|CHAPTER 17. CICHLIDAE (Family 387)
(Kevin Fitzsimmons & Wade O. Watanabe)
17.1 General introduction (Cichlidae)
17.2 Tilapia (Oreochromis sp.) (~20 pages 8-9 500 words)
INTRODUCTION TO Tilapia Culture Chapter
Tilapia, native to Africa and the Middle East, are one of the world’s most important food fishes. People living in the native range of tilapia have caught these fish in the wild for millennia. Tilapia is a common name that is now applied to several genera and species of fish that were formerly classified in the genus Tilapia, in the Family Cichlidae. In the reclassification scheme developed by Trewavas (1983) the several hundred species of Tilapia were split into three genera, Oreochromis, Sarotherodon and some remained as Tilapia. The Oreochromis are maternal mouthbrooders, the Sarotherodon are paternal mouthbrooders and the Tilapia are substrate spawners. The species that are most commonly reared in aquaculture are in the genus Oreochromis. These include the Nile tilapia, Oreochromis niloticus, the Mozambique tilapia, O. mossambicus, the blue tilapia, O. aureus, and O. urolepis hornorum, sometime called the Wami River tilapia. These species will all readily hybridize in captivity. There are now many strains of the parent species along with many hybrid strains available to growers. These will be described in some detail later in the chapter. There are also several species in the genus Tilapia and the genus Sarotherodon that are of interest to aquaculture. Tilapia, like the other cichlids, are of special interest to hobbyists and ecologists. Tilapia in Africa have been intensively studied for the species clusters that have evolved in the Rift Lakes of East Africa. Some lakes contain over one hundred species in a single genus. Some of the tilapias native ranges extend up into Israel and Syria. One of the common names for the fish is St. Peter’s fish. This comes from the fact that two species of tilapia are native to lakes in Israel and are reputedly the fish that were caught by the Apostles and that Jesus used to feed the multitudes as recounted in the Bible.
Domestication of the tilapias started in the 1950’s and 60’s with groups working in several countries (see the section on breeding programs and strains). Tilapia have been important to aquaculture because of the ease with which they can be bred in captivity and the wide variety of water conditions in which the fish will grow. Various strains can be grown in water varying in salinity from fresh water to full strength seawater (35 ppt). They will grow in water ranging from acidic (pH of 5) to alkaline (pH of 9). Tilapia can survive low dissolved oxygen (<2 mg/l) and high ammonia levels (50 mg/l) for longer periods than most other fish. Consequently, they can be grown in densities greater than virtually any other kind of fish. These characteristics make them ideal for aquaculture.
Another characteristic that facilitates selective breeding and domestication is their reproductive behavior. The tilapias used in aquaculture are maternal mouthbrooders. A female lays her eggs in a simple nest prepared by the male, the male fertilizes the eggs and then the female picks the eggs up and incubates them in her mouth. Even after eggs hatch, fry will remain in the mother’s mouth. Once the fry are free-swimming they will return to her mouth for protection. Females can produce several hundred to several thousand young per spawn. The high level of parental care allows breeders to quickly raise thousands of young for directed selection or for stocking into production units. Another advantage is that the adults become sexually mature in less than six months, when they are still a fraction of their potential size. This is an additional advantage for selective breeding, allowing several generations to be produced in the time it takes other fish to reach maturity. The drawback to this high potential for reproduction is that tilapia introduced to new (exotic) locations can quickly spread and impact native fish populations. Likewise in production ponds without predators, tilapia can over-populate ending up with large numbers of small, stunted fish. This can present a serious problem for aquaculturalists who are attempting to rear a large size fish for market. Several methods are used to avoid over-population and stunting that will be reviewed in the section on production techniques.
Eggs of tilapia are relatively large and fry are hardy and omnivorous. Fry readily feed on a variety of foods including periphyton and phytoplankton (attached and floating algae), zooplankton and powdered feed. This allows the culturist to further manipulate spawning by removing the young from the female and raising them independent of the mother. Removal of fry will encourage the female to begin eating again, she eats little while brooding, and be ready to spawn again in several weeks. Sex of fry can be manipulated in several ways. Undifferentiated sexual organs of juvenile tilapia can be induced to produce phenotypic all male or all female populations. Males grow more rapidly and crops of primarily males will avoid problems associated with unwanted spawning. There are several methods and reasons for this “sex-reversal”, that will be covered in detail in the section on reproductive biology.
Another reason that tilapia are prized as aquaculture species is because they are herbivorous or omnivorous, depending on the species. In nature, tilapia receive all of their nutrition from algae, higher plants, detrital matter and/or small invertebrates. This makes it easy to grow the fish in ponds with minimal inputs of feed or fertilizer in extensive aquaculture. If semi-intensive systems are used to generate greater production from a facility, fertilizers can be used to produce algae and zooplankton. In intensive production, feeds containing primarily plant proteins can be fed. These inputs are considerably less expensive than the costly feeds containing high percentages of fish meal or other animal proteins that must be fed to carnivorous fish. Consuming herbivorous fish is a more ecologically efficient transfer of energy and protein to human consumers than using carnivorous fish that require fish or other animal proteins in their diets.
These are just of few of the reasons that tilapia have become one of the most important domesticated fish around the world. The following sections describe some of the most common techniques employed to rear what may become the most import fish in aquaculture in the coming decades.
The tilapias most commonly used in aquaculture, from the genus Oreochromis, are mouthbrooders. The fact that they provide so much parental care has a great impact on the benefits and drawbacks of their reproduction in aquaculture settings. From a practical point there are three major issues regarding tilapia reproduction. First, control of reproduction, for purposes of domestication and other directed breeding. Second, elimination of excessive breeding, to grow a controlled number of fish in a rearing unit. Third, all or predominantly male fish are preferred in production systems.
Regarding the spawning behavior, often fish breeders will isolate males and females to reduce unwanted reproduction. When the fish have reached the desired size, they can be stocked into a spawning unit. Males will exhibit spawning coloration. O. mossambicus males will have very dark pigmentation, a red margin on the caudal fin, and may have a white spot on the throat. Males often have enlarged upper and lower jaws. O. aureus males have a bright iridescent blue or blue-green spot on the throat and a similar red margin on the caudal fin. O. niloticus males are more likely to have a reddish coloration on the throat, and the same red margin. Due to the large number of hybridization events in the past, these color patterns will be intermediate in hybridized strains. Typically, a male will excavate a nest from the bottom sediment. In many culture settings, however, breeders are stocked into tanks in which there is a solid surface. In these instances the male will scrape away all surface algae and leave a cleared spot delineating the nest. A female lays her eggs in the nest, the male fertilizes the eggs and then the female picks her eggs up and incubates them in her mouth.
At this point many culturists intervene and collect fertilized embryos from the female. If a female can be netted with minimal disturbance, she will continue to hold her eggs. A wash bottle can be used to rinse eggs from the buchal cavity. Eggs collected in this manner are typically hatched in a McDonald jar, or various round-bottomed containers with an overflow spout. Hatched fry swim up and out and are accumulated in a screened tray. One of the concerns encountered when incubating eggs is the prevalence of Saprolegnia fungal infections. Fungus tends to grow first on unfertilized or dead eggs, spreading to adjacent viable embryos. The most effective treatment is to carefully remove infected eggs by hand on a daily basis. Keeping eggs in motion is also effective as the fungus cannot easily spread to eggs in motion. There are several chemicals that are known to be effective to control fungus, including potassium permanganate, formalin and salt. Regulations vary on how these compounds are used, so refer to local agencies for instruction. After the embryos hatch, they can survive for several days on yolk sac. Fry will begin feeding before the yolk is completely absorbed.
Many small-scale culturists prefer to allow the female to continue to incubate the eggs. Even after eggs hatch, fry will remain in the mouth. Once fry are free swimming they will still return to her mouth for protection. Tilapias of the genera Tilapia and Sarotherodon, are more likely to be nest builders and care for eggs laid in the nest. However, they also invest in the care of the young by producing large eggs and then protecting the embryos and fry in the nest. Females can produce several hundred to several thousand young per spawn. The high level of parental care allows breeders to quickly raise thousands of young for directed selection, for transgenic research or for stocking into production units.
In some cases precocious juveniles become sexually mature in less than six months, when they may weigh less than 50 g. This can be an additional advantage for selective breeding, allowing many generations to be produced in the time it takes other fish to reach maturity. However, most of the sophisticated large-scale breeding programs developing stocks for international scale breeding programs use fully grown broodstocks to determine desirable growth characteristics through the production cycle. The drawback to precocious sexual development and a high potential for reproduction is that tilapia released in exotic locations can quickly spread and impact native fish populations. Likewise in ponds with no predators, tilapia can over-populate and end up with large numbers of stunted, unmarketable fish.
Research into the reproductive biology of the tilapia has focused on physiological characteristics such as egg development, storage of sperm for use in selective breeding or storage of genes to protect genetic diversity, and water quality and radiation effects on gametes, among other topics. The complex reproductive behavior has been of interest to scientists and hobbyists for decades. More recently, transgenic research and other forms of genetic engineering have included tilapia. Fish eggs including tilapia, are much larger and easier to manipulate than mammalian eggs and do not need to be implanted in the female for incubation. The large number of eggs per female allows for large sample sizes and the problems of working with mammals, which can be expensive to maintain and subject to strict regulations can be avoided.
From a practical farming standpoint, the critical issue is how can the greatest number of young be produced in the smallest area for the lowest cost. Effective hatcheries are a prerequisite for establishment of an industry. Several techniques are used and the success or failure at any given facility will be subject to the skill of the breeder, the quality of the fish, the food and the water available. Simple pond spawning is sufficient in many locations. A shallow pond is built and the males and females are stocked and allowed to spawn at random. Typically 3 or 4 females will be stocked for each male. Seine nets are used on a regular basis to capture the young fish that are then moved to another pond, nursery tank or hapa net. Hapas are fine meshed nets that are suspended from a frame in a body of water. The young fish, usually less than a gram, and only a few days old will be collected and placed in the hapa for initial feeding and to protect the young fish from predators. A variation used in large hatcheries and for selective breeding programs is to place the spawners in tanks or hapas suspended in ponds. After the fish spawn, the eggs or fry are collected directly from the mouth of the female, or the young can be collected as soon as they are swimming away freely from the mother. Eggs and fry removed from a brooding parent can be reared in artificial systems described above.
Reproduction and hatchery operations are some of the most interesting scientific and technological challenges facing the tilapia industry. Development of genetically male tilapia, sex-reversed fry and sophisticated selective breeding programs utilizing dozens of family lines has contributed to a rapid improvement in the growth rates and harvest size of farmed tilapia. Domestication of tilapia is still in the earliest stages and we already are witnessing tremendous improvements with more to follow in the near future.
Male fish are preferred for production fish as they grow more rapidly and to a larger average size. One technique developed to generate all male populations is to utilize hybrids. Certain hybrid crosses, O. aureus x O. mossambicus, and O. aureus x O. urolepis hornorum result in a skewed sex ratio favoring males.
The most commonly used technique to produce all-male populations is to sex reverse fry. Newly hatched fry have undifferentiated gonads. By including a hormone in the feed, or by immersion in a solution containing a hormone, fry can be induced to develop morphologically as male or female, regardless of genotype. The normal technique is to feed methyltestosterone to fry for 28 days. Twenty-eight days is sufficient to induce most if not all the fish to develop testes. The small number of fish that do not reverse will develop as females or hermaphrodites. Temperature can affect the success of reversal, with temperatures below 28 0 C contributing to lower success (95% decreasing with lower temps) and temperatures of 30 0 C contributing to success near 100%.
A novel approach, first developed in the Philippines, involves genetically male tilapia (Mair et al. 1997). In this process, selected fry are fed a feminizing hormone, estrogen, yielding an all-female population. Genetically male but phenotypic female fish are then bred to normal males, yielding a brood of fry with a normal distribution of 1/4 XX, 1/2 XY and 1/4 YY progeny. The YY males can then be found by progeny testing and once identified as such, can be sold to other hatcheries and bred to selected females. The YY males bred with normal XX females should yield 100% XY, or normal males. The primary benefit of this technique is that the production fish are all-male and have never been treated with any hormone. Although the fish have been trademarked as Genetically Male Tilapia, by FishGen of the United Kingdom, they are not genetically engineered. The fish are not transgenic, no genes have been altered and the fish bound for human consumption have not been exposed to any hormones. Transgenic tilapia have been produced in Great Britain and in Cuba (Martinez, et al. 1999). Neither stock has been released outside the lab.
Regulations and Permitting
Tilapias are exotic species to the United States and are subject to restrictions in many states. There are wild populations of O. aureus and O. mossambicus in southern Arizona, southern California, southeastern Texas, and southern Florida. Populations of introduced tilapias have been reported in Alabama, Florida and warm springs in several Western States. Most states now require some type of stocking permit or fish farming license before tilapia can be grown. In Arizona, California and New Mexico, there are geographic restrictions on where tilapia can be grown within the states. In Louisiana and several other states, there are system requirements for growing tilapia. In most cases the requirement is that the fish be grown in enclosed structures with no discharge to open waters. Typically this entails use of greenhouses or industrial buildings that discharge to settling basins or municipal sewage systems.
Environmental and Conservation Issues
Tilapia have become the second most important fish produced in aquaculture. Their spread to countries around the world has been accompanied by environmental externalities, negative impacts on the ecosystem outside the farm. In addition, introductions of various species into the range of closely related tilapias have led to inadvertent hybridization and loss of genetic variability.
Environmental impacts of tilapia can be loosely grouped into two major categories. First is the impact of feral populations of tilapia on native fishes. Introductions of tilapia around the world frequently occur in concert with severe human impacts on local aquatic systems. These impacts often help the tilapia to thrive at the expense of native species. Tilapia may compete for resources with native fish, or they may just thrive in the altered conditions.
The second environmental issue is the nutrient enrichment of local waters from intensive farming of tilapia. Intensively fed fish generate fecal waste and leave uneaten food. Nitrogen and phosphorus dissolved in the effluent and the biological oxygen demand of decaying organic matter can impact the receiving water. Wastewater from processing plants can also impact receiving waters if they are not treated sufficiently. Conventional water treatment plants, constructed wetlands, and irrigation of crop plants are suggested as methods of reducing negative impacts from eutrophication caused by fish farm effluents.
Conservation of genetic variability of wild and domesticated tilapia stocks are also of importance. There are several instances where this is important. First, tilapia stocks have been moved repeatedly and allowed to interbreed with local populations. In some cases this has led a to a decrease in genetic diversity and “pollution “ of endemic populations. The loss of this diversity becomes important because whole genomes may be lost. Some of this genetic variability may be important as a genetic reservoir of material that may be useful for future conservation or breeding efforts.
Several of the most common strains of tilapia came from very small founder stocks. These fish have a high degree of introgression and may be subject to genetic bottlenecks. Development of domesticated stocks that have been selected for certain culture conditions, from an adequate breeding population would be the best way to avoid this problem. The Genetically Improved Farmed Tilapia (GIFT) program is probably the best example of such a genetic improvement effort. Maintaining diversity in feral and captive stocks is important for both the fish in their native environment and for those in captivity.
Impacts of feral tilapia on native fishes.
Tilapia have been introduced into more than 90 countries on all the continents except Antarctica (Pullin 1997). Many of the early introductions were for insect control or aquatic weed control (McIntosh et al., 2003). The agencies responsible for these introductions widely dispersed the fish. In most countries introductions of tilapia for farming purposes came later, or else fish already established in local water bodies were domesticated. Tilapia introductions were often associated with severe environmental change, especially construction of reservoirs and large-scale irrigation projects. Tilapia are often described as “pioneer “ species. This means that they thrive in disturbed habitats, opportunistically migrating and reproducing. Often they were introduced into areas that have severe environmental damage where natives were already at risk. The tilapia are better able to adapt to the new conditions and the natives have been forced to contend with environmental changes and competition from exotic species.
The Lower Colorado River in North America is a good example. A series of dams and diversion of most of the river’s water caused massive environmental changes. The dams stopped the normal flooding cycle, altered the algal community and increased salinity. Introduction of tilapia for aquatic weed control in the irrigation canals allowed them to eventually migrate into the Colorado River mainstream. Tilapia, along with, many other exotics introduced as sport fish, completely changed the fish community. In some areas of the river, tilapia now represent 90% of the fish biomass and virtually all of the native species are endangered.
Many populations of tilapia are now so well established they are a permanent part of the fish community. However there are some steps that aquaculture operations can take to mitigate any additional harm. The eventual goal should be to develop fully domesticated strains of tilapia that will have little chance of surviving outside a culture setting, in much the same manner as most domestic farm animals. The industry is well on its way with tilapia. Red strains of fish are an important step. Red tilapia are only found in domesticated populations and they have very little chance of surviving in the wild. Predation is high from birds, fish and humans because they are so visible in the water. Strains that have been bred to have very large fillets and a more rounded body form are also unlikely to survive outside a farm. Finally, all male populations, developed from hybrids, sex-reversal or genetically male parentage, are less likely to be able to establish a breeding population off farm. All of these techniques should be considered as contributing to the reduction of the ability of tilapia to impact native communities.