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How can I tell if I have male or female koi?
ASKED BY: Ann Marie of Casselberry, FL
ASKED BY: Ann Marie of Casselberry, FL

How can I tell if I have male or female koi?

A: If only koi were as easy to sex as a pair of peacocks! Like those fanciful birds, koi are sexually dimorphic – meaning the males and females look and behave differently – but it's not an easy distinction to discern. They grow to roughly the same size and they both have colorful scales, yet there are subtle differences if you know where to look. With these clues, you'll know whether to name your finned friends Fred or Frieda.

Clue #1: Age
Koi are easier to tell apart when they're mature, and so your first clue will be age – which is related to their length. They're considered mature (3 years old) when they're about 10 inches long. If they're between 3 and 10 inches, they're still juveniles and may be difficult to sex.

Clue #2: Body Shape
Immobilize your koi by capturing it in your pond net and take a look at it from above. 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!

Clue #3: Fin Shape
Next, examine your koi's fins. A male koi's pectoral fins, the ones near his head, will appear pointed and solid in color. In addition, the first ray of his pectoral fin may be more substantial when compared to his female counterpart, which will display rounder fins.

Clue #4: Tubercles
During breeding season, you may see little white growths, called tubercles, on male koi's heads and pectoral fins. They're perfectly natural and will disappear once the fish have finished getting frisky. Females don't develop these protrusions.

Clue #5: Behavior
One final – and obvious! – clue: amorous or fish bully behavior. When viewing your pond, both male and female koi usually act similarly. But sometimes you might wonder why is my fish bullying my other fish.

During mating season, the male koi exhibit aggressive behavior. When male koi compete to vie for the female's attention and establish dominance, they will chase the female, encouraging her (sometimes knocking her against objects) to release her eggs so he can fertilize them.

After all, that frolicking and mating process can lead to minor injuries and stress. Using a product like The Pond Guy Stress Reducer Plus is a good idea, as it helps heal damaged tissue and reduce stress on the fish.

It's not easy to distinguish the male and female koi, but with these tips and clues, you'll know your Freds from your Friedas in no time!