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Fry rearing and culling (senbetsu)

By the koi.wiki editorial team · Updated

Key facts: Fry rearing and culling, known in Japan as senbetsu (選別), is the process of hatching koi eggs, feeding the resulting fry, and repeatedly selecting or discarding individuals so that only those with desirable color and pattern remain in the breeder's stock.[1][2]
A single koi spawning can produce hundreds of thousands of eggs, and according to Kodama Koi Farm a spawning can yield around 400,000 fry[3]. Wikipedia notes that even from champion-grade parents, the large majority of offspring are not acceptable as nishikigoi (錦鯉) because they lack interesting colors or carry genetic defects[1]. Breeders therefore cull offspring at various stages of development based on experience and closely guarded trade techniques[1].Rearing begins with the eggs themselves. The Southern Regional Aquaculture Center (SRAC) states that koi eggs are adhesive and hatch in 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)[2]. The dealer Kodama Koi Farm describes keeping an incubation tank at a constant 77 °F (25 °C), at which eggs hatch in about 3 days[5].

Hatching and first feeding

According to SRAC, larvae carry a yolk sac and begin taking feed on the second or third day post-hatch, readily accepting Artemia nauplii (brine shrimp) or other foods in the 200–300 micrometer size range[2]. Kodama Koi Farm describes a similar practice for hobbyists: fry need food about 24 hours after hatching, and water fleas are the traditional food used by professional Japanese breeders, though brine shrimp is a common hobbyist substitute[5]. To hatch brine shrimp, the farm describes filling a vessel with water at 0.2% salinity, adding dried brine shrimp eggs, and keeping the water at 83–86 °F (28–30 °C) with aeration; the eggs hatch within 24 hours and the shrimp are then siphoned into the fry tank[5]. After about 10 days on brine shrimp, fry are large enough to switch to artificial feed[5].UF/IFAS notes more generally that fry and fingerling diets frequently exceed 50% crude protein, with protein levels and particle size reduced as fish grow, and that rearing young stock requires small, frequent meals[6]. SRAC adds that a standard 25–32% protein diet is typical through growout, with particle size increasing from fine meal to roughly 1/4-inch pellets as fish mature[2].

Why culling is necessary

Wikipedia explains that although a breeder may carefully select parent koi for desired characteristics, the resulting fry still show a wide range of color and quality, and the large majority will not be acceptable as nishikigoi[1][4]. For Showa Sanshoku, Kodama Koi Farm states that mating two Showa yields roughly 70% pink-bodied fry and 30% black-bodied fry; pink fry tend to become Kohaku while black fry become Showa, and sorting hundreds of thousands of fry by this distinction is described as the hardest part of Showa breeding[3]. The Zen Nippon Airinkai (ZNA) notes that within Kohaku, fry with an entirely white ground (Shiro-muji) or entirely red ground (Aka-muji) are usually removed at the first culling[7].Culling is a long-established practice in ornamental fish production generally. The Southern Regional Aquaculture Center notes that most varieties, especially those dependent on color, must grow for several months before their true appearance is known, and that grading and culling for appearance is done manually, representing a significant cost of production[2]. In systems producing the most expensive varieties, SRAC states that as many as 90 percent of fish may be deemed unmarketable[2].

The first cull and later selections

According to Wikipedia, culled fry are usually destroyed or used as feeder fish, often for arowana due to a belief that it enhances the arowana's color, while older culls — within their first year and between 3 and 6 inches (about 7.5–15 cm) long, a size class also called tosai — are often sold as lower-grade, pond-quality koi[1][4][8][9][10][11]. The term tosai and related age terms are discussed further on the page about age terms: tosai, nisai, sansai and yonsai.A Kodama Koi Farm forum discussion describes an early-stage technique for separating black-skinned fry using a suction device, illustrating that some culling methods are developed informally by individual breeders rather than published as standardized procedures[5].

Grading methods

University of Florida IFAS Extension describes grading as the visual or mechanical inspection of fish to select for characteristics such as size, color, fin development, and health indicators including scale loss, torn fin rays, eye damage, and parasite load[12]. A common procedure is to grade mechanically for size first and then by hand for color and other characteristics[12]. Mechanical graders are floating boxes with grates whose spacing, usually given in millimeters, determines which fish pass through versus are retained[12]. Hand grading allows inspection of each fish for multiple characteristics and can involve a wetted grading table with holes leading to separate buckets for sellable fish, keepers, and culls[12].UF/IFAS also notes that careful handling during grading is important to minimize stress, and that sensitive fish species should be graded by hand rather than mechanically, since crowding in a mechanical grader can cause physical damage[12]. In pond harvesting, UF/IFAS distinguishes grading (mechanical separation or visual inspection for size or color) from culling (removal of unwanted fish that do not display desired characteristics)[13].

Culling and genetics

Culling undesirable individuals from a population is a general selective-breeding principle. According to UF/IFAS, culling helps maintain favorable traits in a population and prevents low fecundity, stunted growth, and poor quality, particularly in ponds where fish of varying ages and sizes are mixed together[12]. Wikipedia's general article on selective breeding describes how breeders develop superior bloodlines by "breeding the best to the best," employing a degree of inbreeding and considerable culling, a principle historically applied across livestock and companion-animal breeding and conceptually mirrored in koi breeding practice[14].Culling can also play a role in managing disease. UF/IFAS states that removing infected fish from a population can help reduce overall viral load and infection rate in cases such as lymphocystis, though it notes this is difficult to do completely since some infections are not visible to the naked eye, and culling may be more practical for hobbyists or display aquaria than for large aquaculture populations[15].

Growout after culling

Fish that pass selection continue to grow before reaching market size or show quality. SRAC notes that many ornamentals can take a full year to reach market size or quality, and that the deep red color of some koi can take more than a year to fully develop[2]. It adds that champion koi brood fish command very high prices and are usually accompanied by a pedigree demonstrating purity of bloodline, in contrast to common, inexpensive koi whose purchase cost may exceed the benefit of using them as broodstock[2]. Readers interested in the varieties that selection decisions are made for can consult the koi variety comparison table and the overview of koi varieties.

Common questions

What does senbetsu mean in koi breeding?
Senbetsu (選別) refers to the selection and culling process by which breeders repeatedly inspect fry and young koi and remove individuals that lack desirable color, pattern, or form[1].
Why are so many koi fry culled?
A single spawning can produce hundreds of thousands of fry, and according to Wikipedia the large majority, even from champion-grade parents, are not acceptable as nishikigoi because they lack interesting colors or carry genetic defects[1]. Kodama Koi Farm notes spawnings can yield around 400,000 fry that must be sorted[3].
When is the first cull done?
According to the Zen Nippon Airinkai, within Kohaku bloodlines, fry with an all-white or all-red body are usually removed at the first culling[7]; more broadly, SRAC notes most color-dependent varieties must grow for months before their true appearance can be judged[2].
What happens to culled koi?
Wikipedia states that culled fry are usually destroyed or used as feeder fish, often for arowana, while older culls in their first year (tosai) and roughly 3 to 6 inches (7.5–15 cm) long are often sold as lower-grade, pond-quality koi[1].
How is grading different from culling?
UF/IFAS defines grading as the mechanical separation or visual inspection of fish for traits like size or color, while culling is specifically the removal of fish that do not display the desired characteristics[13].

References

  1. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Nishikigoi. Retrieved November 2026.
  2. ^ 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.
  3. ^ Showa - Koi for Sale and History - Kodama Koi Farm. Kodama Koi Farm (koi dealer). kodamakoifarm.com/variety/showa/. Retrieved November 2026.
  4. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Utsurimono. Retrieved November 2026.
  5. ^ How to Breed Koi. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-to-breed-koi/. Retrieved November 2026.
  6. ^ UF/IFAS EDIS FA096: Fish Nutrition. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA096. Retrieved November 2026.
  7. ^ KOHAKU | Zen Nippon Airinkai. Zen Nippon Airinkai. zna.jp/en/keep-nishikigoi/varieties-of-nishikigoi/kohaku/. Retrieved November 2026.
  8. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Goshiki. Retrieved November 2026.
  9. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Koi. Retrieved November 2026.
  10. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Ghost_koi. Retrieved November 2026.
  11. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Shusui. Retrieved November 2026.
  12. ^ UF/IFAS EDIS FA118: Grading Ornamental Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA118. Retrieved November 2026.
  13. ^ UF/IFAS EDIS FA117: Harvesting Ornamental Fish From Ponds. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA117. Retrieved November 2026.
  14. ^ Selective breeding (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Selective_breeding. Retrieved November 2026.
  15. ^ UF/IFAS EDIS FA181: Lymphocystis Disease in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA181. Retrieved November 2026.