How to Keep Koi from Eating Their Young 

Although koi are known for their friendly demeanors and calm temperaments, like most fish, they are omnivores and will eat fish that are significantly smaller than them. If spawning occurs but fish don’t emerge, it is likely because of opportunistic feeding. 

The probability of any fry surviving in a pond with adult fish is small. This is not an issue for hobbyists not looking to grow their numbers. However, if you want to raise koi fry, you must take steps to ensure their safety.  

Koi Reproduction 

A koi’s spawning period is the aquatic equivalent of land-based animals’ courtship and mating rituals. Fish typically spawn in late spring or early summer when water temperatures are between 65℉ and 75℉. Koi are oviparous, which means that fertilization and embryo development occurs mostly outside the body of the female koi.

A female emits pheromones that signal her readiness to breed. Once a male picks up the scent, he will nudge, prod, and chase her around the pond to encourage her to release eggs. This will continue until the eggs are laid and fertilized, which can sometimes span a few hours. 

Though she might eat them later, it’s the female’s instinct to hide her eggs. Pond plants’ leaves, stems, and root systems provide locations to attach egg clusters. Submerged plants such as fanwort and waterweed are ideal because their dense leaves offer shelter. 

Floating plants will sometimes suffice if they have extensive root systems. Or, you can purchase spawning mops made of nylon or synthetic material that the eggs will stick to. 

Why Do Koi Eat Their Babies? 

Though koi are intelligent (they can identify their caregivers, hand feed, and even learn tricks), they don’t recognize their offspring. Thus, in most cases, the consumption of their own is a case of mistaken identity. Eggs resemble pieces of pellets that comprise most of a koi’s diet. While grazing upon plants, a koi that notices eggs will automatically consume them. 

Similarly, at just a centimeter, newly-hatched koi resemble bugs, tadpoles, or other tiny fish that koi would naturally consume. Once fry grow to about 6 inches, adult koi will find them familiar and stop pursuing them as food. 

Koi eat their young out of ignorance, not aggression. This is a characteristic of most fish species and is not unique to koi. In other words, this behavior is normal. 

How to Prevent Koi from Eating their Babies 

Only one reliable method exists for guaranteeing that koi do not make a meal out of their offspring: separating the babies from the adults as soon as possible. Even waiting the approximately four days it takes fry to hatch might be too long. 

However, a female koi can lay 100,000 eggs for every kilogram of her weight in a single breeding season. So as long as you keep an eye on your pond, you should be able to find plenty of eggs. 

One strong indicator that spawning has occurred is foam accumulation in the pond. This frothy texture at the water’s surface comes from the milt that males release. It typically has an unpleasant, fishy odor. The water may cloud over the course of a day or two. Once it begins to clear, it is time to start gathering eggs.

Recognizing Eggs 

Before transferring eggs to a nursery, it is essential to know whether they are fertilized and are viable. Out of 100,000 eggs laid, only about 20,000 (without the presence of predators) would successfully hatch. Not all koi eggs will be fertilized, even under perfect conditions. During the first few hours after a successful spawning session, koi eggs will remain extremely small, measuring just a few millimeters in diameter. After 24 hours, you can begin to differentiate between fertilized and unfertilized eggs. 

Viable eggs will have a translucent appearance with a slightly brownish tinge. If you inspect them closely, you might see tiny dots inside, which can look like seeds and signal the presence of an embryo. Sometimes you can even see movement and activity occurring within. Unfertilized koi eggs will have a milky white color and, after a few days, will begin to look fuzzy as fungus sets upon them. You should remove these immediately, as the fungus can trigger a systemic infection that can kill the rest of the eggs.

Other Methods 

If you do not want to separate fry but want to maximize the chance that a few will survive to adulthood, there are ways to set up your pond to give the babies their best chance of survival.

Adjust your feeding schedule

If koi are continuously satiated, they are less likely to scan for food between meals. Overfeeding koi can have dangerous consequences, but you can consider breaking meal portions into small sizes and feeding throughout the day. You also can add healthy treats to the diet and provide those intermittently. 

Add hideouts 

Pond plants provide shelter and a food source for fry, so more plants in your pond means a better chance of survival. However, don’t go overboard; too many plants can harm your pond. Plants require oxygen and nutrients to survive, so they will eventually compete with your koi for resources. Plants should not comprise more than 60% of your pond. 

Rock arrangements also can provide a safe place for young koi, primarily if small grooves and gaps exist that adults cannot reach. 

Preventing Koi from Spawning 

If you are uncomfortable with any loss of life, then you will need to prevent your koi from spawning in the first place. The most obvious way to do this is to cultivate an all-male or all-female pond. If your pond is already established, you can determine each koi’s sex and separate the two during spawning season. 

An alternative is to trick your fish into thinking that spawning season never arrives. This is done by keeping the water temperature low throughout spring. You may also need to keep the pond shaded to simulate reduced daylight hours.

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