Koi come in many beautiful colors and patterns, but brilliant scale color doesn’t distinguish gender and size in koi it’s more related to age than to gender. So how can you tell? Well a few visible physical differences exist between male and female koi. The older the fish, the easier the task should be.
Koi reach maturity once they are about 10″ in length (Measure Body Length, Not including the tail), but some fish may get to that size in a year or it may take 3 or more. Both of these factors can vary as you will see over time, if you have a small pond or a large one and how many fish you have. When examining your fish from above, on a mature Koi, the females tend to be noticeably plumper than the males in the spring to early summer. This is because adult females become full of eggs, making them appear much more rounded. The difference in body shape isn’t as apparent in juveniles.
The subtle differences in the fins appear in adult koi males and females, and a lesser extent in nearly mature juveniles. The pectoral fins (the side pair near the head) tend to be more pointed and solidly colored in males.
Other Physical Differences
During the breeding season, male koi show little white growths (tubercles) on their heads and pectoral fins, whereas females do not. It’s not a sign of illness and will disappear once the fish have finished spawning. Female koi usually do become larger than males, although this is hard to know unless you know the fish are the same-age.
Both genders behave in much the same peaceful manner but once they start breeding, you can easily tell which is which. The males will be the ones chasing the females through the water. Don’t worry too much about them multiplying unless you have setup your pond for breeding. The adult koi will eat most of the eggs and fry. In a traditional Koi pond with no rock just liner the eggs really don’t stand a chance of surviving. (See Koi Breeding for more Info)