How to Tell Male and Female Koi Apart? Methods to Identify Males and Females

Once koi grow into adult fish, males and females differ, and we can distinguish them by the head, body shape, belly, pectoral fins, vent, spawning behavior, and other aspects.

1. Head

The female's head is slightly narrow and longer; the male's head is wide and short, with the forehead slightly protruding.

2. Body shape

The female's body is short, thick, and plump, with a swollen belly, and the closer to spawning time, the more obvious the belly swelling; the male's body shape is leaner and longer.

3. Pectoral fins

The female's pectoral fins have rounded tips and are larger; the male's pectoral fins have slightly smaller and somewhat rounded tips, and are thick and stiff. During the breeding season, the difference between female and male fish is very obvious: the male's first pectoral fin ray and gill cover develop several small white granular protrusions, called "nuptial tubercles," which disappear on their own after the breeding season.

4. Belly and vent

The female's belly is swollen and soft; the male's belly is small and hard, and can be identified by touch. The male's vent is small and sunken, and milky white semen flows out when lightly pressed by hand; the female's vent is slightly wider and flat, with a slight outward protrusion, and eggs are released when the belly is lightly pressed by hand.

5. Spawning behavior

The easiest way to distinguish female from male fish is to observe spawning behavior. During spawning, the female swims constantly, luring the male to chase her.