Koi and Goldfish Breeding Basics
Only selective breeding in koi and goldfish will result in specific physical traits.
Stephen M. Meyer
Q. Every spring I look at my pond fish and hope the ones that look fatter than normal will lay eggs. My real hope is that the ones that produce babies will be the koi, the beautiful fantails, or even the long-tailed comets. But every year, with hundreds of tiny offspring, the babies are always short-tailed common comets. I've looked everywhere and cannot find information on the mating of goldfish and their offspring. I would love to learn how to tell the males from the females, which to mate with which, and all the other details of controlled spawning.
A. Goldfish and koi are the products of selective breeding. The many varieties that grace our aquariums and ponds are the result of genetic manipulation. Thus, the controlled breeding you are interested in is natural and appropriate for this particular type of fish.
I could not possibly do justice to the topic here, so let me provide the basics and suggest you check out KOI USA and The Goldfish Report, two very valuable sources of information. The Goldfish Society, which publishes The Goldfish Report, also has an excellent handbook that covers goldfish breeding (Goldfish Society of America, P.O. Box 851282, Richardson, TX 75085-1282).
The traits we like in fancy goldfish and colorful koi come about by genetic manipulation through selective breeding. Breeders choose particular characteristics they like, such as long flowing tails, and then select males and females that strongly exhibit those characteristics, and mate them.
Most of the traits we find very attractive are recessive rather than dominant. Controlled breeding of successive generations with particular traits can often increase the proportion of babies showing the characteristic, but it may take 10 or 20 years to fix a trait well. Patience and a good eye for fish are musts. This is why "breeder" fish are so expensive.
Another factor weighing against fancy goldfish and brightly colored koi is natural selection. Fish with long flowing tails, bizarre body shapes and bright colors are just not competitive with their more natural brethren. They cannot get to the fish food as fast, so they tend to be smaller and underfed. Moreover, if anyone is going to be eaten by other pond critters or by larger fry, it is going to be the most unique specimens — those that stand out and are not fast enough to save themselves. Therefore, breeders cull fancy fry very early and segregate them in special breeding ponds where their competitive disadvantages can be minimized.
So, your lack of success in breeding a fancy variety is not too surprising. Left to her own devices, Mother Nature will try to produce normal, natural, ancestral variants of goldfish and koi, not the fancy ones you desire. If you want to experiment with selective breeding then you should find a male and female with the identical characteristics you want in the offspring. These animals should be kept separately from your other fish during breeding season. You can improve the spawn further by hand spawning and carefully culling out all the fry that do not have characteristics you bred for.
In my previous series on goldfish in Aquarium Fish International, I suggested some ways for distinguishing male and female goldfish. However, the most reliable way is to see who chases whom at breeding time. The chasee is the female, the chasers are males. Of course, even this is not 100 percent guaranteed because same-sex breeding behavior is often observed among fish.