Category: Breeding Techniques

Tips on breeding your fish.

Inbreeding Tropical Fish, Good or Bad

According to some aquarists inbreeding is an evil thing while other hobbyists seem to believe that an inbreeding philosophy is a neutral thing which is neither good nor bad. To me it depends upon the application in which line breeding, a form of inbreeding, is being applied and the reasons for inbreeding the species. Generally, the idea of selective inbreeding is to reproduce purebred strains of a species by preserving hereditary characteristics that are desirable, and getting rid of undesirable traits. Also by selective inbreeding, a species’ hidden genetic characteristics will appear in the offspring. Many of these traits are generally desired and bred for by aquarists. This form of selective breeding is used by tropical fish breeders because they often hope to build up strains of fish with superior qualities. Usually these traits are caused by an unnatural mutation or a recessive form of a gene(s) which effects the size, coloration and/or finnage of the species.

The body size of a fish is a characteristic that breeders often try to inbreed into a strain of fish. Usually, the intent is to increase the body size, but if the breeding stock are not carefully chosen the opposite result can occur. Some breeders have suggested that inbreeding can lead to dwarfism of a species.

The coloration of a fish species is determined by the presence or absence of pigment containing cells called chromatophores. These are categorized into the color pigments that they represent: melanophores, brown, black or both; erythrophores, red; leucophores, white; xanthophores, yellow; and irridophores, reflective pigment cells. Consequently, color mutations can occur through the presence, absence or blending of these chromatophores. An example of a color mutation that was unnaturally bred into a strain through inbreeding is the pink convict cichlid. It lacks the original melanophores of the wild caught convict. The pink convict represents a condition known as leucomorphism, (white coloration).

Another desirable mutation that is often bred for is albinism. Albinism is a total absence of pigment. Albinism usually does not occur naturally, or if it occurs the fish usually does not survive for long. In this case chromatophores are absent.

Mutation of finnage is also another trait that breeders try to manipulate. Breeding stock needs to be selected very carefully for the desirable gene that represents the traits being bred for.

However, with all the desirable characteristics being bred for, the breeder needs to be aware that undesirable traits will be included in the genotype as well. These undesirable genes may cause weakened body metabolisms, hereditary diseases, and congenital abnormalities such as spinal problems and loss of finnage. These situations generally occur because of the breeders inexperience and the lack of culling undesirable breeding stock and fry. If culling does not occur, and line breeding of these undesirable fish continues, these undesirable traits will occur more frequently in the fry thus severely weakening successive generations. This brings up cross breeding within a species. Cross breeding is when several similarly related strains are bred together to create greater genetic diversity. However, these other strains can have genetic problems that the breeder should be aware of. As with all forms of breeding, the breeder needs to be very selective when choosing breeding stock. One difficulty may be the difficulty in obtaining closely related strains.

Even with desirable purebred strains of a species that show no signs of genetic weakness I still question whether the traits being bred for are beneficial to the species. I also question if these mutations would occur naturally within a population? Although a variety of mutations occur within a species very few of these survive. Even fewer of these natural mutations are caught by humans. I was lucky to observe an albino form of a killer whale at Sealand in Victoria in 1969. Other natural mutations that I am aware of were recently caught and put on display at the Sydney Aquarium in Australia. These included a Port Jackson shark and several long-finned eels that showed a condition known as xanthomorphism (yellow coloration). Ichthyologists at the aquarium are amazed how these conspicuously colored fish have managed to survive their hostile and highly competitive environments. However, all of the above conditions are rare.

Most mutations occur due to selection for recessive forms of genes. In captive strains these recessive traits can be consistently bred for but in the wild the consistent occurrence of recessive traits is unlikely. Therefore, one question that needs to be asked is would these captive bred, recessive form fish survive if introduced back into the wild? I believe that survival would be difficult for most human manipulated strains of a species in the wild. However, if these manipulated strains did survive they would likely revert back to the wild form of the species through introduction to new populations of the same species.

This brings me to Darwin’s theory of natural selection that explains the occurrence of mutations within a species. The process of natural selection selects individuals that have beneficial traits, which help them survive in their natural environment, while allowing those individuals with less beneficial traits to be lost over time. Eventually, through several generations, all individuals of the species will retain the beneficial genes. Through Darwin’s theory of evolution, natural selection continually works to determine the genetic traits that help a species ensure its continued survival.

Even though line breeding of a species does have its problems, inbreeding a species will not pollute the gene pool of that species as no new fish are introduced. Cross breeding strains of the same species will also not pollute the gene pool since cross breeding occurs within the same species. Gene pool pollution occurs when two species hybridize. Hybridization should be discouraged by hobbyists because unnatural genes are introduced into each species’ gene pool, a generally undesirable occurrence. However, there are studies and theories which suggest that Mother Nature has been involved in natural hybridization. While this may occur naturally, humans should leave creating these new hybrids to Mother Nature.

In closing I wish to tell you how I feel about the subject of tropical fish breeding. First of all, I believe in the hobbyist’s responsibility to breed a species as close as possible to its wild form. Therefore, I disagree with line breeding most species. However, if I bred line bred strains of a species I would endeavor to crossbreed that species back to a similar existing purebred strain before I would continue inbreeding. I do believe it is very important to properly select the breeding stock (related or not) and also to properly cull the fry. I do not agree with hybridizing species because if hybrids were intended to exist we would already find them in nature. However, a guppy is still a guppy, long fins, albinism or not.



Dr. Axelrod’s Mini-Atlas (1987)

Tropical Fish Hobbyist, Vol. XLIV, 4

World Book Encyclopedia, (1965)