There are two ways a fish farmer can increase fish production: The first is to increase the the size of the fish farm. The second is to increase yield, which is the weight of fish produced in each pond. It is often impossible to increase the size of the farm, because either the land is unavailable, the land is prohibitively expensive, or the land that can be used or water supply associated with that land is so poor that the costs of production would exceed the value of the crop. Even when a farmer can increase his land holdings, it often makes sense to improve production efficiency and produce greater yields (kg/ha). There are two ways a fish farmer can increase yields: The first is by environmental manipulations, such as the increased use of lime, fertilizers, feeds, and/or improved water quality management. The second is by growing genetically improved fish. If both approaches are used, yields can increase dramatically.
There are a number of breeding programmes that can be used to improve a population of fish genetically. Selective breeding and crossbreeding (also called “hybridization”) are the two traditional approaches that have been used for thousands of years, and they have been used to improve all major crops and livestock grown by farmers. Inbreeding is often combined with hybridization to improve the results of the crossbreeding programme. Three additional approaches have been developed recently: chromosomal manipulation, production of sex-reversed brood stock, and genetic engineering.
Although farmers have conducted breeding programmes on livestock for thousands of years, fish farmers are only beginning to use selection, hybridization, or other breeding programmes to improve aquacultured species of food fish. Although some progress has already been made, many fish farmers are culturing fish that are essentially wild and unimproved.
Selective breeding is a breeding programme that tries to improve the breeding value of the population by selecting and mating only the best fish (largest, heaviest, those with the desired colour, etc.) in the hope that the select brood fish will be able to transmit their superiority to their offspring. If this occurs, the next generation will be more valuable because the fish will grow faster, which will increase yields; the fish will grow more efficiently, which will lower feed costs; or all fish will have a more desired body colour, which will increase their market value.
Crossbreeding is a breeding programme that tries to find mating combinations between different populations of fish which produce superior offspring for grow-out, offspring that are said to exhibit hybrid vigour. Although crossbreeding is a tried-and-true method of increasing yields, the results of crossbreeding programmes are impossible to predict (unless the mating has been made previously), so the production of superior offspring is a hit-or-miss proposition. Many combinations often have to be evaluated before one is discovered that produces offspring with hybrid vigour. Crossbreeding programmes usually involve different strains within a species (intraspecific hybridization), but different species can also be hybridized (interspecific hybridization). To date, much of the breeding work in aquaculture has been devoted to hybridization among the different species of tilapia in an attempt to produce all-male hybrids for grow-out.
In general, crossbreeding is used to produce superior fish for grow-out (production fish), while selection is used to create superior brood fish. The hybrids that are created in a crossbreeding programme are usually grown and sold as food. A farmer rarely retains and spawns hybrids to produce a new generation of production fish. On the other hand, select brood fish that are created in a selective breeding programme are created for one purpose-to produce the next generation of fish for grow-out-and their offspring can, in turn, be retained and selected to continue the process.
In recent years, biotechnological research has led to the development of three additional breeding programmes that can be used to increase yields. One of the most common breeding programmes in aquaculture is the production of sex-reversed brood stock to produce monosex populations for grow-out. This is done either because one sex is superior or more desirable or to prevent reproduction during grow-out. For example: female sturgeon are more valuable than males because they produce caviar; female salmon are the more valuable sex, because sexually precocious males die before they can be harvested; male tilapia are more desired than females because they grow twice as fast. The major goal in tilapia farming is to prevent reproduction during grow-out; this can be best accomplished by producing a monosex male population.
The production of sex-reversed brood stock is usually accomplished by feeding either estrogens or androgens (sex hormones) to sexually undifferentiated fry to sex-reverse them. Sex-reversed fish are individuals that are one sex phenotypically but the other genetically. If sex reversal is done properly, sex-reversed fish are capable of producing monosex populations for grow-out. The type of hormone used-estrogens to produce sex-reversed females or androgens to produce sex-reversed males-depends on the sex-determining system of the species and whether you want to produce an all male or an all-female population.
Another breeding programme that is becoming more commonplace is chromosomal manipulation. This is usually done to produce sterile fish. The most common form of chromosomal manipulation is to use temperature or pressure to shock newly fertilized eggs (chemicals can be used, but they are less effective). If this is done properly, the shock prevents the second polar body from leaving the egg, so the newly fertilized egg contains a haploid sperm nucleus, a haploid egg nucleus, and a haploid second polar body nucleus. These three haploid nuclei fuse and produce a triploid zygote which, in turn, produces a triploid fish. Triploids are sterile. This type of breeding programme is used to enable farmers to grow exotic species whose culture might otherwise be illegal or to induce sterility in species that become sexually mature before they reach market size. For example, grass carp culture in most of the United States is legal only if a farmer raises triploids. This technique can also be used to improve the results of interspecific hybridization.
Chromosomal manipulation can be used to produce animals that have only a mother (gynogens) or only a father (androgens). This is done by creating haploid zygotes and by then shocking the zygotes to produce diploid zygotes. Haploid zygotes are produced in one of two ways: a normal egg is fertilized by sperm whose DNA has been destroyed by UV irradiation (gynogenesis); a normal sperm is used to fertilize an egg whose DNA has been destroyed by UV irradiation (androgenesis). Gynogenesis and androgenesis are techniques that can be used to produce highly inbred lines for breeding purposes. It can also be used to produce supermales; such males are capable of producing all-male populations.
In recent years, a new, high-tech breeding programme has been developed: genetic engineering. This is a breeding programme that transfers a single gene or a set of genes from one individual into another. This transfer can be within a species, between two species, or even across kingdoms. Although genetic engineering has generated lots of publicity, to date it has not produced genetically superior fish for farmers. Furthermore, this type of breeding programme is very expensive, highly regulated, and requires highly trained scientists. This type of breeding programme should be conducted only by scientists working at universities, at governmental research stations, or at agribusinesses that are capable of supporting expensive research projects with secure containment facilities.
Although all breeding approaches that can be used to improve yields are important and although they can be used either singly or in combination to achieve specific goals, this manual will describe only those procedures that can be used to improve fish by selective breeding.
The decision to conduct a selective breeding programme is a decision that must be made for each farmer or each fry/fingerling production center on a case-by-case basis. The decision to incorporate selective breeding into a farmer's work plan should not be a general decision made at a regional level. If it is, most of the selective breeding programmes will be failures, because selective breeding programmes require dedication, a certain level of sophistication, record keeping, and the investment of extra labour. Additionally, selective breeding programmes are not free; they also require the investment of money. Finally, these programmes usually do not produce immediate improvements. Improvements are usually not seen for at least one growing season, so 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 selective breeding programmes.
A final requirement that must be met before a farmer can conduct a selective breeding programme is the existence of proper facilities. This manual outlines selective breeding programmes that can be used to improve growth rate and other phenotypes on a medium-sized farm or fingerling production center, which was arbitrarily defined as a farm with about 2 ha of ponds. The breeding programmes outlined in this manual can be conducted in 1–150 ponds, depending on the type selection that will be used and the culture system that is typically used to grow the fish. Additional ponds will be needed to hold brood fish and to spawn them; other types of facilities, such as holding tanks, will also be needed.
Finally, common sense must prevail when choosing the most appropriate breeding programme. Even if a farmer has the ability and the desire to conduct a selective breeding programme, the biology of the species and the way it is grown should be carefully considered before the decision is made to conduct a selective breeding programme to improve growth rate. Even though most farmers would like to have faster-growing fish, in some cases greater yields can be achieved by improving other phenotypes via another type of breeding programme. For example, the biggest problem in tilapia culture is the fact that tilapia become sexually mature before they reach market size and, as a result, reproduce in the grow-out ponds. This uncontrolled reproduction means that a significant percentage of yield is unmarketable. Tilapia farmers may benefit from breeding programmes that can produce monosex male populations far more than from selective breeding programmes that might improve growth rate.
