# Created on May 13, 2013 7:57:49 PM Do I have a female or male Koi? This article will help you determine if you have a feamle or male Koi.
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Sexing Koi Fish

Sexing your Koi

Sexing Koi is difficult if they are not in breeding condition and more so when they are young. As the males come into condition, they develop little white spots, called tubercles, on their pectoral fins. Do not confuse them with white spot which is an ailment. The tubercles will be seen about March and subsequently, depending on how warm the water is.

tubercles on male koi

The breeding Koi female will start to swell in the midriff area and the swelling will tend to be one sided, this is the developing eggs in her ovaries. Your female will regularly feed more eagerly than males.

The opercula (the bony covering to the gill chamber) and head on the males become quite rough, like sandpaper. Male fish usually feel rough to the touch on their opercula (the bony covering to the gill chamber) and head, they can also have a slightly yellow tinge to the white skin on the head. You can run your hands gently over the cheeks and sides of the Koi and if it is male they will often feel rough to the touch, rather like sandpaper, and if it is female they will feel completely smooth. The roughness is caused by tubercles which grow along the sides of male Koi and which they use to stimulate the female fish to release her eggs when spawning.

Another way to sex your Koi: Female Koi have a line running from head to tail which has another line running across it at the tail end. Male Koi do not have this cross-piece. Instead, they just have a simple line running from head to tail. However, this difference is not easy for a non-expert to distinguish, you need a very experienced eye to do this accurately.

If you have Koi and Golfish in your pond they will mate. Do Goldfish and Koi Interbreed? The answer is yes but the offspring are not what you would call quality. In fact, they are not the prettiest of fish and thank goodness they are sterile.


Article by Randy LeFever

Use these helpful tips to figure out the sex of your Koi.

  • The Shape
  • The sex of a young Koi does not affect its body shape until it begins to reach sexual maturity, which is usually at around three years of age. Most females at this age will start to carry eggs and will tend to be more rounded and "blimp" shaped. Males are more streamlined and torpedo shaped. Males lack the swollen abdomen of females. Generally speaking, females will grow to a larger size than males. Take a look at the pictures below for an example of the two.

  • The Feel
  • Generally by two years of age, male koi will start to feel rough to the touch, especially during the spring season as temperatures are rising. If you rub your fingers on the gill plates and along the side of the body, it feels like sandpaper. Females will be slick and smooth to the touch.

  • The Vent
  • The male vent will be more con-caved in appearance where-as the female vent will be convex and slightly protruding with a slight pink appearance. Below are an example of the male and female vents.

  • The Behavior
  • During the spring spawning season sexually mature males will chase the females, nudging them with their nose to encourage her to release her eggs.

    Randy LeFever owner of Blue Ridge where our beautiful Koi come from.

Baby Koi

Once the spawning and Koi breeding (fertilization) has taken place, the parents must be removed otherwise they will eat the eggs. Koi eggs will usually hatch within 4-7 days depending on the temperature, the warmer it is, the quicker the hatching. The recommended temperature should be 68-77° F. The eggs do not hatch all together, and some may fail to hatch at all. Once the eggs hatch the baby Koi are able to live by absorbing their yolk sac, they will not need to eat for 2-3 days.

The spawning tank can be the rearing tank as well in the first weeks of the fry’s (baby Koi) life, just make sure and remove the parents.

When the fry have hatched, they will instinctively seek shelter and hide in any cover they can find. This is what the spawning medium is for. Using a special sticky pad on their heads, the fry attach themselves to the spawning medium, or to the vat wall. At this stage in their development, the young Koi have no swim bladder, mouth or vent. They breathe by absorbing oxygen through the fine blood capillaries that surround the yolk sac, which is still attached to the gut. It is essential that there is plenty of oxygen entering the water at this stage, as a reduction in the quantity of dissolved oxygen in the incubator could lead to mass mortality.

The Koi fry have only one fin, which encircles the posterior end of the body. As the Koi grow, feeding on their yolk sac until all the yolk is utilized, they develop paired fins, a mouth and other organs. After two or three days, the young Koi swim up to the surface and take two or three gulps of air, which they force into their swim bladder. They then start to swim freely in mid water, usually gathering around the air stones, a sign that they are ready to be fed.

Conversion Tables Click Here
Nishikigoi Terms Click Here

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