Medaka Fry Growth Stage Management Guide: Raising from Newly Hatched Fry to Juvenile Fish | ブリちょく
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Medaka Fry Growth Stage Management Guide: Raising from Newly Hatched Fry to Juvenile Fish
Explains growth stage-based management methods for medaka fry. Introduces feeding, water change frequency, container size, and sizing timing for newly hatched fry, fry, juvenile, and young fish stages.
Key Takeaways
Explains growth stage-based management methods for medaka fry. Introduces feeding, water change frequency, container size, and sizing timing for newly hatched fry, fry, juvenile, and young fish stages.
The most delicate stage in medaka breeding is raising fry immediately after hatching (newly hatched fry). Newly hatched fry are extremely small at just 4-5mm, requiring completely different management from adult fish. By correctly understanding the growth stages of fry and managing feed, water quality, and containers according to each stage, survival rates can be significantly improved. This article divides medaka fry growth into four stages and explains the management points for each in detail.
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※If growth variance appears, separate individuals with body length differences of 1.5x or greater into separate containers. Adults must be completely separated from fry as they will eat them.
Newly Hatched Fry Stage (Hatching to 2 Weeks)
Medaka fry immediately after hatching are called "newly hatched fry" (hairiko), with transparent bodies measuring 4-5mm in length. Newly hatched fry deplete the nutrition from their yolk sac within 2-3 days and must then forage for food on their own. The greatest challenge during this period is securing "appropriately-sized feed." Newly hatched fry have mouths only about 0.3mm wide, making standard medaka flake food impossible for them to eat. The optimal feed is green water (phytoplankton culture), which contains microorganisms small enough for newly hatched fry to ingest and maintains a constant food supply suspended in the water. Paramecium is also excellent as an initial feed for newly hatched fry and is not difficult to culture. Commercial newly hatched fry powder foods (such as Medaka no Mai Baby) can also be used, but since uneaten food degrades water quality, feed only small amounts. A 1-5L small container is sufficient for newly hatched fry, and water currents must never be created. Aeration is unnecessary, and even surface ripples can exhaust newly hatched fry. Water changes should either not be performed or limited to removing bottom debris with a dropper.
Fry Stage (2 Weeks to 1 Month)
After two weeks of growth, fry reach 7-10mm in body length and begin to look more like medaka. This stage is called the "fry stage." As mouth size increases, feed variety can expand. Brine shrimp nauplii have extremely high nutritional value and greatly promote growth during this period. Brine shrimp eggs hatch when placed in salt water (1 tablespoon of salt per liter of water) with aeration at 25-28°C for 24 hours. Collect the hatched nauplii with a dropper, rinse them in freshwater, then feed them. You can also switch to artificial feeds with slightly larger granule size than powder types for fry. Feeding 3-4 times daily is ideal, and frequent small portions are key to maximizing growth during this stage. Expand the container to 5-10L, with a target density of no more than 5 fry per liter of water. Growth rate variation begins to appear at this stage, so if large and small individuals coexist, smaller individuals may not get enough feed.
Juvenile Stage (1-2 Months)
When fry reach 1-1.5cm in body length, they enter the "juvenile stage," and management becomes significantly easier. From this point onward, adult feed can be finely crushed between the fingers and fed to them. Pinch flake food to create powder or lightly crush it with a mortar until it reaches the appropriate size. Be careful, as completely adult-sized pellets still won't fit in their mouths. Reduce feeding to 2-3 times daily, with the guideline being the amount they can consume within 3 minutes. Water change frequency can increase at this stage. Exchange approximately one-fifth of the water once per week. Allow new water to reach the same temperature before adding it slowly. Expand containers to 10-20L or move juveniles to outdoor plastic tubs or wooden boxes. When transitioning to outdoor cultivation, gradually acclimate them while monitoring temperature differences. In the latter half of the juvenile stage (around 6 weeks after hatching), body color begins to appear and breed characteristics become visible. If planning for selection, begin sorting by body shape and color criteria starting at this stage.
Young Fish Stage (2-3 Months) and Sizing
Fish measuring 1.5-2cm in body length are called "young fish." At this stage, they can eat adult feed as-is, and management methods match those for adults. However, keeping them with adult fish still carries risks of feed competition or chasing due to size differences, so it is safer to manage them separately from adults. Sizing (size selection) is the most important task during this stage. Growth rate varies considerably between individuals, and siblings from the same parents can show body size differences exceeding 2x. Since larger individuals suppress smaller ones, separate individuals with body length differences of 1.5x or greater into different containers. Perform sizing using small fry nets, keeping contact time minimal and avoiding direct hand contact. In the latter half of the young fish stage (around 3 months after hatching), males and females become distinguishable. Identify them by the presence or absence of the notch in the dorsal fin and the shape of the anal fin.
Five Tips to Improve Fry Survival Rate
Here are important tips for improving medaka fry survival rates. First is "complete separation from adults." Medaka parents consume their own fry, so manage eggs in separate containers from the start. Second is "utilizing green water." Green water creates an ideal environment where feed is constantly present in the water, dramatically improving newly hatched fry survival rates. It can also be created by exposing adult fish water to sunlight. Third is "maintaining water temperature at 25-28°C." At this temperature range, fry metabolism is active and growth is rapid, with low mortality rates after hatching. If heating is difficult, place the container in a sunny location. Fourth is "avoiding overcrowding." Excessive density causes rapid water quality deterioration and growth variance. Target no more than 5 fry per liter of water. Fifth is "appropriately managing water change frequency." Progress from minimal changes during newly hatched fry stage, to 10% weekly during fry stage, to 20% weekly during juvenile stage, increasing gradually as the fry grows.
Choose Breeding Medaka at Brichoku
The skills to raise medaka fry healthily directly connect to selective breeding and breeding as a side income. At Brichoku, you can purchase quality parent medaka from breeders with proven breeding records, and receive concrete advice on fry rearing methods and selection techniques. Building connections with breeders who support your first breeding attempts is the shortcut to success.
