Koi are egg-laying fish and can reproduce prolifically under the right conditions. A single spawning event can produce tens of thousands of eggs, though survival rates vary widely.

Sexual maturity

Koi typically reach sexual maturity at 2 to 3 years of age and around 10 to 12 inches in length. Males generally mature slightly earlier than females.

How to tell males from females

Feature	Male	Female
Body shape	Slender, streamlined	Rounder, fuller belly
Pectoral fins	Thicker, more rigid	Softer, more flexible
Vent area	Concave, narrow	Convex, slightly protruding
Behavior in spring	Chases females	Being chased

Spawning season

Koi spawn naturally in late spring to early summer when water temperatures rise consistently to 65 to 70°F. In Minnesota, this typically falls in late May through June.

Spawning process

• Males chase the female vigorously, sometimes for hours or days
• The female releases eggs onto aquatic plants, spawning mops, or pond surfaces
• Males immediately fertilize the eggs with milt
• Eggs are small, sticky, and transparent to pale yellow
• Hatching occurs in 4 to 7 days depending on water temperature

Egg and fry survival

In a community pond, survival is very low. Adult koi, including the parents, will eat eggs and fry aggressively. Without intervention, very few fry survive naturally.

To improve survival

• Move spawning mops or plants with eggs to a separate tank or pond immediately after spawning
• Use an air stone to keep water oxygenated around the eggs
• Remove unfertilized eggs quickly as they turn white and can spread fungus
• Feed hatched fry infusoria, liquid fry food, or finely crushed flake food
• Move fry to progressively larger grow-out tanks as they develop

Fry development timeline

Stage	Timeframe
Eggs hatch	4 to 7 days after spawning
Fry become free-swimming	2 to 3 days after hatching
Color begins to appear	4 to 8 weeks
Culling for quality possible	6 to 8 weeks
Resemble miniature koi	3 to 4 months

Culling

Serious breeders cull fry early to focus resources on the highest quality fish. Culling is based on body shape, color pattern, and fin development. This is a standard practice in Japanese koi breeding and is done humanely.

Minnesota consideration

The shorter warm season in Minnesota means spawning is concentrated into a narrower window compared to warmer climates. Water temperature monitoring in late May is key to anticipating spawning activity so we can be prepared with spawning mops or a separate fry tank if we want to raise offspring.

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