produce?

A large female koi can churn out as much as 100,000 eggs per kilogram of her weight!

A single female koi in her prime can produce hundreds of thousands of viable eggs. A large female can churn out as much as 100,000 eggs per kilogram of her weight!

She must produce this much as the chances of egg fertilization, attachment, and survival are extremely slim in the wild. Some breeders attempt to increase these chances by removing fertilized eggs and caring for them in a tightly controlled environment, devoid of potential predators and other sources of harm.

Fertilized vs. Unfertilized Eggs

Microscopical view of carp eggs 3-4 hours following fertilization. ResearchGate

During the first few hours after a successful spawning session, koi eggs will remain extremely small. In the beginning, they measure just a few millimeters in diameter and resemble tiny beads. It is inevitable that not all koi eggs will be fertilized, even under perfect conditions. Just 24 – 48 hours after spawning, unfertilized eggs can easily be distinguished from fertilized ones as they eventually turn milky white in color and look fuzzy (due to fungal growth). Over time, these eggs decompose if they aren’t consumed by your fish beforehand.

In contrast, fertilized eggs remain clear and are light yellow to brown in color. These don’t necessarily expand to accommodate a growing embryo. Up close, they will appear to contain small black specks. These first well-defined specks are actually the embryo’s eyes. As early as 40 hours after fertilization, developing fry can begin to be visible. If all goes well, they will hatch in just 4 – 5 days!

Here is a very useful video documenting each step of  the development process of fertilized koi eggs:

Requirements for Egg-Laying

Group of koi swimming together
The quality & viability of koi eggs are largely influenced by the health of the sexually mature koi, so it’s very important that you care for them properly. randychiu / CC BY 2.0

To ensure that your sexually mature females are able to produce viable eggs, you must provide them with quality, high-protein koi food. Males should likewise have access to quality fish feeds, though a “pregnant” female must consume much more to support the growing set of eggs in her belly. The quality of water is equally important because a weakened pregnant female will produce many bad eggs, even if she is allowed to recover afterward.

Female koi are confident enough to spawn and can safely deposit their eggs when the pond is diversified with aquatic plants. Those with underwater foliage and floating root systems would work well as these will eventually aid in protecting eggs and hatchlings. Moreover, these can aid in oxygenating your pond water. Maintained aeration and pH levels are absolutely necessary to increase the chances of egg survival.

If you don’t wish to cultivate plants in your pond, you can make use of a spawning rope or brush. This contraption resembles a soft wreath with fine grass and provides a lot of surface area for egg attachment.

When to Anticipate Koi Eggs

Koi in a pond
Water temperature should ideally be between 65 – 70˚F (18 – 21˚C) for female koi to spawn. jen / CC BY 2.0

When reared alongside sexually mature males, healthy females can spawn as much as twice per year. If exposed to naturally fluctuating ambient conditions throughout the year, they will generally spawn just once. This normally occurs in May – June, when the weather is most agreeable and when your koi’s energy and metabolic levels peak. Water temperature at this time should ideally range from 65 – 70˚F (18 – 21˚C).

The spawning event usually takes place early in the morning and tends to occur in stages. Koi will likely consume a large number of their spawned eggs to compensate for spent energy prior to releasing more. At breeding facilities, spawning may be artificially induced.

Caring for Koi Eggs & Fry

Koi fry underwater
These tiny little things are koi fry! You should care for your koi eggs in a separate chamber or tank, so that when they hatch, the fry don’t get eaten by other koi. Adam / CC BY-SA 2.0

When left to hatch in a pond, a huge percentage of fertilized eggs will likely be eaten by your koi. If your pond contains other animals, less than 10% will manage to hatch. This is why koi breeders prefer to collect the koi eggs and care for them in a separate, well-aerated chamber or tank. This will also ensure that the hatched fry are spared from predation.

If the koi eggs are attached to a spawning rope, you can simply scoop it out of the pond and move it to a nursery tank or some other water feature meant for fry. Similarly, if the eggs are attached to vegetation or moveable structures (bricks, stones, etc.), you can transfer these into the nursery. If the eggs are attached to immovable structures, you can use a fine net or your own hand to very gently collect them.

Make sure your incubation or nursery tank has optimized conditions for koi egg development. The water pH level should be neutral or slightly basic (7.0 – 7.5) and should be highly oxygenated. Your water’s temperature should be maintained at 68 – 71˚F (20 – 21.6˚C) to ensure that the eggs are able to hatch. Temperatures outside of this range can increase the occurrence of birth defects.

Separating Unfertilized From Fertilized Eggs

Breeders prefer to separate the unfertilized milky white eggs from the fertilized ones. Unfertilized eggs can encourage fungal growth, which may spread to the surrounding viable eggs and compromise the developing embryos within. Egg separation isn’t a necessary task, but it can certainly improve the hatch rate of fertilized eggs.

This task can prove to be quite difficult for clumsy first-timers. The white eggs need to be dislodged from the attachment site very carefully. Viable eggs can easily get dislodged as well. This risky process can even reduce, instead of increase, the hatch rate if some of the fertilized eggs get squished or torn in the process. Definitely leave this optional step to the experts, or get the help of a professional aquarist if you must separate the eggs.

Leaving Eggs in the Koi Pond

If you don’t intend to breed your koi and would instead prefer to take your chances, you can opt to leave the eggs in your pond. It’s highly likely that the majority of the embryos in the fertilized eggs will fail to make it to adulthood this way. Concerned about your pond being overcrowded? You can simply rely on your fish to decimate most koi eggs and fry as these are great sources of protein.

In case you would like to increase the chances of survival but must leave the eggs in the pond for practical reasons, you can make use of makeshift barriers and plants to protect them. A fine mesh net or wire used as a partition or placed around the area with eggs may keep the hungry fish away. Creativity is key here. Unfortunately, there’s no guarantee that this will protect the eggs from other potential predators, such as birds, frogs, or insect larvae.

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