Koi in a Pond

Determining whether a koi is male or female involves some detective work. Even an expert with trained eyes can miss some of the physical distinctions. As koi lack external sex organs, we must rely on the subtle differences in external features. Fortunately, as they age, koi provide several clues that combine to offer a clearer picture. 


Age is important when sexing koi, as many differentiating characteristics do not develop until maturity. Koi are typically considered mature when they reach 12 inches in length. Depending on the variety, this may take up to three years. At this point, koi are biologically able to breed. However, koi in healthy pond environments typically won’t mate until age 4 or 5, so it can take quite a while for spawning-related characteristics to manifest.


An average koi grows between 24 and 36 inches. Genetics and environment are the main determinants of size, but females are typically larger than males. This size difference is thought to stem from Japanese breeding practices, where a single large female was placed with several smaller males to minimize injuries. Generations of breeding in this fashion have led to male koi generally being smaller than females.

Body Shape 

Mature female koi have larger midsections and appear rounder (often described as “blimp shaped”), while male koi appear more streamlined and torpedo-shaped. This results from the expansion of females’ ovaries to accommodate a growing number and size of eggs. Without the ability to produce eggs, males will not bulge in their midsections (unless they are overfed). Female koi also develop broader shoulders. 

When assessing body shape, view your koi from above. You might need to remove the koi from the pond and use a showing bowl to examine its contour. You can also try feeling the underside of the koi. Females will have a soft abdomen between their bones, while males’ abdominal bones are fused. 


In general, male koi develop color faster and exhibit slightly brighter palettes than female koi. Similar to other aquatic species, this trait is linked to mating. You can see male koi become more colorful as water temperatures rise. These colors subdue in the weeks after spawning. Due to their larger size, females tend to develop more intricate patterns. Color alone should never be used as a determinant of sex as multiple factors affect it, including genetics, diet, environment, and health. 

Pectoral Fins 

Pectoral fins are on both sides of a koi’s body, just below the head. A male koi’s pectoral fins will appear pointed and solid in color. Females will display rounder fins that are more transparent. The fins’ first ray may also be more substantial in the males. If you choose to handle the fish, you can assess the oyabone, which is the bone closest to the head that runs from the pectoral to the tail fins. Males have thicker oyabones than females. 


During spawning, male koi will grow small white spots called tubercles on their heads and fins. To the touch, they feel rough and gritty, like sandpaper. You won’t see these on females, which will continue to feel smooth. Tubercles will subside after the season, so only use this criterion during active spawning. 


The vent, located on the underside of the koi, is a key physical trait that can distinguish male koi from females. The vent of a mature female will protrude slightly and feel soft, with a pinkish slit running crosswise. Males have firmer vents that are either flat or concave. 

Breeding Behavior 

One hallmark of the male and female distinction in koi is spawning behavior. Fish typically spawn in late spring or early summer when water temperatures are between 65-75°F. A male will target an egg-bearing female and chase her around the pond to encourage her to release her eggs. This behavior can go on for several hours but will cease once the female accomplishes this. 

In some cases, distinguishing the chaser from the chasee can be difficult because it occurs at high speeds. Males will also harass each other to assert dominance and the right to breed with the females.

Feeding Habits 

Contrary to behavior during spawning, females are the primary aggressors at mealtime. They tend to stay near the surface and eat continuously during feeding, while males nibble and swim between bites, darting to the surface and arrowing back down. However, the variety of the koi also can affect feeding behavior. 

Inspecting Koi

If you need to handle your koi to inspect certain characteristics, it is important to do so safely. Corral the koi and guide it into a viewing bowl using a pan net (a shallow net resembling a frying pan) with slow and deliberate movements. Stay as quiet and calm as possible when approaching. Never chase the koi around your pond.

Before holding koi, wash your hands with dechlorinated water. This will help protect the slime coat. Remove any jewelry that could scratch or injure the fish. When lifting the koi out of the bowl, grasp it tightly enough that it cannot escape your hands in mid-air and become seriously injured. If the koi is jumpy, rotating it around slowly in the bowl or tucking its nose towards your armpit can help to settle it. 

Implications of Koi Gender 

The ideal ratio of males to females will differ depending on your pond’s purpose. Breeders may want a higher male-to-female ratio, with larger females to reduce the chances of injury during aggressive spawning.

If your pond is primarily for personal enjoyment, a ratio of two males to one female will help prevent stress and aggression. Or you can opt to cultivate an all-male or all-female pond. The desirable ratio may also change depending on the size of your pond. Females tend to be larger, so they will place more demands on the pond’s water volume.

Finally, the price can be a determining factor as sexually mature females are almost always more expensive than males. 

Whether male or female, Next Day Koi offers an impressive variety of fish for sale. Shop our full selection of koi and goldfish.

2 responses

  1. Beth Merithew :

    What an incredibly informative article, I see so many people thinking their tiny koi are spawning, and they are not. Your fish are incredible and I am lucky enough to have some and they went from 4 inches to over 20 in 3 years.

  2. Christine Creason :

    Great informative articles! I am on my third koi pond, it’s an addiction! I keep making them bigger!😍 Our koi are our zen, I did my first pond in 1998. I started with goldfish and 1 rescued koi, which I still have! Bella the koi and Cuda the goldfish are appro. 30 years old! I have 7 – 10 year old koi and we’ve had some successful spawns, resulting in almost 100 more koi! I love your great articles and your koi are gorgeous!

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