Koi Gender Identification – How Can You Tell? [Pics & Tips]

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Koi Carp Sex Identification – How Can You Tell?

Selection of Kohaku koi
To the untrained eye, it can be very hard to tell the difference between male & female fish, especially when they’re of the same breed! KoiQuestion / CC BY-SA 2.0

Telling apart male and female fish can be quite tricky for the untrained eye. When purchasing young fish, it can often be impossible to tell the sex. Until a fish reaches a point of sexual maturity, all koi within the same breed or type look morphologically similar. As they don’t have external gonads, we must rely on the subtle differences of external features in mature koi.

One of the main reasons for distinguishing your koi’s sex is to ensure that you have a proper ratio of males to females in one pond. This is especially important for breeders who wish to reap the most out of their koi community. The sex would also be important when naming ornamental koi and selecting individuals (broodstock) to sustain a purebred or hybrid lineage.

Identifying the sex can often be a waiting game, especially for breeders who wish to adequately value their young fish. This is partly why many purebred Japanese koi are sold when they are at least 2 years old. The domestic koi varieties that pet shops often carry are usually unsexed, especially as they are sold when they are just a few months old or 4 – 8 inches (10 – 20 cm) long.

How Early Can You Sex Koi?

Marudo Showa koi
Usually, koi become sexually mature when they reach a length of 30cm. This Showa koi is 32.5 cm and may already be able to mate, despite its young age. From a fish health standpoint, it is best to wait until the koi is 4 or 5 years old before letting them mate. KoiQuestion / CC BY-SA 2.0

On average, koi become sexually mature once they have reached a length of about 12 inches (30 cm). Depending on the variety you own and the optimization of environmental requirements and food, it can take approximately 2 – 3 years for a koi to grow to this length. At this point, they are biologically able to start breeding. From a fish health standpoint, however, koi should be at least 4 or 5 years old when they undergo their first mating event.  

Experienced breeders are often able to sex koi prior to full sexual maturity, but novice koi breeders are advised to wait to be able to sex them accurately. When their morphological features are larger, it is easier to pinpoint the subtle details that indicate koi sex. When the fish is too small, determining sex is often a gamble as external features can undergo morphological changes as the fish lengthens.

Sexual Dimorphism in Koi

Woman holding koi
Morphological differences between male & female koi are often hard to tell, so you may need to physically handle your fish to look for some of the external indicators of sex. KoiQuestion / CC BY-SA 2.0

Koi are sexually dimorphic fish – this basically means that males and females are systematically different and that sex is genetically determined from birth rather than influenced by external factors. In koi, sex is indicated by a combination of traits apart from just gonadal features and the production of gametes (sperm or egg). Even the spawning behavior between sexes is markedly different.

Unlike other sexually dimorphic species (e.g. humans, peacocks), morphological differences between koi sexes are extremely difficult to tell from a distance. To accurately sex koi without relying on spawning behavior, you will have to closely observe your fish. In some cases, and because males and females may grow to similar lengths, you must physically handle your fish to inspect some of the indicators listed below.

Main External Indicators of Koi Sex

1) Body shape

Female koi in pond
Female koi tend to have more rounded bodies compared to males of the same age, and get even more round when they reach sexual maturity. Alan L / CC BY 2.0

Experienced breeders can tell a mature male from a female relatively quickly by looking at the differences in body shape. Females have a more rounded body and generally become larger than males of the same age. Males are usually more streamlined and slender. When females are sexually mature, their roundedness becomes more apparent as their ovaries expand to accommodate the growing number and size of eggs. In contrast, male testes don’t expand all that much to accommodate millions of developing sperm.

This indicator is not the most reliable one and should only be used with older, healthy koi. Young females may often appear just as slender as males due to several factors, such as a lack of food, stress, or disease. In the same way, the abdomens of males can seemingly expand outward if the fish are overfed or are experiencing digestive abnormalities.

2) Pectoral fin shape

Yellow male koi
Male koi tend to have more pointed pectoral fins, whereas female pectoral fins are more rounded. KoiQuestion / CC BY-SA 2.0

A koi’s pectoral fins are located on both sides of its body, just behind the head region. Comparable to a bird’s wings, these open outward and are flat on the horizontal sphere. In males, the pectoral fins tend to be solidly colored and are slightly more pointed. The oyabone, which is the thickest part of the fin and is located closest to the head, is usually thicker and may feel quite rough (like sandpaper) due to fine bumps called tubercles. These bumps become more marked during summer, when water temperatures rise to above 18˚C (64˚F).

In females, the pectoral fins are more rounded. They are generally soft and smooth as well, regardless of the season. The oyabone is not significantly thicker or rougher compared to the rest of the fin’s rays. Thus, this indicator is more accurately used to distinguish males. Note that it is not always reliable, especially when used outside of the spawning period. Also, relying on this indicator means physically handling the fins of your fish. To prevent injury, be extremely gentle and make sure the fish is relaxed throughout the inspection process.

3) Ventral region (vent)

Male koi vent
The vent is sometimes referred to as the anus. Male koi (pictured) tend to have a tighter vent that may appear to be receded. Photo from koiphen.com

The vent, sometimes referred to as the anus, is located on the ventral region of the fish. This is where gametes are extruded. With separate gonadal and waste pores, it is located just before the anal fin. The vent is the most definitive external indicator and should always be checked prior to determining the value of Japanese koi.

The vent of a mature female koi appears to protrude from the body. It has a slit that runs crosswise and tends to be softer to the touch. In contrast, males tend to have a tighter vent that does not markedly bulge away from the main body. It can appear to be receded. Although this indicator is the most reliable for mature fish, it cannot be used to sex young koi. The vents of young males and females remain morphologically similar prior to sexual maturity.

4) Gills

Female koi
Male koi have tubercles around the gill area during spawning season, whereas females, like this Tanchoshowa koi, don’t have them. KoiQuestion / CC BY-SA 2.0

Tubercles, which are tiny bumps that increase the roughness of the koi’s skin membrane, increase in density around the gill area (and the pectoral fins) during the spawning season. The rough texture of tubercles supposedly aids in arousing the female fish. These are only present in males and are a decent indicator of their maturity. Outside of the spawning season or in the absence of sexually viable females, this indicator is not reliable.

5) Color development

Group of tosai koi
Some hobbyists purchase tosai koi if they’re willing to take a gamble on their sex and future appearance. KoiQuest / CC BY-SA 2.0

The pace of color development can differ between male and female koi of the same breed. Males tend to develop their color quicker. This can make young females look lighter in comparison. While some breeders are able to pinpoint color differences between sexes of healthy koi, the use of this indicator is not recommended as color can be influenced by diet, growth period, environmental conditions, and disease. Additionally, the patterns and colors on young female fish can change over the first few years of life as a result of morphological changes associated with breeding.

Some hobbyists opt to purchase unsexed tosai (koi that are below a year of age) as they are interested in the lineage of koi and are willing to take a gamble on their sex and future appearance. These are a more affordable option, though it is said that many turn out male as the fish that sell best are those with more vivid colors at such a young age.

6) Female enlargement and spawning behavior

Male koi pushing into female koi
Male koi will repeatedly push and prod females that are producing lots of eggs, in an attempt to get her to release them. Photo from Max Pixel

Females that are producing a large number of eggs will have abdomens that rapidly expand in size until their bodies are distinctly rounded. At this point, they are extremely easy to distinguish as males will look markedly slim next to them. Males will repeatedly chase, prod, push, and graze the female in an attempt to get her to release the eggs. The female will stay close to floating or marginal plants as it would be safer to release her eggs under the vegetative cover.

Ideal Ratio of Male to Female Koi in a Pond

Large koi tank
Be careful not to stock your pond with too many mature females, as your pond can very quickly become overcrowded. KoiQuestion / CC BY-SA 2.0

Depending on your purpose for investing in koi, the ideal ratio of males to females will differ. Breeders who wish to maximize the value of female koi will often stock a single female with several carefully selected males. Note that this may be dangerous for the female, especially if she is smaller than the males. As the males can chase females quite aggressively during the spawning period, they can become extremely stressed. Large females are often paired with smaller males to reduce the chances of severe injury.

If you aren’t too keen on genetic selection, a ratio of 2 males to 1 female would help prevent a single female from becoming overwhelmed. Be wary of stocking ornamental ponds with many sexually mature females, however, as just a few males can fertilize a fair number of their eggs. Though a majority of fertilized eggs and fingerlings may eventually become consumed by their parents and other fish, you may end up with an overcrowded pond in no time and be pressured to rehome many young koi.

The desirable ratio may also change depending on the size of your pond and the number of fish you would like to have. As females tend to be larger, they will place more demands on the pond’s water volume. If you don’t intend to breed koi at all, you can even opt to cultivate an all-male or all-female koi pond. An all-male pond would definitely be a cheaper investment but may not be as appealing to koi enthusiasts. Acquiring a large number of hefty mature females would be very heavy on the pocket as they can often cost twice the price of males, but you would have a pond that you could unarguably use for show!

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