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Mud ponds (Niigata field ponds)

By the koi.wiki editorial team · Updated

Key facts: Mud ponds, known in Niigata as doro-ike (泥池), are outdoor earthen "field ponds" used by Japanese koi breeders to grow young koi during the warm months, before the fish are harvested in autumn and moved to indoor tanks for winter[1].
In Niigata Prefecture, the main production area for nishikigoi, breeders raise fry and young koi by releasing selected individuals into outdoor earthen ponds, locally called mud ponds or field ponds, where they grow from spring through autumn before being harvested and moved indoors for the winter[1]. This cycle is part of a production process first developed in the historic Nijūmuragō (二十村郷) region spanning Ojiya and Yamakoshi, where farmers originally kept food carp in terraced ponds attached to rice paddies as early as 1781[6][2].According to prefectural production records, after spawning the fry are sorted (selected) multiple times as they grow, and are kept in mud ponds during summer and autumn before the surviving, selected fish are lifted from the ponds in autumn in a process called ikeage (池上げ) and transferred to indoor tanks for the winter[1].

What is a mud pond?

A mud pond is an outdoor, earth-bottomed pond used seasonally to grow young koi, as distinct from the concrete or lined tanks used for winter storage or retail display[1]. Niigata's mud ponds trace back to ponds originally built for food-carp farming near rice terraces in the Nijūmuragō area spanning Ojiya and Yamakoshi, where carp were already being raised by 1781[2][6]. Ojiya City, which still produces a large share of Japan's nishikigoi, states that colored koi arose from mutations among food carp bred in this mountainous area during the early 19th century, and that selection and breeding by local farmers over generations produced the ornamental koi known today[7].

Why breeders use mud ponds

According to the dealer Kodama Koi Farm, koi raised in a Japanese mud pond take in natural minerals from the mud and water that are often missing in concrete ponds, which the source states can support growth, body structure and immune health, and also benefits color development; the same source notes that mineral supplements are sometimes added to pond water elsewhere to approximate mud-pond water quality[3].Kodama Koi Farm, a koi dealer, also states more generally that koi growth depends heavily on environment rather than genetics alone, and that spacious ponds, strong filtration, consistent feeding and low stress allow koi to grow much larger than they would under poor conditions[8]. The same source describes summer as the most important growth period of the year, when warm, stable water allows koi to metabolize food efficiently and grow fastest, while growth slows in autumn as fish condition for cooler weather and nearly stops in winter[8].

The seasonal cycle: spring to autumn

Niigata's published production process begins after spawning, when hatched fry are selected (a culling process) several times as they grow[1]. Selected fish are then kept in outdoor mud ponds from summer into autumn, during which breeders must manage weather exposure, predators, feeding amounts and disease risk[1]. In autumn, as water temperatures fall, the grown koi are lifted from the mud ponds in the ikeage process and moved into indoor tanks for the winter[1]. This matches the broader seasonal pattern described for koi growth, in which spring brings a return of appetite and accelerating growth, summer is the peak feeding and growth period, autumn is a conditioning phase of reduced feeding as fish build reserves, and winter brings minimal feeding and near-dormant metabolism[8].

Autumn harvest practices

General guidance on harvesting ornamental fish from outdoor ponds, published by University of Florida IFAS Extension, notes that ponds stocked with fish of similar age are typically harvested within a single day or a few days using seine nets, and that water levels may be lowered or ponds drained to concentrate fish before netting[4]. The same source cautions that repeated pulls of a seine net disturb bottom sediment, increasing turbidity that can clog fish gills, and can also release hydrogen sulfide gas produced by decaying organic matter in pond mud, which is lethal to fish at high concentrations; it recommends harvesting quickly and efficiently to minimize these risks as well as scale loss, abrasions and fin or eye damage[4].

Pond bottom management and water quality

Earthen pond bottoms require their own maintenance considerations. The Southern Regional Aquaculture Center notes that new or renovated ponds are sometimes partially filled with water from adjacent, established ponds as a disease-prevention measure, since the actinosporean stage of proliferative gill disease is carried by worms living in pond bottom mud[9]. Separately, University of Florida IFAS Extension describes the use of hydrated lime to sterilize pond mud between production cycles: the lime raises pH of the mud and water to 12 or higher for about a week, killing most disease agents, but ponds must be drained and cleaned first, and lime cannot be applied while fish are present because the pH spike is lethal to fish[5]. Water-testing guidance also warns that samples taken from ponds should avoid contamination from surface mud, debris or algae, since this can affect readings used to interpret pond water quality[10].

Historical and regional context

The practice of raising koi in earthen ponds in Niigata has roots in the Nijūmuragō region, an area spanning present-day Ojiya and Yamakoshi where food carp were farmed in terraced ponds near rice paddies, according to multiple secondary sources, with systematic breeding of ornamental carp beginning in the 1820s[2][6][11]. A severe drought around 1781 caused these ponds to run dry, and surviving carp were moved to shrine ponds to escape the disaster, according to the same accounts[2][12]. Ojiya City states that it continues to produce roughly 45% of Niigata Prefecture's nishikigoi and holds annual judging events in spring, for koi that have overwintered successfully, and in autumn, for the season's outstanding koi[7]. Readers interested in this background can consult the history of Nijūmuragō: Ojiya and Yamakoshi, Niigata.

Common questions

What is a mud pond in koi breeding?
It is an outdoor, earth-bottomed pond used in Niigata to grow young koi outdoors during the warmer months, as opposed to the concrete or indoor tanks used for winter storage[1].
Why do breeders believe mud ponds help koi grow?
According to the dealer Kodama Koi Farm, mud ponds expose koi to natural minerals often lacking in concrete ponds, which the source associates with better growth, body structure, immune health and color development[3].
When are koi moved out of mud ponds?
Niigata production records describe koi being lifted from mud ponds in autumn, in a process called ikeage, and transferred to indoor tanks for winter[1].
Are mud ponds cleaned or disinfected between uses?
University of Florida IFAS Extension describes hydrated lime being used to sterilize pond mud between stocking cycles, raising pH sharply to kill disease agents, though this can only be done when no fish are present because it is lethal to fish[5].
What risks are involved in harvesting koi from mud ponds?
University of Florida IFAS Extension notes that seining disturbs bottom sediment, raising turbidity that can clog gills and potentially releasing lethal hydrogen sulfide gas from decaying mud, so harvest should be done quickly[4].

References

  1. ^ ニシキゴイ (Wikipedia). Wikipedia (Japanese). ja.wikipedia.org/wiki/錦鯉. Retrieved November 2026.
  2. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Nishikigoi. Retrieved November 2026.
  3. ^ Essential Nutrients & Supplements for Koi Growth & Color. Kodama Koi Farm (koi dealer). kodamakoifarm.com/essential-nutrients-supplements-for-koi-growth-co…. Retrieved November 2026.
  4. ^ UF/IFAS EDIS FA117: Harvesting Ornamental Fish From Ponds. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA117. Retrieved November 2026.
  5. ^ UF/IFAS EDIS FA028: The Use of Lime in Fish Ponds. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA028. Retrieved November 2026.
  6. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Utsurimono. Retrieved November 2026.
  7. ^ 錦鯉の里 - 小千谷市ホームページ. Ojiya City. city.ojiya.niigata.jp/site/kanko/nishikigoinosato.html. Retrieved November 2026.
  8. ^ How to Grow Big Koi: Guide to Large Koi Growth with a Healthy Pond. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-to-grow-big-koi/. Retrieved November 2026.
  9. ^ SRAC Publication No. 475: Proliferative Gill Disease. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/475%20Proliferative%20Gill%20Di…. Retrieved November 2026.
  10. ^ SRAC Publication No. 4606: Interpretation of Water Analysis Reports for Fish Culture. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4606%20Interpretation%20of%20Wa…. Retrieved November 2026.
  11. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Goshiki. Retrieved November 2026.
  12. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Koi. Retrieved November 2026.