Distinguishing between a male and female koi is not very easy. The fish, known for their brilliant colours and low maintenance, don't show a lot of gender differences until they attain sexual maturity and grow to 10 inches long or larger. This takes about two years. There are several subtle differences in the koi's physical characteristics that can give an indication of the gender. A few of them can be seen before the koi reaches maturity.
Observe the shape of the sexually mature koi's body. The male koi has a slender, torpedo-shaped body. The female koi, which holds eggs in her abdomen, is large and wide at the sides.
Look at the koi's pectoral fins. The female has a large, round-edged pectoral fin, which is opaque with less colour than the male. The male's pectoral fin is pointy and coloured. The first ray of this fin is thicker than the female's fin.
View and touch the skin around the lower jaw. The texture of the skin in this area is noticeably rough in mature male koi. The roughness is caused by the growth of tubercles along the sides of the male koi's body -- which he uses to stimulate the female to release eggs during mating.
Look for elevated white spots or nuptial tubercles on koi during breeding season, which occurs in spring. Only male koi develop these spots. They appear on the head, gill plates and pectoral fins. They will fade at the end of the breeding season.
Observe koi during breeding season. A koi purposely bumping the back end or side of another koi is definitely a male.