Koi are close relatives of goldfish. Koi have been bred in captivity for centuries to produce fish in many colors. Modern pond koi scarcely resemble their drab, wild-type ancestors. Since domesticated koi breed in captivity, researchers have been able to extensively study their reproduction, including the survival rate of hatched baby koi, or fry.
How Many Eggs
Koi produce quite a few eggs. A typical koi spawning can yield anywhere from 2,000 to 50,000 eggs. The exact number depends on several factors. For example, larger parents will generally produce a greater number of eggs. Additionally, koi can usually spawn several years before they reach their full adult size. Adult koi usually produce more eggs as they get older and larger. On top of this, some breeds/strains of koi may produce more eggs than others.
Koi spawn through a method called egg-scattering. In this type of spawning, fish release eggs and sperm in the same general area and hope for the best. Because of this, only about half of koi eggs hatch. But that's only one aspect of hatched koi survival rate. Once the eggs that do hatch become fry, various factors influence survival rates. According to a study published in the December 2006 issue of "Aquaculture International," a peer-reviewed journal, the type of pond greatly influences they survival rate of koi. Artificial ponds, such as concrete or plastic ponds, foster a survival rate of 67.83 percent, while earthen ponds have a survival rate of 95.5 percent, possibly because earthen ponds better encourage the growth of tiny organisms the fry can eat.
If every single koi fry survived to adulthood, a single spawning would overcrowd ponds. For this reason, every koi breeder uses a process called culling. In culling, breeders kill off koi. Experts recommend that koi farmers cull 60 to 80 percent of fry a month, starting at three months. Ideally, sick and undersized fish are culled first. Then, remaining koi are culled based on lack of traits the breeder is seeking for that particular strain.
Koi and Genetics
The koi in ornamental ponds have many traits, including long fins and exotic color patterns. Careful selective breeding has produced the koi you see today. Many of the desirable traits in koi are recessive. This means fish need to receive two copies of a recessive gene to express it. This also means that uncontrolled breeding tends to produce a reversion to wild-type, drab, short-finned fish. Culling is necessary to produce the colorful fish.