Inbreeding is a genetic concept that most aquaculturists have heard about, but few understand what it is or know how it can be used or abused. Inbreeding is one of the few natural biological processes that evokes deep-seated prejudices and invokes legal sanctions; as a result, it is probably the least understood and least appreciated aspect of animal and plant breeding. Most have heard of inbreeding because of legal and moral laws against certain consanguineous (“blood-related”) marriages and because extension agents and others have repeatedly preached about the evils of inbreeding. However, inbreeding is no different from any other piece of technology. If used properly, it can improve things; if used improperly, it will make them worse.
Inbreeding has been a concept that has plagued and intrigued humanity for thousands of years. The Torah, Bible, and Koran all contain proscriptions against incest. Most societies have laws pre-dating constitutional rights that prohibit marriages between certain relatives, although feudal rulers routinely ignored these laws, because it was a prudent way of consolidating money, land, and power.
The major reason why inbreeding was considered to be immoral was that no one knew anything about genetics, and consequently no one knew how inbreeding worked. It was easy to observe that when close relatives mated, some offspring were deformed or were weak and died. Since genetics was unknown, people associated the deformities, etc. with the act of incest itself and thus concluded it was evil and the deformed offspring were some form of divine punishment.
Our aversion to consanguineous marriages and abhorrence of incest is probably rooted in our evolutionary past and is controlled by our genes. Studies with several species of animals have shown that various behaviours, such as dispersal patterns-either forced or voluntary-of juveniles who are about to become sexually mature prevent parent-offspring, brother-sister, or first cousin matings which produce high levels of inbreeding. Since inbreeding produces some subviable or inviable offspring, genetically controlled behaviour which prevents such matings will be favored by natural selection.
Prohibitions and prejudices against inbreeding were so deeply rooted in Europe that as recently as 200 years ago European animal breeders were told that they should not use inbreeding in their brood stock management plans because it was against the Laws of God and Nature. However, animal breeders ignored these injunctions when they discovered that inbreeding was a breeding technique that could be used to develop new breeds, improve herds, and produce outstanding animals.
Once the science of genetics was established, geneticists were finally able to explain why some inbred offspring were deformed and were able to show that this was a problem of heredity, not of morality. They were also able to explain how inbreeding produced better animals and plants. An understanding of the genetics of inbreeding enabled geneticists to devise regular systems of inbreeding so animal and plant breeders could use inbreeding to improve growth rate and other phenotypes in a reliable and predictable manner.
Inbreeding is the mating of relatives, or the mating of fish more closely related than the population average (which is another way of defining relatives). Some geneticists want to restrict the definition of inbreeding and say it is the mating of “close relatives,” and that means the mating of two individuals more closely related than second cousins. The restricted definition might be more practical as a breeding definition, because it is based on the amount of inbreeding in the offspring that are produced by a mating. However, the mating of any relatives, whether they are closely related or distantly related, is inbreeding, and it will produce inbred offspring.
Inbreeding is, for all practical purposes, the opposite of crossbreeding. While inbreeding is the mating of individuals more closely related than the population average (between relatives), crossbreeding is the mating of individuals less closely related than the population average (between individuals from two populations).
Inbreeding, along with selection and crossbreeding (hybridization), is one of the three major breeding programmes that have been traditionally used to improve livestock and plants. While selection and crossbreeding are more widely known and appreciated, inbreeding is a powerful breeding technique that has been used to: establish new breeds and varieties; improve the results of selection; create better brood stock; and improve the results of crossbreeding programmes.
There is a wealth of information about inbreeding in livestock and laboratory animals and an even greater treasure trove for plants. However, comparatively little information exists for farmed fish; most of the information that does exist is spotty or consists of warnings about why inbreeding should be avoided and explanations about how it can be prevented.
While inbreeding is an important breeding programme that can be used to improve a population when it is planned and directed, unplanned and uncontrolled inbreeding can ruin a population through something called “inbreeding depression,” which is a decrease in growth and viability coupled with an increase in abnormalities. Genetic aspects of fish stock management are still in their infancy. Many farmers are still raising stocks that are either wild or only a few generations removed from the wild. Most farmers worry only about the environmental aspects of management-nutrition, water quality management, disease control and prevention-and ignore the genetic aspects. Since the modern science of aquaculture is still in its infancy, these management decisions make sense, because the best way to raise a crop has often not been quantified. Vast and immediate improvements can be obtained by refinements in fertilization regimens, feeding practices, or water quality management. These changes are often very cost effective because the changes are inexpensive.
The genetic aspects of fish farming have received far less attention because they require sophisticated managers, are long-term forms of management, require extra facilities, and add additional financial burdens; furthermore, the results are often unpredictable. While great improvements are possible, they are usually more costly than environmental improvements, and the return is not immediate.
Those who are interested in incorporating genetic improvements into yearly work plans usually want to conduct selective breeding or crossbreeding programmes; inbreeding is ignored. There are two probable reasons for this: The first is the deep-rooted prejudice against inbreeding. The second is the warnings against inbreeding that come from fish geneticists and aquaculture extension agents.
These warnings have been sounded because fish farming is a form of animal husbandry where inbreeding can be a more serious problem than it is with livestock. For the most part, fish farmers work with small populations. When new stock is acquired, the number of fish acquired is usually small because: shipping fish is a lot of trouble; it is costly, especially since water is heavy; and it is difficult to ship fish great distances. Consequently, many shipments contain only a handful of fish. Often, the fish that are acquired come from one or two matings, which means the genetic size of the population is quite small. Finally, fish are highly fecund compared to traditional livestock, which are capable of producing only one to a dozen offspring each year. Because of this, there is often a great temptation to spawn as few fish as possible in order to save money and labour and to lower the cost per fingerling produced. The fecundity of some fish is so great and the size of many farms is so small that a single spawn will often produce enough fish to stock many farms.
A number of warnings against inbreeding have been published because aquaculture is a unique form of animal husbandry, in that it produces fish for the sea as well as fish for the table. Many fish culture programmes are conducted not to raise food, but to raise fish that will be stocked in lakes, rivers, and oceans to help sustain or re-establish fisheries. These programmes are successful only if survival of the stocked fish is large enough to improve the creel or if the stocked fish spawn and increase the size of the next year-class. Unfortunately, many stocking programmes have not been successful, and a major reason for this is improper brood stock management at the hatchery. Brood stock management plans that make fish farming economically successful are plans that make stocking programmes unsuccessful. Much of the blame can be attributed to the small numbers of fish that are spawned every year which lead to unwanted levels of inbreeding that will, in turn, lower survival.
Even when fish are raised for food, the inbreeding produced by small populations can cause problems. If unwanted inbreeding occurs, it can reach levels that cause growth rate or other production phenotypes to decrease. This means yields will decline unless management is intensified; in other words, the environmental aspects of management must be increased to counteract genetic mismanagement.
Furthermore, small population sizes produce genetic drift, which is random changes in gene frequency. The ultimate effect of genetic drift, particularly in small populations, is the loss of alleles. The loss of genetic variation can make future selective breeding programmes ineffective. Selection improves productivity by exploiting a population's genetic variance; genetic drift decreases a population's genetic variance.
Inbreeding can occur when a farmer conducts a selective breeding programme; in fact, it is inevitable. Selection is a breeding programme that improves a population by allowing only superior animals to produce offspring. When this happens, the size of the breeding population is reduced. When only the best are allowed to mate and the only criterion for mating is a certain phenotype, relatives are often mated. This causes unwanted inbreeding. While inbreeding will be produced during selection and while it can cause some problems, it is of secondary importance. The major genetic goal during this type of breeding programme is to improve the population by selection. However, inbreeding should be moderated when conducting a selective breeding programme, because there is no point in conducting selection simply to counteract inbreeding depression.
Consequently, the loudest and most persistent message that comes from geneticists and extension agents is: avoid inbreeding and do what it takes to prevent it from occurring. And that is the major thrust of this manual. Most of the manual describes what inbreeding is, what causes inbreeding, how it works genetically, how it is measured, and what can be done to prevent it from occurring. These subjects are developed in order to compile a list of recommendations that farmers and hatchery managers can incorporate into yearly work plans to better manage their populations and prevent inbreeding from ruining the population genetically. Since the problems associated with genetic drift and inbreeding are linked, part of this manual is devoted to this topic and to management techniques that can be used to minimize the effects of genetic drift.
A manual simply stating that inbreeding is bad and should be avoided would be only partially useful. Since inbreeding is an important breeding programme, one that has been used to produce superior animals and plants, there is no reason why aquaculturists cannot use inbreeding to produce superior stocks of fish. Consequently, one chapter is devoted to the ways inbreeding can be used to improve growth and other phenotypes and to the development of regular inbreeding programmes that can be used to produce inbred lines which can be used in selective breeding and crossbreeding programmes.
Few farmers will want to or be able to use inbreeding programmes. The decision to conduct any breeding programme is one that must be made for each farmer or each fry/fingerling production center on a case-by-case basis. Breeding programmes are expensive and time consuming. They require a certain level of sophistication, because good record keeping is a requirement. This is especially true for inbreeding programmes. Inbreeding is the mating of relatives, and you cannot mate relatives if you cannot follow pedigrees. Also, these programmes require facilities. Ponds that are being used for inbreeding programmes are ponds that cannot be used to raise food. Finally, breeding programmes usually do not produce immediate improvements. Improvements are usually not seen for at least one growing season; when inbreeding is used, there is often a 2- to 3-generation lag before improvements are noted. Consequently, a farmer must be able to incorporate long-term planning into his farm management programme, and he must be patient. As a result, within a region, only a small percentage of farmers or fingerling production centers should or will ever conduct inbreeding programmes.
But even if few farmers or hatchery managers will conduct inbreeding programmes, an understanding of how inbreeding can be used to improve productivity will enable the rest to better understand what inbreeding is; how it occurs; what is does; and most importantly, how it can be controlled.
