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how to tell if a fish is male or female (sexing koi)

how to tell if a fish is male or female

how to tell if a fish is male or female (sexing koi)

How can you tell if a koi fish is male or female?

A mature male koi will have a slender looking body, while a female koi will have a rounded body, particularly when it’s spawning season and she’s carrying a nest full of eggs!

Next, examine your koi’s fins. A male koi’s pectoral fins is thicker, pointy and solid color


How can you tell if a fish is a boy or a girl?

Look for tubercles. One of the main tell-tale signs that your koi fish is male, is the development of tubercles (small white spots) on their gill-shields head and pectoral fins it is well emphasized during the breeding season.

Look for a thinner, more streamlined body shape

Look for a concave vent

Look for a midline ridge

Watch for chasing behavior.

Koi fish are a great addition to any garden koi pond or aquarium.

But whether you just brought your koi fish home or its spawning season fish friends

determining the sex of your koi fish can prove difficult for new owners.

This is especially true if the koi are still young.

However, determining koi fish gender is important for the health of your koi.

If you can determine the gender of your koi, you can more easily watch for gender-specific sickness and disease in the koi’s life.

Male and female koi differ slightly in shape and size.



Sexing, or identifying the koi fish  sex, can be difficult if the koi is under a foot long.

However, with older koi  fish you can easily identify the gender by comparing size.

Generally, male koi are slimmer and smaller than their female counterparts.

Female koi have a fuller body and broader shoulders.

However, at a young age, male koi are typically larger than female.

Male koi may also have larger pectoral fins than females. T

hese fins are more pointed than round female fins.

You can also identify a koi’s gender by examining the vent area on the underside of the fish.

A male koi will have a line from head to tail.

A female koi will have the same line, but will also have another crossing in a “T” shape.

A female’s vent will also be more round and more pink than a male’s.

Observe your koi during feeding to see if they skirt away or fight for the food.

If you carefully feel your koi’s pelvic bones, females will have a soft abdomen in between the bones for holding fish eggs.

In male koi, however, these bones are fused. Again, these methods are difficult when koi are young.

Also take note of your koi fish’s eating habits. Females tend to be more aggressive during feedings, staying near the surface and continually eating, while male koi swim around more throughout the session.

During spawning season, male koi will develop some tiny white spots on the sides of their face.

These are called tubercles and feel rough to the touch, like sandpaper. Females do not develop tubercles.