Preparing for Reproduction
Before Koi reproduce they must reach a length of 10 inches or longer. Once the Koi have reached that length they will reach sexual maturity and their sexual organs will develop fully. Male Koi will also begin to develop small uniform ridges along their head and the fin bones of their pectoral fins. These ridges may appear similar to the fish infection Ichthyophthirius, but are much more uniform and will feel rough to the touch. The ridges will be used in breeding to assist with the female spawning.
Beginning to Spawn
Koi generally spawn during the summer, as they require the water temperature to be close to 68 degrees Fahrenheit (though they may begin spawning in water as cool as 63 degrees.) When it comes time to spawn, the male Koi will begin rubbing up against the female and nudging her with the breeding area ridges that he has developed. This nudging assists the female in releasing the eggs from her body so that the male can fertilize them, with the released eggs being deposited on plants, the bottom of the stream or pool that they are in, or any other solid surfaces nearby. The female will generally release an average of 100,000 eggs per 2.5 pounds of her body weight.
Fertilizing the Eggs
Once the female Koi has released her eggs, the male will release his sperm in an attempt to fertilize them. Though there are a large number of eggs released, a surprisingly small number of these will actually be fertilized. This is due in part to some predators and even the parent fish eating the eggs, but is also attributed to the difficulty that the sperm can have in finding the eggs if a current is present. The eggs which are fertilized will develop baby Koi within them, hatching forth into the water after 3 to 4 days.