culling is a pretty vague term, when developing a strain or whatever, most of the fish spawned will get culled. As to what happens to them at that point, just depends on the breeder. Many end up as feeders or are euthanized, but if they are desirable enough they may be resold. Depending on the species, something like guppies, the culls are sold and the choice specimens are kept to further develop a line.
Culling just means removing fish from a group, its based on whether they fit the desired criteria.