How to sex Koi. Male or female?

In our current times of political correctness and equal opportunities, gender should no longer be considered an issue. The view held by society is changing, accepting that the sex of an individual is less of an issue when considering roles in secular work.

Perhaps a little paradoxically, this levelling of the playing field is occurring at the same time as the outward appearances of men and women, afforded by fashion and cosmetics have become even more polarised.

In the world of koi, the opposite is true, where male and female koi maintain the differentiation between sex roles, yet at times may be different to tell the sexes apart.

As koi are nearly always viewed from the top, and the number of koi bought and sold are immature, it can prove difficult, if not impossible to tell the difference between male and female koi.

Things get easier as koi mature and develop into their true potential with definitive identification possible at certain times of the year. For a relative newcomer to koi keeping there is real satisfaction to be gained from knowing the sex of each of your fish. Likewise, if a breeder, either amateur or professional, it is imperative to be able to sex your own koi.

Furthermore, if viewing koi as potential show fish it is to your benefit to be able to identify a female koi with their well documented greater potential for growth and volume over male koi. Show winning and champion koi are nearly always female on account of their volume and size and it will pay you dividends if you are able to identify one.

Having spent a good number of years breeding koi commercially, it is vital to be able to sex koi accurately and quickly. The process of breeding koi, involving a period of artificial manipulation and inducement requires the sexes to be segregated to prevent a premature spawning. The koi farmer must avoid a flock spawning at all costs, insisting on complete control over which broodfish are matched, where segregation of the sexes is key to his success.

The differences between male and female koi

The differences between the sexes become more apparent as they age, particularly so in the spawning season. Male koi are slender, compact and torpedo-like in appearance, with very trim lines and smooth body contours. They also show behaviour that is particular to male koi and is particularly noticeable when all male koi are segregated into a single tank. Male koi tend to dart around energetically, with sharp turns of speed, showing real ‘attitude’ and a skittish character. They are less trusting than the opposite sex and this shy temperament is all the more apparent at feeding time when they will take considerably longer than females to feed at the surface.

The key distinguishing physical feature that male koi possess develop in late spring as they come into breeding condition. Often mistaken for whitespot, breeding tubercles appear on the head, flanks and tail of mature male broodfish. To the touch, these tubercles make the typically slippery koi feel ‘as rough as sand paper’ giving them better purchase for when they drive and push against gravid females in an attempt to expel their eggs. In darker koi, white tubercles will be clearly visible but on paler or metallic koi, tubercles may only be evident to the touch.

Females on the other hand, do not develop tubercles nor will they retain the sleek lines of a male as the breeding season approaches. Eggs that develop in the ovaries will swell, leading to an increase in volume to the abdomen. In addition, female koi tend to behave in a less timid manner to a tank full of males, feeding voraciously as soon as the food hits the water. Whether these characteristics are as well defined and identifiable in a pond of mixed koi is debatable, but males are still less likely to be the first to feed of the surface.

The Hands-on Method.

The sure method of sexing koi is to roll up your sleeves and exert some timely pressure in the right place. As long as the water temperature is above 15 degrees C, males should be producing milt, which can be expressed by applying finger and thumb pressure either side of the vent. A ripe and mature female when turned upside down (having been sedated) will show a swollen abdomen and a pink and fleshy protruding vent.

The right ratio.

If you are happy to allow your fish to flock spawn in your pond (with the associated risks of damaging broodfish) then there should be fewer males than females. Males are extremely attentive and will chase any fish (including other males!) that they suspect is ripe. If you can divide the attentions of such devoted and obliging males by increasing the ratio of females, your ladies are less likely to suffer stress.

Professional Breeding.

Professional breeders can use a number of spawning methods, ranging from inducing koi with hormone injections and hand stripping, hand stripping broodfish that have matured naturally and are actively releasing eggs through to allowing koi to flock spawn in a mud pond. Each method not only relies on matching the koi numerically, but also genetically to ensure that the offspring are of the highest quality.

The power of pheromones!

The powerful effects of pheromones (hormone-like compounds that are released by koi into the water) can be observed very graphically when managing two isolated tanks of segregated sexes prior to spawning. By simply removing a bucket of tank water from the mature female broodfish and adding it to thousands of gallons of water in the male tank, male koi can be stimulated to show spawning behaviour in the absence of any females. It can be quite amusing to observe a tank full of frustrated male koi trying to mate with each other, convinced by the presence of female pheromones in the water, that there is a ripe female in their tank.

Kill blanketweed and string algae.