Cull: Verb: to examine carefully in order to make a selection from; pick over.
Noun: something picked out, especially something rejected as not up to standard.
Culling is a necessary evil, one of those things that we often don’t want to think about, but that’s essential if we’re trying to breed strong, healthy fish. If you’re only a person who keeps a few fish, culling is probably a subject of little importance and of less interest. But if you’re actively breeding fish, it’s absolutely essential for the continuing improvement of the fish you are breeding, and for the overall good of the species.
“The future health of any strain of fish depends upon just how handy we are with the culling net,” says Greg Niedzielski, discussing this subject. “Culling refers to the process of selecting the best individuals of a strain of fish for future reproduction, and the disposal of inferior specimens. [It]… is an important technique for maintaining the quality of a strain of fish and preventing the circulation and distribution of inferior specimens. Not rigorously culling … may, to some, seem to be a more ‘humane’ approach, but … over time [it] can do great harm to … a species.” Culling eliminates the weak and the deformed, removes weaknesses from the breeding strain, and allows selection of the best possible individuals as breeding stock.
“It goes without saying that only the best individuals should be used for breeding purposes, but what many people don’t realize is that the corollary to this is that inferior specimens should be destroyed,” Greg continues. “Selling [culls] to local pet stores is not indicated, as this merely distributes inferior fish to unwary customers.”
Why Should We Cull?
In our artificial aquarium environments, we often are able to hatch a high percentage of the eggs produced in a spawn, and often raise a high percentage of the fry. This is not the norm in nature, where successful reproduction by a pair of fish may result in the survival to adulthood of only a few of the strongest offspring; the weaker, slower, less healthy or less well adapted young ultimately fall prey to predators or disease.
Culling mimics this natural selection process, but other factors may influence our decisions with the culling net as well: size and growth rate, coloration, and personal preference. For the continuing vigor of a species or a strain, we must cull for deformity (missing fins, bent spines, deformed mouths, etc.), lack of vigor, slow rates of growth, and poor overall quality.
Culling for deformity or major weakness is self-explanatory: you never want to perpetuate such problems. But if you’re breeding a strain of one of the more popular species, particularly one with a number of strains based on color or fancy pattern, such as angelfish, swordtails or guppies, what you may consider “culls” may in fact be quite acceptable in the “pet” market, just as “pet quality” puppies are acceptable in the dog market.
Culling is an ongoing process, based on observation. One of the earliest things the aquarist can observe is the condition of the fry. Sickly, undersized, or deformed fish are often apparent long before other selection criteria can be applied. Some hatches of fry grow at such varied rates that they will ultimately “self-cull,” the larger fry preying upon the smaller. Often up to half of a large spawn can be culled early on for size or weakness, allowing more room and better growing conditions for the rest.
As the fry develop they can continue to be culled, both for overall quality and for poor color or color pattern. In killifish, such culling for color and type should always be based on the colors and patterns representative of the wild type. Occasionally a color sport may appear that is worth reproducing, such as the Gold Lineatus or the various colors of Australe. In fish that mature late, or that develop long finnage, culling for all but the most obvious problems may be delayed well into maturity, and even up to show, sale, or breeding times.
There are a number of ways to dispose of the culled fish, depending on the size and the number of culls and on your own personal situation.
Culled fish may be fed to other, larger fish (having a large, hungry cichlid or other predator in the fishroom, or having a friend who has one, does sometimes have advantages). Culls may also be humanely destroyed. Many breeders recommend freezing cull fish in a small amount of water as a painless method of euthanasia. Others suggest decapitation using a sharp razor blade. If you make your own paste or frozen fish food, healthy culls may be used in your favorite recipe as part of the protein requirement.
Releasing culls into local waters is never an acceptable alternative. “Flushing” is also to be avoided, since it subjects the culls to prolonged distress before death, and provides the (hopefully remote) possibility that the fish will survive and get into local native waters.
Culling may not be a pleasant task, but it is a necessary one that serves a number of useful purposes. Cull carefully, dispose of the culls humanely, and then sit back, relax, and enjoy watching those beautiful, healthy fish that remain.
Reference: Niedzielski, Gregory J. “A Few Words About Culling (A Necessary Evil?),” Journal of the American Killifish Association (JAKA) 31 (6) 192-195 (1998).
G.C.K.A. Newsletter, October 1999