How to…Identifying a Koi’s gender

Your greatest priority as a Koi keeper should always be the health of the Koi in your pond, whether they are pets or prize winners and regardless of how much you might have paid for them. But once you feel you have provided the best environment for your Koi to flourish, you may feel that you wish to learn about the other issues that arise when keeping Koi.

One subject that has become more important to Koi keepers in recent years is the sex of the Koi in their pond. But why is it important to know whether your Koi are male or female? For most Koi keepers it isn’t important if their interest is in simply keeping their Koi healthy and having beautiful fish in their pond. The sex of Koi, however, can affect the quality of the Koi that you buy and the number that you can keep in your pond long-term. It is often possible to buy high-quality male fish for less than a female fish of the same quality simply because female fish, on average, are likely to grow bigger than male fish. That isn’t to say that male fish always stay small or that female fish always grow big! It just means that the perception that they have is keeping the price of male fish lower than perhaps they really deserves to be – a shrewd Koi keeper could use this to their advantage and buy better-quality male fish for less money than a female of the same size.

But if your interest is in showing your Koi, then once a Koi begins to grow and mature, its sex can influence how big it is likely to grow and determine the way its body shape is likely to develop. At shows in the UK, the sex of the Koi isn’t an issue in itself and there are no categories at the BKKS National Koi Show that recognise the sex of the Koi being shown nor have we heard of any at regional shows. However, at some Japanese shows, categories just for male fish have been introduced which recognise the outstanding qualities that they can display and which would otherwise be overshadowed by their generally bigger female counterparts.

Growing up
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. It is possible to do well with male fish at smaller sizes in shows, but as the fish reach maturity and go above 30–40cm in length, the difference in body shape between the sexes becomes more obvious. It is natural for female fish to develop the appearance of a ‘fat tummy’ when carrying eggs and this can spoil the line of the fish, whereas male fish don’t suffer from this problem. Some females manage to maintain their gently tapering shape even in the breeding season and it’s these that fair better in the show vat.

Female Koi generally (but not always) grow larger than male Koi and this is another reason why Koi at bigger sizes tend to be female.

Detective work
While pattern tends not to vary between the sexes, on Go-Sanke the hi is often stronger on male fish than on females, particularly when they are younger, making them appear more ‘finished’ at an earlier age than females – another reason why male Koi can often do well at shows in the smaller sizes.

Even after a Koi has reached sexual maturity, there is always the odd Koi that fails to display any of the normal characteristics of its sex, making sexing Koi very difficult for breeder, dealer and hobbyist alike! In mature Koi, the most obvious sign is body shape – female fish are usually more rounded and males generally slimmer and more streamlined. Male fish usually feel rough to the touch on their cheeks and sides and can have a slightly yellow tinge to the white skin on the head. There are sometimes other subtle differences in relation to the size and shape of the fins (male fish sometimes have noticeably bigger, less rounded pectoral fins for example) but these can often be imperceptible or misleading. The sex of most mature Koi can be determined by body shape and skin texture. When catching Koi in order to examine them it is important to bear in mind that the net should be used only as a means of guiding the Koi and should never be used to lift it as this can cause damage to the fish. It is easiest to use a floating basket when examining Koi, but the next best thing is a large tub floating in the water with enough water in it to support the Koi once caught. The edge of the bowl should be dipped under the waiting Koi contained in the net, and the Koi guided into it.

Once in the bowl, run your hands gently over the cheeks and sides of the Koi – 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. If your Koi does feel rough to the touch along it’s cheeks in particular, then it is definitely male.

If your Koi feels as smooth on its sides and cheeks as it does on the rest of its body it’s likely to be female but there are the odd exceptions to this rule so if you want to be 100% sure, the next step is to examine the vent on the underside of the fish. There may be a few among you with the skills to turn a fish upside down to examine it, without risking bruising the fish in the process, but the method we recommend is that you place your Koi into a plastic bag with a small amount of water and hold it up so that you can look at the underside of the fish. The female vent is slightly protruding or convex and pale pink in colour, while the male vent is flat or concave – this is the definitive way to sex a Koi.
Male Koi shouldn’t be written off. They could be considered more of a challenge if you’re planning to show your Koi and want to go for the bigger size prizes, as it is possible
for some males to overcome the odds and outshine their female competition.