All koi carp are colorful, so you can’t distinguish gender by noting which has the more brilliant scales. Size in kois is more related to age than to gender, so size rarely helps determine sex. But a few visible physical differences exist between male and female koi fish. Unless you are breeding koi fish, you probably need to know the sex only in order to decide whether to call an individual Karen or Kevin. The older the fish, the easier the task should be.
Koi reach maturity once they are about 10 inches long, at an age of about 3 years, although both these factors vary. Examine your fish from above, easy in a pond. Once they are mature, the females tend to be noticeably plumper than the males. This is because adult females become full of eggs, making them appear much more rounded. The difference in body shape isn’t apparent in juveniles, though.
Subtle differences in the fins appear in adult koi males and females and, to 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. The first ray of these fins may be more substantial in the boys.
Other Physical Differences
During the breeding season, male koi show little white growths -- tubercules -- on their heads and pectoral fins, whereas females do not. These are not a sign of illness and will disappear once the fish have finished spawning. Female koi usually do become larger than males, although the method serves to help determine gender only for fish you know are same-age adults.
Generally, both genders behave in much the same peaceful manner. Once they start breeding, you can easily tell which is which. The males will be the ones chasing others, the females, through the water. Don’t worry too much about them multiplying: The adult koi will eat most of the eggs and fry. Unless you take steps to ensure that the majority of the eggs and fry survive to adulthood, it’s highly unlikely you’ll end up with too many koi. It is advisable to under-stock a pond so you have space for the odd koi baby who lives to adulthood.
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