Koi will breed annually if your pond has males older than two and females older than three.
Fish typically spawn in late spring or early summer when water temperatures are between 65 and 75 degrees Fahrenheit and the days are longer.
You will typically notice spawning when it happens. A male will target an egg-bearing female and chase her to encourage her to release her eggs. This can go on for several hours but will cease once the female accomplishes this. After she lays the eggs, the male fertilizes them, and the baby koi will begin to grow.
A frothy texture at the water’s surface, which comes from the milt the male releases, also indicates spawning has occurred. It has an unmistakable, pungent odor that smells “fishy” and “earthy.”
If you want to raise some of the fry, you will need to remove eggs from the pond as soon as the female releases them. Koi will voraciously devour any eggs they find. However, some strategies will make it more likely for a few babies to survive if you choose not to separate.
Finding And Separating Eggs
Female koi will attach their eggs to a surface area, so the sides of the pond and undersides of leafy pond plants are the first places to look for them. (If you are hoping to “hide” eggs from their cannibalistic pond mates, the more plants, rocks, and shelters, the better.) Or, you can purchase spawning mops, made of nylon or synthetic material that the eggs will stick to.
A female koi can lay 100,000 eggs in a breeding season, so you won’t be able to move all of them to a nursery pond or tank. If you transfer too many, you will need to go through the culling process later—eliminating some fry to keep the population manageable. People not seeking distinct characteristics find it easier to separate a limited number of eggs and leave the rest in the pond as a food source.
Koi eggs hatch around four days after fertilization. Viable eggs will have a translucent appearance with a slightly brownish tinge. You’ll see tiny dots inside them, which can look like seeds. Sometimes you can even see movement and activity occurring within.
Unfertilized koi eggs will turn a milky white color, which signals the presence of fungus. You should remove these immediately, as the fungus can trigger a systemic infection that can kill the rest of the eggs.
Characteristics of Fry
Upon hatching, yolk sacs weigh down koi fry, which will need to develop a swim bladder and fins before they can swim, which occurs after about three days. At that point, they work their way toward the water’s surface to take their first few gulps of air, which expands their swim bladders and regulates buoyancy.
Fry have no pigmentation at this stage, making it impossible to distinguish one from the next–that is, if you can see them at all. They can be challenging to spot without magnification, as they are only about 7mm once they lose their yolk sacs.
If you decide to let nature run its course in your pond, you can consider fry that reach 3 inches to have outgrown the most vulnerable period for predation.
Using a filter in a nursery pond or tank is not a good idea because it can suction the fry. You can help prevent this by placing a fine net around a filter’s water intake, but it isn’t foolproof. It is best to initially control water parameters with regular water changes of 20 percent and by regularly testing ammonia, pH, nitrate, and nitrite levels.
A nursery pond or tank will need higher oxygenation than a typical koi pond, so you need to add an air stone.
For the first 24 hours after birth, the retained yolk will nourish the baby koi. After they lose their yolk sacs, you should feed them liquid or suspended-particle food for the first week. After that, they will be able to eat progressively larger, non-liquid foods.
If you are not separating the fry from your pond, you don’t need to worry about providing special food—any surviving fry will feed off the microorganisms in your pond.
For the first month, feed baby koi four times a day. When they grow to about an inch long, they are ready to eat commercial fish food. We recommend Blue Ridge Koi & Goldfish Food Growth Mini Pellet, which provides a balanced nutritional system for fry under 5 inches. If you’re using bigger pellets, grind them up so the babies can ingest them.
You can expand your koi’s diet to include certain human foods, such as fruits, vegetables, and even hard-boiled eggs, once they’re grown.
Returning Fry to the Main Pond
When your koi are around 4 to 6 months old, they will likely be large enough to return to your main pond. To do this, place the koi in bags with water from their nursery and float the bags on the pond’s surface, allowing the koi to acclimate to the water temperature. Release them after about an hour, and monitor them to ensure they are doing well.
You may notice that the newly introduced fish will shy away from the bigger koi fish in the pond. This is normal and will pass as they get used to their pond mates. Just make sure that you remember to include them during feeding time.
Also, keep an eye on your water parameters after introducing new fish. New additions will add to the filtration system’s overall load, so frequent checks are necessary for the first few weeks.
Leave Plenty of Room for Growing Koi
Remember that as the baby koi continue to grow, they will take up considerably more space in your pond, potentially causing overcrowding. A koi pond should contain at least 1,000 gallons of water with 200 gallons per additional fish. If you’re like many hobbyists, you will probably want to expand your pond to make more room.
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