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Spawning

By the koi.wiki editorial team · Updated

Key facts: Spawning is the process by which koi release eggs and sperm into water for external fertilization, typically triggered by rising spring water temperatures.[1][2]
Koi (Cyprinus rubrofuscus, an ornamental variety of common carp) reproduce through spawning, the general term for fish releasing eggs and sperm (milt) into water for external fertilization[3][1]. Like other cyprinids, koi are egg-layers whose sticky eggs attach to plants, ropes, or mats rather than being carried by the parents[4][2]. Hobbyist breeding is commonly described as a springtime event that koi keepers get only one chance at each year koi breeding[2].Both home aquaculture sources and extension publications describe spawning as closely tied to water temperature, the use of artificial spawning substrates, and a short incubation period before hatching[1][2].

What is spawning

Spawning refers broadly to the release of eggs and sperm into water by aquatic animals, a reproductive strategy used by the majority of bony fishes[3]. Koi and common carp are classified as obligatory plant spawners, meaning they scatter adhesive eggs onto submerged vegetation or other substrate rather than building nests or guarding young[4][5]. During spawning a female is typically followed and "bumped" by several males in shallow water, with multiple males fertilizing the eggs as they are released, a polygamous pattern also noted for common carp generally[1][4].

Temperature triggers

According to the Southern Regional Aquaculture Center, koi and goldfish generally spawn as water temperature approaches 68°F (20°C), and to reduce spontaneous breeding, males and females are often kept separated until it is time to spawn[1]. FishBase similarly reports that common carp breed at a temperature range of 15–20°C (59–68°F)[4]. The koi dealer Kodama Koi Farm describes placing spawning ropes once water reaches about 75°F (24°C), and separately advises maintaining a stable range between 65°F and 75°F (18–24°C) to encourage spawning and support koi health[2]. The Southern Regional Aquaculture Center also notes that koi and goldfish have an optimum temperature range around 65–75°F (18–24°C), though they can survive temperatures from 32°F (0°C) to 95°F (35°C)[1].

Spawning ropes, mats and brushes

Because koi eggs are adhesive, breeders provide artificial spawning substrate so eggs do not clump on the tank or pond floor[1]. Kodama Koi Farm describes preparing 10–15 spawning ropes, folding long ropes and hanging several strands from a top rope inside a round show tank filled to about 2 feet (0.6 m) deep, with aeration and a cover net in place before introducing the parent koi[2]. The same source notes that synthetic spawning mats can be used to mimic plant textures and give koi a better surface for egg-laying[2]. In commercial koi and goldfish culture, the Southern Regional Aquaculture Center describes spawning mats placed around pond or tank edges, which are moved to a separate pond or tank once spawning is complete to protect the eggs, since both koi and goldfish consume their own eggs after spawning[1].

Broodstock and spawning behavior

Kodama Koi Farm recommends using one female with two to three males, all older than three years, for a hobbyist spawning attempt[2]. The Southern Regional Aquaculture Center notes that although koi reach sexual maturity around age one, commercial brood stock are usually at least two years old, and that larger females produce more eggs[1]. According to the koi dealer Kodama Koi Farm, male koi become more active during spawning season and chase females to prompt the release of eggs, and keepers may need to swap in a different parent koi if a pair does not spawn after several nights[2]. The same dealer notes that males can develop white breeding tubercles on the gill covers and fins during spawning season, while females appear swollen in the abdomen and move more slowly; see sexing koi for further detail on these seasonal cues[6]. The All Japan Nishikigoi Promotion Association notes that nishikigoi lay tens of thousands to several hundred thousand eggs in a single spawning, and that some breeders use over one hundred parents, producing millions of eggs[7].

Egg care and disinfection

After eggs appear on the spawning ropes, Kodama Koi Farm describes disinfecting them by dissolving malachite green (2 grams per 265 gallons) into water and soaking the egg-covered ropes for 30 minutes before moving them to a separate incubation tank of matching temperature, in order to reduce contamination[2]. The Southern Regional Aquaculture Center describes an alternative commercial method in which eggs and sperm are mixed in a bowl (the "dry method"), then treated with a salt-and-urea solution to allow fertilization and water hardening without activating the adhesive layer, followed by a brief rinse in a tannic acid solution that eliminates adhesiveness so the eggs can be hatched in upwelling jars[1].

Hatching

Kodama Koi Farm states that keeping incubation water at a constant 77°F (25°C) is key to healthy fry, with eggs hatching in about 3 days at that temperature[2]. The Southern Regional Aquaculture Center gives a broader range, reporting that koi and goldfish eggs hatch within 2 to 9 days depending on temperature, hatching in 46 to 54 hours at 84°F (29°C) and in 5 to 7 days at 70–75°F (21–24°C)[1]. FishBase reports a similar figure for common carp generally, with eggs hatching in about 4 days[4]. Newly hatched fry carry a yolk sac and begin taking feed on the second or third day post-hatch, and are large enough at first feeding to immediately accept brine shrimp nauplii or similarly sized foods[1]. Kodama Koi Farm notes that fry need food starting about 24 hours after hatching and describes hatching brine shrimp (Artemia) eggs in a separate heated vessel as a food source, after which fry can be switched to artificial feed after about 10 days[2]. For the subsequent stages of rearing and selecting young koi, see fry rearing and culling (senbetsu).

Koi spawning in broader context

Koi reproduction follows the broadcast- and substrate-spawning pattern typical of many bony fish, in which eggs and sperm are released externally with no subsequent parental care of the brood[3]. This differs from livebearing fish kept by other aquarists, which produce a small number of larger offspring directly[8]. Goldfish, close relatives of koi within the carp family, show a very similar pattern: breeding after a seasonal temperature change, with males bumping and nudging females to release eggs that adhere to vegetation or a spawning mop, hatching within 48 to 72 hours[9]. Fish that have just finished spawning are generally weaker and less resilient to environmental changes, and spawning exhaustion is a recognized cause of fish kills in some species[10].

Common questions

At what water temperature do koi spawn?
The Southern Regional Aquaculture Center reports that koi and goldfish generally spawn as water temperature approaches 68°F (20°C)[1], while the koi dealer Kodama Koi Farm describes setting up spawning ropes at around 75°F (24°C) and recommends a stable range of 65–75°F (18–24°C) to encourage spawning[2].
What are spawning ropes used for?
Spawning ropes and mats give koi an adhesive surface, similar to aquatic vegetation, onto which the sticky eggs can attach and be kept from clumping on the tank floor[1][2].
How long do koi eggs take to hatch?
Hatching time depends on temperature: Kodama Koi Farm reports about 3 days at a constant 77°F (25°C)[2], while the Southern Regional Aquaculture Center gives a range of 2 to 9 days depending on temperature, with faster hatching (46–54 hours) at 84°F (29°C)[1].
Do koi eat their own eggs?
Yes; the Southern Regional Aquaculture Center notes that both koi and goldfish consume their own eggs after spawning, so brood fish and spawning mats or ropes with eggs should be separated from the parents quickly[1].
How many eggs does a koi lay in one spawning?
The All Japan Nishikigoi Promotion Association states that nishikigoi lay tens of thousands to several hundred thousand eggs in one spawning, and that breeders using over one hundred parents can produce millions of eggs[7].

References

  1. ^ SRAC Publication No. 7201: Species Profile: Koi and Goldfish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/7201%20Species%20Profile-%20Koi…. Retrieved November 2026.
  2. ^ How to Breed Koi. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-to-breed-koi/. Retrieved November 2026.
  3. ^ Spawning (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Spawning. Retrieved November 2026.
  4. ^ Cyprinus carpio, Common carp : fisheries, aquaculture, gamefish, aquarium. FishBase. fishbase.se/summary/Cyprinus-carpio.html. Retrieved November 2026.
  5. ^ Common Carp (Cyprinus carpio) - Species Profile. US Geological Survey. nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=4. Retrieved November 2026.
  6. ^ Male vs Female Koi Fish: How to Identify Gender and Koi Sexing Tips. Kodama Koi Farm (koi dealer). kodamakoifarm.com/male-koi-vs-female-koi-fish/. Retrieved November 2026.
  7. ^ Valuable reasons - All Japan Nishikigoi Promotion Association. All Japan Nishikigoi Promotion Association. jnpa.info/en/nishikigoi/reason/. Retrieved November 2026.
  8. ^ Fishkeeping (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Fishkeeping. Retrieved November 2026.
  9. ^ Goldfish (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Goldfish. Retrieved November 2026.
  10. ^ Fish kill (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Fish_kill. Retrieved November 2026.