Determining the gender of a koi fish requires careful observation and attention to detail. Because koi lack external sex organs, we rely on the subtle differences in their external characteristics. Fortunately, over time, koi fish display a range of signs that combine to give a better overall understanding of the koi and if they are male or female.
Male and female koi of the same species display distinct physical traits that are unrelated to their reproductive functions. Variations, such as their size, weight, or how they behave and think differ from male to female. It does require handling the koi to sex a koi, so to limit stress we dont normally sex our koi till they are 2.5-3 years old. Until then sexs are together in ponds
Male koi fish typically have a more streamlined physique in comparison
to females. Their pectoral fins (fins just below the head) are larger
compared to their body size, and their head ratios are significantly larger.
Male fins typically have a slightly more pointed edge. In spring, the texture
of their cheeks becomes rough to the touch as they prepare to breed. Male fish
can sometimes have small white tubercles on different parts of their body, like the gill covers, face, head, and pectoral fins.
In the spring, females develop a more rounded appearance as their ovaries fill
with eggs before spawning. It's interesting to note that females generally tendency
to grow larger than males. The shape of the pectoral fins is rounded but that is very slight in some koi and in others more profound.
As fish mature and reach the breeding age, their unique characteristics start
to become noticeable. The vent, located on the underside of the koi, is a
significant physical feature that can be used to distinguish between male and
female koi. When a female is mature, her vent will have a slight protrusion and
a soft texture. It will also have a horizontal slit that appears pinkish. Male
vents are typically more solid and can have a flat or concave shape.
All female koi in one of the ponds at windsor fish hatchery
Koi for sale, Goldfish for sale. Windsor fish hatchery breeding and selling since 1984