Culling Fish Fry | How, What and Why
Culling fry is a term that refers to removing fish from the gene pool. How a breeder chooses to do this depends greatly on the breeder. Some breeders adopt out culls on the condition that they are not bred from, a fuzzy feel good methodology that does not particularly assure high quality in the gene pool, and other breeders kill the fry in some fashion.
Whether you cull or not, and how you cull very much depends on you. There is no right or wrong way to approach the issue. Personally, I believe that in the wild a very small percentage of fish, only the fittest, would survive to adulthood. It is true that most fish will spawn hundreds of young in a single spawn, however it is in no way natural that all these fish survive to adulthood.
The easiest method of culling is to remove excess eggs. If fish fertilize too many eggs, then some can be removed, limiting spawn size immediately. This is perhaps the best way to limit the size of the spawn, however it does not select for quality progeny, only for fewer progeny.
In most spawns, the front runners will establish themselves quite early on in the piece. They grow larger than the other fry, they are more vigorous and they usually look more impressive than their brothers and sisters. Culling at this point, feeding the fry to larger predator fish, allows a breeder to keep the spawn small and allows them to focus their efforts on raising a few prime specimens rather than a great deal of average fish.
There are many ways to euthanize fish, it's a topic I have expounded on here, but I think that euthanizing spawns is a waste of money when young fish fry make such nutritious food for bigger predator fish. If the fish are large enough, death is fairly instantaneous and painless.
Culling is something all breeders should think about before they breed. It's an important and oftentimes essential part of the breeding process, (space constraints alone can often make culling necessary,) and one that will rear its head eventually. How soon it becomes an issue depends on the type of fish you are breeding. Schooling fish will often do well without the need for culling as they will happily form communities together. Solitary, aggressive fish, on the other, hand, like betta fish, may pose more problems as the time comes to separate them out.
Personally I am a believer in small spawns keeping only quality specimens, however others believe that all fish should be kept unless sick or deformed. Make your own mind up, but make it up before you decide to breed.