By Carolyn Weise

Sexing fish is difficult to impossible until they are mature enough to spawn. With koi, the females will develop a larger, wider body, capable of producing thousands of ova (eggs). The males will remain slender, long-bodied fish. Unlike birds or other wildlife, the color or eventual size will not distinguish male from female in koi. Both are capable of the same beautiful colorations. Unlike dogs, or cats, there is no personality difference either. Unlike other fish, koi do not remain small given a small container in which to live. They grow to their genetic predisposition, up to 36” in length. Koi are oviparous, or egg laying fish. Unlike breeding purebred rottweilers or collies, the parent fish do not necessarily bring forth young that look like them. Some will revert back to the original black/brown colored Magoi which used to inhabit the Japanese rice paddies. Others may show color combinations that are unique and wonderfully unexpected.

The only way to control breeding is to isolate two parent fish and remove them once they have spawned. Once spawning takes place the parents will consume many of the eggs. Likewise, the only way to prevent unwanted spawning is to keep only same sex fish together in the pond.

In spring, you will see the first signs of which sex is which. The females that have reached maturity (around 3 years of age, however they can and have bred at 2 years), will begin to swell with eggs. As the water temperature warms, the males will also begin to actively chase the females. Generally when the water reaches the 72ºF point, and especially if there are plants in the pond, the chase begins in earnest.

The males will push the females against rocks, plants, and the sides of the pond to squeeze out the eggs as they release their milt to fertilize them. Injuries and deaths have occurred from spawning due to crowded ponds or too many males. What is obvious to the pond owner (in the morning) is froth on the top of the pond and a strong fish smell. That is the excess of protein from the spawn during the night. As a rule, the spawn will continue throughout the day, for up to two weeks. Or it can end after a couple of days and begin anew the week after.

After the eggs are released and fertilized, the will attach to the rocks, plants or liner of the pond. In 3-4 days they fry will hatch. Any eggs that turn white will have been killed by fungus. The fry that hatches becomes free swimming after hatching. Within one week colors can be seen, although the fish will change colors as it develops for the first 2-3 years of its life. The parent and larger koi will no longer try to eat the babies once they are able to identify them as fish. They are not cannibalistic.

Nor are they protective parents. These fry are on their own from the moment of their birth. They will face predatory insects, birds, frogs, and illness. Yet, many will survive. It is difficult to realize that each of these tiny beings may grow to 36” and may need a new home…. Please consider this when you decide to breed your fish.

Other methods of spawning are artificial, such as squeezing the eggs and milt out of two parent koi, into a container for fertilization, then adding them to an incubation tank to develop. Koi breeders (farmers) have a succession of ponds to “grow out” these babies, and cull those that aren’t worth keeping. They keep only the best of the best.

Beside plants and natural pond environments, hobbyists have used breeding mats, clean mops and brushes as receptacles for eggs during spawn to collect the eggs for sale later on. Many a koi dealer has started out in just this way!