【Professional Tips】Medaka Selection Techniques | How to Identify Quality Specimens

The Importance of "Selection" in Determining Improved Medaka Quality

In improved medaka breeding, selection (senbetsu) is one of the most important tasks. No matter how beautiful the parent fish you have, if you neglect selection, quality will decline with each generation. Conversely, by continuing proper selection, you can gradually produce ideal specimens from the fish you have on hand.

Selection is the process of sorting out specimens for breeding, specimens to release, and specimens to exclude. It's not simply "keeping good fish," but rather a strategic decision that determines the breed's direction. It may seem difficult as a beginner, but anyone can develop an eye for selection if they understand the key points. If you search for medaka from breeder direct sales, you can meet high-quality specimens refined through careful selection.

Selection Timing by Growth Stage

The basic approach is to select medaka at each growth milestone. Rather than trying to sort everything perfectly at once, accuracy improves by narrowing down in stages.

Fry Stage (Hatching to 1 Month)

Selection during this period focuses on "exclusion." Early removal of deformed specimens—such as those with curved spines, missing fins, or significantly delayed development—is essential. Most deformities are hereditary; overlooking them and raising them to adulthood risks using them for breeding. Additionally, deformed specimens mixed in can hinder the growth of other fry.

Juvenile Stage (1-2 Months)

When body length reaches approximately 1-2 cm, breed-specific coloration and patterns begin to emerge. At this stage, check the intensity of body color and the direction of color development. While not yet fully expressed, this is when hints of future beauty begin to show. Specimens with weak coloration or off-target color directions are culled at this stage.

Young Adult Stage (2-4 Months)

This is the main selection period. Breed-specific characteristics—body iridescence, scale shimmer, body shape, fin morphology—are nearly fully expressed. Breeding candidates are narrowed down at this stage. Be especially careful when selecting parent fish for the next generation.

Selection Points for Body Iridescence

Body iridescence refers to light reflection appearing on the skin surface of the back, one of the characteristic beauties of improved medaka. When selecting, confirm the following points:

• Length of iridescence: Specimens with "full body" iridescence extending from the head to the caudal peduncle are considered highest quality. Specimens where the iridescence breaks midway may be difficult to improve in the next generation.
• Width of iridescence: Specimens with iridescence spreading across the full width of the back are higher quality. Specimens with only a thin line of iridescence are rated lower.
• Color and uniformity of iridescence: The color of iridescence varies by breed—silver, blue, gold, etc. Confirm whether it matches the breed's standard color and whether it shines evenly on both sides.
• Water temperature and iridescence visibility: Body iridescence is more visible at lower water temperatures. During selection, lowering the water temperature to around 15-20°C makes judgment easier.

Selection Points for Scale Shimmer

Scale shimmer refers to the characteristic where iridophores in the scales reflect light and sparkle. When selecting shimmer-type breeds, focus on the following points:

• Quantity and density of shimmer: Ideal specimens have shimmer densely distributed across the entire body. Confirm whether it spreads evenly across the head, belly, and near the tail fin.
• Size and uniformity of shimmer: Specimens with large, evenly-sized particles look more beautiful. Those with a mix of sizes are rated somewhat lower.
• Checking from different light angles: Shimmer appearance changes greatly depending on how light strikes and the viewing angle. Confirm from multiple angles—from above and from the side.
• Hybrid breeds with shimmer and body iridescence: For breeds with both characteristics, evaluate each criterion separately to ensure both are met.

Selection Points for Body Shape and Fins

No matter how beautiful the color or iridescence, specimens with body shape issues should be excluded from breeding candidates. Body shape is a genetic foundation that directly affects the next generation.

• Straightness of the spine: When viewed from the side, select specimens with straight spines. Even minor spine curvature is hereditary and must be excluded.
• Body depth (height): Standards vary by breed, but specimens with good body depth and robust proportions appear healthy and beautiful.
• Fin morphology: Confirm that each fin matches the breed standard (Hikari body type, Swallow, Daruma, etc.). Fins with missing portions or adhesions are excluded.
• Bilateral symmetry: The pectoral and pelvic fins should be evenly matched on both sides—another key checkpoint.

Generational Management and Utilization of Culled Specimens

Avoiding Inbreeding Depression

Repeated breeding of individuals from the same bloodline can lead to inbreeding depression several generations later. Specific problems include weakened immunity, reduced fertility, and increased deformity rates. It's important to introduce new blood every 3-4 generations by bringing in the same breed from other external lines.

Management and Utilization of Culled Specimens

Specimens culled during selection (colloquially called "hane") are not without value. Manage them in separate containers and find uses for them—fostering programs, discounted sales, or crossbreeding trials with other lines. Also, if selection standards are too strict, you may end up with too few specimens on hand, so balance the number you keep with your goals.

Keeping Records

Recording which parent fish produced which offspring and which generation they are significantly simplifies generational management. Simple notes or photos are fine. As records accumulate, the direction of your own breeding program becomes clearer. As of 2026, more breeders are using smartphone apps to manage breeding records; consider using these tools.

Summary

Improved medaka selection is a steady accumulation of effort toward quality improvement. Ideal specimens don't appear overnight, but by continuing careful selection at each growth stage, the fish in your care gradually become more beautiful. Understand the criteria for body iridescence, scale shimmer, and body shape, and approach selection with generational management in mind.

At BreederDirect, breeders who carefully continue this rigorous selection offer specimens they have confidently raised. You can see their commitment to fish care through seller profiles and comments, so even beginners can select with confidence. Meeting high-quality improved medaka can become a valuable hint for your own breeding program. Please visit BreederDirect's medaka page to find your ideal specimen.