Complete Guide to Koi Spawning (Signs, Behavior, Advice)

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Guide to Koi Spawning (Behavior, Signs, Season, & Advice)

Koi frenzy
The spawning season starts when mature fish reach peak energy levels in response to sexual pheromones. Rob Chan / CC BY 2.0

The spawning season is one of the most exciting times of year for pond enthusiasts and koi breeders. It starts when the energy levels of mature fish peak in response to sexual pheromones and environmental cues. Koi may appear to endlessly chase one another as they get a sense of the readiness of their partners. The flurry of movement enlivens the pond while flashes of brilliant koi colors dance in the water.

The spawning period of koi is the aquatic equivalent of the courtship and mating periods of other land-based animals. As koi are oviparous fish, with eggs that are fertilized externally, the females don’t get pregnant in the typical sense. They produce unfertilized eggs, which they then release into the wild once they are ready for fertilization. Successful mating does not necessarily require contact as long as the males release their sperm onto fresh eggs.  

To encourage successful spawning, aquatic conditions must be optimized. Best results are achieved when the fish are comfortable and receive high-quality sources of nutrients. Professional koi breeders usually prepare for the spawning period several weeks in advance. Keep in mind that before male and female koi are even allowed to spawn in a single pond or tank, you must be committed to caring for their young.

Readiness for Breeding

Group of koi fish
You may wish to separate sexually mature females from the rest of your koi, as sexually mature males may act aggressively around them & increase their stress levels. frank green frankg, CC0, via Wikimedia Commons

Once koi reach an age of about 2 – 3 years old, they are usually sexually mature. It’s best to wait until they are 3 – 4 years old and at least 10 – 12 inches long to encourage breeding.  Their capacity to produce healthy eggs and sperm peaks once they are closer to their maximum size and have fully developed their colors.

If koi begin spawning as early as 2, they may have difficulty recovering from the stress of breeding. As most of their energy stores would be directed toward producing gametes, their growth may also be affected. The chances of producing high-quality offspring coupled with quick recovery are increased when slightly older koi are used. They tend to be moderately fecund until they are around 6 – 7 years old, after which they should be allowed to retire from breeding.

Sexually mature females that begin to search for ideal spawning areas can be singled out and transferred to a dedicated spawning pond or large tank. They can also be left in their resident ponds, though the presence of other fish will significantly reduce the survival rates of their young. Moreover, if there are multiple sexually mature males in the pond, they may behave roughly around the female and increase her stress levels.

Best Ratio for Successful Spawning

Koi pond
Male koi should be a similar size to female koi (or even smaller) to help reduce stress. Nullumayulife / CC BY 2.0

Ideally, sexually mature male and female koi should be present in the pond at a ratio of 1:1. This reduces the stress on pregnant females and prevents males from being aggressive with one another in an attempt to chase the pregnant female. A ratio of 2:1 may also be acceptable in large ponds that are equipped with ample protective vegetation or potential hiding places for stressed females.

To reduce stress, males and females should be evenly sized. Smaller males can also be used as they are less likely to generate enough force to push or trap larger females. Although it may produce more variation in offspring, a higher number of males may be detrimental to the survival of mature females. It may also result in the loss of more eggs as a huge percentage of them, including the fertilized ones, may be consumed by the mature koi.

Optimal Spawning Conditions & Season

Aquatic plants
Female koi like to release their eggs close to aquatic plants, as they provide protection from predators. High Contrast, CC BY 3.0 DE, via Wikimedia Commons

The spawning period usually takes place from May to June as these pleasantly warm months allow for a higher metabolic capacity. In late spring or early summer, mild temperatures serve as an environmental cue for courtship. Ideally, the temperature at this time of year should range from 18 – 21˚C (65 – 70˚F). In warmer temperate, subtropical, or tropical zones, koi may spawn more than once in a single year. In hatcheries, spawning may be induced with the use of injectable hormones.

Throughout the spawning period, water parameters should be optimized. Dissolved oxygen levels should be at least 7 ppm to prevent the fish from becoming stressed throughout the process. Water clarity should be at an all-time high and the pH should be neutral. Nitrate levels should be kept at a minimum, with ammonia levels close to 0 mg/L. A pump and filter would help maintain desirable parameters throughout the pond so that clean water can circulate around the eggs wherever they are released.

Females prefer to release their eggs in areas with heavy vegetation. Floating plants with submerged root and shoot structures may suffice. The eggs, which are sticky, usually become attached to the leaf surfaces. When the aquatic plants are more complex and crowded, the eggs are afforded more protection from predators. Some breeders opt to use artificial structures, such as a spawning mop or brush, that can easily be moved out of the pond and into a separate tank for rearing fry. 

Appearance of a “Pregnant” Female

Pregnant female koi
A “pregnant” female koi may look bloated because their abdomen has to stretch to accommodate hundreds of thousands of eggs! Alan L / CC BY 2.0

The presence of sexually mature males incites the production of eggs in mature females. As the eggs, which can easily number in the hundreds of thousands, increase in size, the ovaries expand. The abdomen of the female koi must stretch to accommodate them.

Closer to the spawning date, the female may appear markedly bloated and rounded. At this stage, it would be very easy to tell her apart from the males. She must be fed high-quality, protein-rich feeds with a well-balanced nutritional profile. Inadequate food may lead to poor egg quality, a reduced number of eggs, or malnourished females. Breeding koi may need to be fed up to 4 times a day during the spawning season.

Koi Courtship Behavior

Koi swimming
Male koi are extremely clingy towards females when their eggs are almost ready. Bernard Spragg. NZ / No copyright

When the eggs are almost ready, the female koi begins to release pheromones that signal her readiness to breed. Males respond to the pheromones by approaching the female and staying very close to her. They may begin to push and prod the female while chasing her around the pond or tank, though most of the action will take place around the spot that the female has chosen for spawning. In planted ponds, the female may repeatedly touch or brush her body against the vegetative structures.

As the spawning period approaches, male koi become increasingly more aggressive. It may be necessary to place a net above the pond or tank, as they may lunge at the female or violently push her toward the shallow edges of the pond or the foliage. The female may attempt to jump out of the water, especially if there is more than one male.

Up until the actual spawning event, males remain exceedingly clingy and continue to give the female their full attention in the form of bumps and head-butts. The entire courtship process is exhausting for all the fish involved, which is why pond conditions must be optimized. Avoid conducting heavy pond maintenance activities during the spawning period as these may cause unnecessary stress.

The Spawning Event

Pond foam
If foam begins to accumulate in your koi pond, it may mean that your koi are spawning! Kristopha Hohn / CC BY-SA 2.0

The morning after the peak of male courtship, which takes place when ambient temperatures and daylength are finally adequate for spawning, the female koi begins to release her eggs. They may not be released all at once, particularly if an exceedingly large amount has been produced. The female will take time to spread her eggs over vegetation or the spawning brush. The males follow suit, releasing their milt onto the same areas.  

Foam accumulating close to aerators, filters, or water features is usually a sure sign of spawning koi. The foam may have an unpleasant or fishy odor that may waft around the entire pond area for a few days. The water may also become increasingly cloudy while the koi move in spurts through or above the submerged vegetation. It may take quite some time until the female has released all of her eggs. A single female that begins spawning in the early morning hours will usually release the last of her eggs by noon. If there are several females in the pond, the process of releasing eggs may take up to 3 days.

As the fish are very preoccupied during the main spawning event, it would be best to withhold feeding. You may feed very small amounts – just enough to satisfy individuals that may take the chance to feed. It is advisable to check the water parameters every few hours as nutrient levels may spike due to the expelled materials. If there are several spawning couples in a single pond, large amounts of foam may accumulate and compromise the water quality. Be prepared to conduct partial water changes in a manner that would not disrupt the spawning event.

Post-Spawning Care

Koi feeding
After spawning, you can go back to feeding your koi as usual. Jack Dorsey / CC BY 2.0

Males begin to lose interest in female koi once all of their eggs are released. This means that the females can once again swim around the pond in peace. Both koi sexes will soon begin to eat the majority of the eggs in the pond. More often than not, this leaves around 10% of the full number of released eggs. As even fewer eggs will have been fertilized successfully, just a handful of koi fry are likely to survive. This is why breeders prefer to remove the eggs from the pond and care for them in a separate enclosure.

After the spawning event, you can resume feeding the fish at a regular frequency and with the normal amount of feeds. Females may take much longer to recover from spawning compared to males, so take time to observe them and note unusual behavior or immobility. Healthy females are usually able to recover quickly. Over the course of a few days to weeks, their bodies will return to their usual form.

Make sure to inspect and clean out any submerged pond gear, pumps, and filters after the main spawning events. Eggs may gather there and may, unfortunately, begin to rot if left unchecked. Continue monitoring the water parameters and perform the necessary partial water changes to prevent drastic increases of ammonia and nitrates. The post-spawning period can be quite demanding. It calls for much attention and heavy water maintenance, but rest assured that your efforts will be rewarded by the memorable experience.

Preventing Spawning

Group of koi fish
You can try to prevent spawning in a number of ways, such as “tricking” your koi into thinking it isn’t the right time of the year! Alabama Extension / No copyright

Whether or not you intend to breed koi, keeping both sexually mature males and females in a pond will inevitably lead to spawning. Successful fertilization will likewise lead to more young fish, which can eventually crowd out the pond in the absence of natural predators. Moreover, spawning can be stressful for koi fish and cause them to be extremely vulnerable to diseases.

Some fish enthusiasts wish to house koi in their ornamental ponds but have no interest in breeding them. If you intend to avoid the entire spawning/mating process, it may be best to maintain single-sex ponds or separate the males from the females during each spawning season.

Another way to prevent koi from spawning is by tricking them into thinking it isn’t the right time of year. This is done by keeping the water temperature low all throughout spring. You may also need to keep the pond shaded to simulate reduced daylight hours. Regardless, these attempts may still fail to prevent spawning. If you begin to spot courtship behavior or the enlarged abdomen of females, temporarily house either the male or female fish in a separate tank or pond.

Using a Dedicated Tank for Spawners

Koi in spawning tank
You should add spawning mops or aquatic plants to a tank so that the koi are encouraged to spawn. KoiQuestion / CC BY-SA 2.0

Many professional breeders prefer to isolate their gravid females and pair them off with selected males to secure a bloodline or create hybrids of superior quality. They usually house their spawners as a lone couple or at a ratio of 3:1 (more males) in a tank with a surface area of about 6 – 9 square feet. This way, the female’s eggs would be fertilized only by the sperm of the selected males. The tank must be filled with clean freshwater and disinfected (e.g. with methylene blue) before the koi are introduced.

The tank should be outfitted with foliage-rich shoots of aquatic plants to encourage spawning. These can also be replaced with spawning mops to serve as attachment points for the eggs. Once the tank is ready, the spawners should be acclimatized for at least 20 minutes before releasing them into the water. Make sure the water is adequately aerated throughout the process.

Once the spawners have been released into the tank, you can partially cover it with a dark-colored net or shade cloth to allow them to calmly become acquainted with their new surroundings. After a few nights, if the temperature is right, the koi will begin spawning. They may begin releasing their gametes a few hours before dawn.

Towards the end of the spawning period, some experienced breeders may opt to “hand strip” or “milk” females in an effort to facilitate the quick release of all the eggs.  The milt of males may also be forced out of the caudal peduncle. Once the fish are spent, they can be removed from the tank and returned to their ponds. The spawning tank, devoid of adult koi, can then be used to care for the eggs and fry!

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