The Fins
They are a Koi's means for propulsion. Female and Male Koi have different looking fins. Female fins tend to be larger, but will have less color. Female fins will have a rounded edge and will be opaque.
Male fins will have a pointy edge and will have a solid color look to them.
Body Conformation
Male and female Koi have different body conformations. Male Koi will be long and skinny, like a cigar, female Koi will look more like a blimp. Female Koi get wider than male Koi because they carry tons of eggs. Many Koi collectors make it a point to only have female Koi in their ponds, since the large and wide body displays patterns much better. That is why most of the award winning Koi at shows are female.
Female Koi grow larger than male Koi.
This is the reason why:
When the Japanese breed Koi, they pair a group of males to one female. They do this to get a wide variety of genetics, boosted immunities, and a nice selection of colors/patterns. Now, if you are going to breed one female with a bunch of large males, she is going to get pretty beaten up. This is not good, since a nice breeding female can cost more than a decent house. In order to prevent the female breeders from getting beaten up, they pair a large female with a bunch of smaller males. Since the process is repeated every generation, the males will always be smaller than the females.
Telling the Difference:
It is much easier to tell the differences between sexes when Koi get larger and older. When they reach sexual maturity, at around 2 years of age and around 12" of length, the females will fill up with eggs. This will give them the blimp look. When Koi are larger, it is easier to tell if their fins are pointed or rounded and if they are clear or opaque.
make from that what you will!
