The tilapias are members of the Cichlidae family and are amongst the most widely produced food fishes in the world, second only in importance in aquaculture terms to the carps. Although the Cichlidae family is widely distributed throughout the tropics, tilapias are an exclusively African and Levantine association, and including the genera Tilapia, Sarotherodon and Oreochromis as well as many lesser known genera.
The subdivision into genera is largely made on the basis of breeding behaviour. Members of the genus Tilapia are substrate spawners and nest guarders, whilst members of the genus Sarotherodon are generally paternal or biparental mouth-brooders. Oreochromis species are arena spawners and maternal mouth-brooders. Mouth-brooding is generally recognized as having evolved from the more primitive substrate-spawning behaviour.
Of the 90 or so known species of tilapia, there are around 30 belonging to the genus Tilapia, 18 are paternal/biparental, and 42 belong to the genus Oreochromis.
The natural distribution of tilapias reflects both historico-geographical factors which lead first to isolation and then speciation, and the environmental requirements/preferences of different species.
The genus Tilapia is widely distributed throughout western Africa with the exception of two species, T. zillii and T. rendalli, which have extended their ranges into central and eastern parts of the continent. By contrast, members of the genus Sarotherodon tend to occur in small, specialized, locally abundant populations with limited geographic distribution, and are common in the Rift lakes, rare in West Africa and absent from the Zaire basin. Oreochromis species are widely distributed throughout Central and East Africa and several of the lakes in the Levant.
The natural distribution of the tilapias has radically changed over the past fifty years by introductions which have been widely documented (Figure 10.1). Species have been stocked into lakes and reservoirs in order to try and increase fishery yields or control weeds and for aquaculture purposes into ricefields, fishponds and cage systems in lakes. Accidental introductions have also occurred, with the result that the tilapias are now widespread thoughout the tropics and subtropics. They are even used in several heated-water systems in temperate countries.
1 Based on manuscripts prepared by Drs K.J. Rana and M.C.M. Beveridge, Institute of Aquaculture, University of Stirling, Scotland
Fig. 10.1 Introductions of tilapias (six principal species) outside Africa (modified from Philippart and Ruwet, 1982)
General biology of tilapias
The tilapia body shape is typical of the Cichlidae: very deep and laterally compressed. They have large, usually cycloid scales and a double lateral line system, one of which runs from the posterior of the operculum to a point level with the posterior end of the dorsal fin, whilst a second lateral line begins immediately below this point and continues to the tail. The anterior half of the characteristically long dorsal fin is spiny whilst the posterior half has soft, branched rays. At the base of the posterior section is a black patch, the “tilapia spot”, which is particularly apparent in juveniles and is more pronounced in some species than in others.
The tilapias are all principally herbivorous and this is reflected in anatomical adaptations. There are both bicuspid and tricuspid jaw teeth and small, sharp, pharyngeal teeth which vary in size and coarseness depending upon fish size and dietary preferences, and which are used for shredding coarser material and partially disrupting plant cell walls. The gill rakers are surprisingly small and stubby and not particularly numerous (a standard count of only 8–12 in some species) belying the ability of many species to filter-feed on small algae and other planktonic material. The intestine is long and coiled (up to 14 times the body length) in order to help in the absorption of nutrients from a largely plant-based diet.
Tilapias occur in a wide range of aquatic habitats, and occupy a variety of niches, including pools, lagoons, river margins and floodplains, reservoirs and lakes. In lakes and reservoirs, different species often show preferences for particular habitat types, some preferring open waters whilst others are morè closely associated with inshore areas.
As stated above, tilapias are generally herbivorous (Table 10.1). Some species, particularly those belonging to the genus Tilapia (e.g., T. rendalli, T. zillii), are macrophyte feeders whilst others, such as S. galilaeus, browse on benthic algae. Most species exhibit some plasticity in their dietary preferences, O. niloticus, for example, feeding on either epiphytic or planktonic algae depending upon prevailing conditions.
Tilapias show an unusual degree of inter- and intra-specific variation in growth rates and reproductive strategies. Within a water body various species will grow at different rates, the faster-growing species tending to attain a larger maximum body size. However, the same species will grow at different rates in different water bodies, size at maturation and maximum body size tending to be smaller in estuarine and small water bodies. Maturation size can also respond to changes in environmental conditions. Many fish species exhibit similar trends; however, under adverse conditions the decline in size at maturation is also accompanied by a decrease in age, which is unusual in fishes, and this has a number of serious implications for aquaculture which will be discussed below.
Although tilapias have the capacity to breed throughout the year in tropical environments, they often do not, and instead there can be marked differences in the seasonality of spawning. In rivers, tilapias generally spawn during the period of most stable environmental conditions (i.e., the dry season). In the major lakes, a large proportion of the fish may be in spawning condition any time of the year, whilst in small or shallow lakes distinct breeding seasons are often observed. The frequency of spawning also seems to be highly variable.
DIETS REPORTED FOR TILAPIAS IN NATURAL HABITATS
(based on several sources)
|O. aureus||Adults feed mainly on phytoplankton and zooplankton|
|O mossambicus||Adults omnivorous, but feed mainly on plankton, vegetation and benthic algae. Juveniles feed entirely on zooplankton|
|O. niloticus||Adults omnivorous, but feed predominantly on phytoplankton and can utilize blue-green algae. Juveniles consume wider range of food items|
|T. rendalli||Adults fed principally on macrophytes and attached periphyton. Juveniles feed on benthic vertebrates|
|T. zillii||Adults feed principally on macrophytes and benthic invertebrates|
Tilapias are becoming increasingly important components of tropical inland fisheries production as species continue to spread, either by deliberate introduction or by accident. It is estimated that in southeast Asia alone, the area devoted to reservoirs will have increased 20-fold between the mid-1970s and the year 2000, and it is likely that many of these dams will support tilapia populations. It has been suggested that tilapia culture began as long ago as 2500 BC, although the current industry has its origins in Kenya in 1924 with the experimental culture of O. spilurus niger. Other African countries slowly began to farm tilapias in the 1940s, although the period of growth was during the 1950s and 1960s.
O. mossambicus was being widely cultured in Indonesia by the mid-1940s and the species was subsequently introduced into Malaysia and hence spread throughout southeast Asia. During this same period, tilapia farming trials were under-taken in southern USA and Central and South America. Perhaps the greatest successes, however, were in Israel, where, since the late 1940s, a concerted programme of research and development in conjunction with commercial interests has been largely responsible for the state-of-the-art in tilapia polyculture methods.
The reasons for the great interest in tilapia farming were that they were tough, easy to breed, and could be successfully grown on poor quality feedstuffs, such as agricultural by-products. However, in many parts of the world, disillusion set in after only a few years and production, particularly in Africa, declined. One reason for this was that early maturation and, consequently, overpopulation and stunting were common among pond-reared populations. It has since been learned that these problems could, in part, have been reduced through a more careful choice of species. For example, O. niloticus is much more suitable for culture than most strains of O. mossambicus, the species favoured thirty years ago. During the 1980s the culture of tilapias has enjoyed a renaissance, largely as a result of the application of research results to the design and management of farming systems, and today tilapias are more widely cultured than ever. Much tilapia culture is to provide food and the species most widely cultured are O. niloticus, O. aureus, and, still, O. mossambicus. Most workers would now concur with the view that O. niloticus and O. aureus are in many cases the best species for aquaculture purposes, although T. zillii and T. rendalli have some potential for polyculture in conjunction with plankton feeders. The red or golden strains of tilapias which are usually a cross between O. mossambicus and one or two other species such as O. hornorum, O. aureus or O. niloticus are also increasing in popularity as many people dislike the dark coloration and black peritoneum common to most tilapias. Moreover, a number of other species may be more tolerant of low temperatures or high salinities than O. niloticus or O. aureus. There are also often locally available species or strains such as O. andersonii in Zambia, O. mossambicus in South Africa, or O. spilurus in Kenya, which are worthy of consideration.
Tilapias are also widely cultured in order to establish or replenish fisheries. In Sri Lanka, for example, the stocking of perennial and seasonal tanks or man-made reservoirs since the mid-1950s has had a tremendous effect on the fisheries. The macrophyte feeding T. rendalli and T. zillii have also been stocked in irrigation channels and dams in many parts of the world in attempts to control weed growth. Fish (T. rendalli, T. zillii, O. aureus) stocked at a rate of 500–3 000/ha are reported to be effective in controlling the growth of submerged weeds, although they are much less successful in dealing with the tougher floating and emergent vegetation.
Tilapia species which are able to feed on floating-leaved plants have some-times been stocked in ponds and other static water bodies, including rice fields in order to help control Anopheles mosquitoes. The plants act as a refuge for the mosquito larvae which, when exposed, may be eaten by the tilapias or by the mosquitofish Gambusia which may also be stocked.
Tilapias have also been cultured as baitfish, as forage species, for sport, and the smaller, more attractive species, such as S. melanotheron, have been cultured for the aquarium trade. Pilot projects have also been carried out in Thailand and South Africa to evaluate their possible role in treating sewage waste.
In summary, the tilapias are best described as thermophilic. Although they can tolerate temperatures of 6°–10°C for short periods, mortalities due to cold can occur below 12°C. Growth and reproduction are increasingly impaired at temperatures below 20°C, whilst feeding ceases below 11°–15°C. For most species, growth occurs between 20°C and 35°C, whilst above 37°C, increasing mortalities are likely to occur.
1 Includes contributions by Drs M.J. Phillips and L.G. Ross, Institute of Aquaculture, Stirling, Scotland, UK
There are some differences between species (Fig. 10.2). T. zillii and O. aureus are probably the most cold-tolerant of the commercially important species, whilst O. mossambicus and O. niloticus are the most heat-tolerant. Hybrids possess temperature-tolerant characteristics intermediate to those of the pure stocks. But a number of factors such as size, thermal history and salinity, can influence the above generalizations. Fry and fingerlings prefer warmer temperatures than adults, for example, whilst there is some evidence that tilapias are better able to withstand cooler temperatures in slightly brackish-water than in freshwater.
Since the aim of aquaculture is to achieve maximum production at minimum cost, it is of prime importance to know at which temperatures maximum growth occurs. Although information on the tilapias lags behind that on other cultured species such as salmonids, nevertheless studies to date show that for most of the commercially-important species maximum growth occurs between 28° and 30°C.
Fig. 10.2 Temperature tolerances of tilapias normal range of temperature variation in natural habitats; ---- extreme temperatures tolerated in some natural habitats and in ponds; .... physiological limits of tolerance as found in laboratory experiments
Above 32°C, growth rate and food intake decrease exponentially with increasing temperature. As a rule, tilapias do not grow well below 16°C, and under conditions several degrees above lethal will often develop fungal and bacterial infections resulting in high mortalities. In sub-tropical situations, where temperatures may fall below 16°C for part of the year, deeper ponds or covered ponds are recommended. For reasons of economics it often makes most sense to overwinter broodstock or small fingerlings, or both, although it is the practice in Israel to overwinter intermediate size fingerlings for grow-out the following spring. Handling of tilapias below 15°C is not recommended due to reported high mortalities.
In intensively fertilized ponds where high levels of primary production can occur, it is not unusual for dissolved oxygen concentrations (DO) to fall to near zero at dawn. Oxygen fluctuations can also be observed in intensively-managed raceway or tank systems as a result of high BODs, caused by accumulations of uneaten food and faeces. Filamentous algae growing on the sides of these systems can also influence DO levels. Periods of prolonged anoxia follow the collapse of an algal bloom. In practice, tilapias are highly tolerant of low DO concentrations and survival for short periods of time in less than 0.5 mg/l have been widely reported. But it has been shown that growth of tilapia is adversely affected if DO saturation falls below 25% for prolonged periods. Exposure to DO levels of less than 20% saturation for 2 or 3 days can lead to mortalities. Tilapias are thus well adapted to survive and grow in most aquaculture situations although they will not, of course, survive complete anoxia. During low oxygen conditions, fish should not be fed and handling should also be avoided as both cause dramatic increases in the respiration rate.
Salinity (Table 10.2)
As a group, tilapias are widely reported to be tolerant of a wide range of salinities, a possible reflection of their supposed marine origins. The culture of tilapias in brackishwater is of considerable interest in many areas of the world where supplies of freshwater are limited. Moreover, there may be advantages in marine/brackishwater culture in terms of a reduction in energy costs of ionic and osmotic homeostasis, thus enhancing growth, and in reducing or inhibiting reproduction.
Many of the commercially important species are highly tolerant of saline conditions, some even being able to grow and breed in full-strength seawater. On the basis of experimental and field observations, a “league table” can be tentatively drawn up, with T. zillii, O. aureus, and O. spilurus as the most euryhaline species, followed by O. mossambicus, then O. niloticus and T. rendalli. This list is by no means exhaustive, and a number of other saline-tolerant species such as S. melanotheron and O. hornorum have also been considered with a view to culture. Hybrids seem to possess characteristics intermediate to those of the pure species.
Salinity tolerance is greatly dependent upon size, age, temperature and acclimation. Small fish are less tolerant of high salinities than large fish whilst extremes of temperature have been shown to adversely affect salinity tolerance. However, the most important factor is acclimation. Even O. niloticus, a species intolerant of marked changes in salinity, can survive, grow and reproduce in full strength seawater - albeit to a limited degree - provided that it is properly acclimated. A standard method is to hold fish in 50% seawater for four days, then 75% seawater for a further four days, before transferring to full strength seawater. However, care in handling, good water quality and the nutritional status of the fish are of paramount importance in ensuring the successful transfer of tilapias from freshwater to saltwater.
The differences between species in salinity tolerances reflect differences in osmoregulatory abilities. For many species, growth appears to be unaffected or even enhanced in salinities up to a few parts per thousand. Optimum growth should occur when energy expenditure on osmoregulation is minimized, i.e., when the body fluids and external medium reach the same osmotic pressure.
SALINITY TOLERANCES OF TILAPIAS
(from several sources)
|Species||Natural Distribution||Performance in saline conditions|
|O. niloticus||Does not occur widely in salt water conditions||Can survive direct transfer to 20‰ and by acclimation, to full strength conditions. Growth unimpaired by salinity. Spawns in full strength seawater and egg and fry production possibly enhanced at salinities of 5–15‰.|
|O. mossambicus||Occurs in brackish and hypersaline conditions||Can survive direct transfer to 20‰, and hypersaline conditions if acclimated. Growth unimpaired and possibly enhanced at salinities of around 10‰. Some evidence that it can reproduce in full strength and hypersaline conditions.|
|O. aureus||Can survive direct transfer to 18–20‰, and full strength conditions if acclimated. Growth unimpaired in full strength seawater. Reproduces in salinities up to 19.5‰.|
|O. spilurus||Occurs widely in brackish waters||Growth not impaired in full strength seawater. Can be spawned in full strength seawater, although fecundity is markedly reduced.|
|T. rendalli||Occurs widely in brackish waters||Can survive direct transfer to 50‰ seawater.|
|T. zillii||Occurs widely in brackish and even hypersaline waters||Can survive direct transfer to 75‰ seawater and can be acclimated to hypersaline conditions. Grows well in full strength seawater, although conflicting evidence regarding breeding.|
Since several species such as O. mossambicus and T. zillii, can reproduce prolifically in full strength seawater, overcrowding and stunting in brackish-water stocked with these species will be a problem unless ponds are carefully managed. In selecting a species for culture in brackish or seawater, it may be better to choose a comparatively fast-growing and marketable species, rather than a species which, although more tolerant of saltwater conditions, is slower growing and perhaps less marketable. O. aureus has been recommended as a possible choice for culture in brackishwater since it is fast-growing and clearly does not reproduce above salinities of 19.5 parts per thousand. An alternative choice for brackish conditions might be O. niloticus or one of the red tilapia hybrid strains. O. spilurus is probably the best choice for culture in full strength seawater.
In captive tilapia stocks, sexual differentiation of the primary germ cells into immature male or female gonads occurs very early. In species such as T. zillii and O. niloticus this may occur between 15 and 30 days after hatching at 30°C. Further sexual development and maturity of gonads will be determined by environmental factors such as nutrition, temperature, light and salinity.
Age and size at sexual maturity
The age and size at which tilapias mature and breed varies with environmental conditions. Generally natural or wild populations in large water bodies (e.g., lakes) breed at an age of between 2 and 3 years. Captive hatchery stock in ponds and similar shallow water bodies mature and are capable of breeding at an age of only 2–3 months. Under hatchery conditions sexually mature fish breed at 5–6 months.
Morphological and coloration changes during breeding
A knowledge of the external characteristics during breeding is an important additional tool for sexing and selection of sexually active, healthy broodstock.
Juvenile tilapias are generally silvery in colour. During sexual maturation and breeding their body undergoes both morphological as well as coloration changes. The extent of these changes depends on the sexes and species.
Both male and female of T. zillii and T. rendalli undergo coloration changes of similar intensity. In the genus Sarotherodon, for example S. galilaeus where both parents orally rear eggs, the differences in colour and their intensity is minimal whereas in Oreochromis, for example O. niloticus, O. aureus and O. mossambicus, there is a marked sexual difference between males and females. The breeding coloration of some commercially important species is given in Table 10.3.
A notable feature of the mouth-brooders such as O. placidus, O. mossambicus and O. hornorum is the enlargement of the upper and lower jaws of males which are used in jostling to assert male dominance.
During breeding, the genital papillae of both sexes become enlarged (Fig. 10.3). The papilla of Tilapia spp. is mottled grey and conical, the males generally having a longer and more pointed papilla. In Sarotherodon spp. the papilla is simple and conical. In both these groups the sexes are not easily distinguishable.
Fig. 10.3 External structure of the genital papillae in tilapias. A) Tilapia spp., B) Sarotherodon, C) Oreochromis spp. (non-tasseled species)
BREEDING COLORATION OF HATCHERY REARED TILAPIA MALES
|Lips normal. Head pale pink. Throat white||Pale pink. Deeper coloured belly||Vertical fins - blue black edges.|
Caudal fins - vertical band markings
|Lips normal. Lower lip white. Head blue grey with white throat||Flank blue-grey. White belly||Upper margin of dorsal fin - orange caudal fin unpigmented. Pelvic and anal fin - pale yellow/white|
|Lips enlarged. Head black. Lower cheek and throat white||Black||All fins black with orange-red free edge|
|Lips enlarged. Head black||Black||All fins black|
|Lips enlarged. Head pale pink. Throat pale yellow||Flank - grey, scales with yellow edges. Belly pale yellow||Vertical fins blue-black edges. Caudal fin checkered|
|Lips normal. Head blue grey||Flank and belly - blue-black. Genital papillae modified into tassels||All fins blue-black. Dorsal fin - irridescent blue|
|Lips normal. Head golden yellow, irridescent scales on top of head. Throat white||Flank and belly grey-gold with occasional black markings||Pelvic and anal fins black|
|Lips normal. Head grey-black. Lower cheek and throat red-orange||Flank grey-black. Belly red-orange. Genital papillae elongated and pointed||All fins grey-black|
The differences between sexes in Oreochromis species are more obvious. The papilla may be simple, conical and whitish but in some species, e.g., O. macrochir, it is modified into a tassel-like structure to attract the females.
The genital papilla is situated posterior to the anus. The male has only one common opening to the exterior, the urogenital pore, into which enter the sperm duct and urethra. The papillae of all female tilapias have two apertures: an anterior opening to expel ovulated eggs and a separate posterior urinary pore.
Spawning behaviour and parental care
In natural waters spawning normally occurs in shallow waters at depths of between 0.3 and 3 m. Under captive spawning conditions such as ponds, tanks, net enclosures, etc., successful spawning can be achieved in water depths of 30– 60 cm.
The male of all Tilapia species establishes a territory which is vigorously defended. The size of the territory in natural waters is variable and may depend on factors such as social dominance, size of fish and availability of suitable substrate.
Within this secured territory a nest, which serves as a stage for spawning activity, is constructed. Tilapia nest in firm substrates such as plant material or rocks.
Spawning patterns differ between the tilapias. The females of T. zillii and T. rendalli assist in nest construction and a pair-bond which may last several spawnings develops. The eggs, which are sticky, are laid onto a firm pre-cleaned substrate in rows and fertilized by the male. The single-layered mat of eggs is guarded, cleaned, aerated and protected by both parents for 2–3 weeks.
Among Sarotherodon species such as S. galilaeus a similar pair-bond is formed and both sexes construct the nest. Batches of eggs are shed into the saucer-shaped nest and fertilized by the male. When all the eggs are shed either the male (e.g., S. melanotheron) or both parents (e.g., S. galilaeus) pick up the eggs into their buccal cavity for rearing.
Among Oreochromis only the males construct the nests in a common spawning ground which is visited by ripe females. During spawning only a temporary pair-bond lasting for the duration of spawning is formed. Eggs are shed in batches into the nest and each batch is fertilized by the male and immediately picked up by the female into her buccal cavity. After all the eggs are shed and collected, the female retreats to a nursery area to rear her clutch. The rearing period varies between females and between spawnings of the same female and may last between 10 and 21 days. During this period the clutch is reared through a range of developmental stages until they are free-swimming. There then follows a transitionary phase during which fry are released for increasing lengths of time until they are independent.
Major environmental factors affecting spawning:
Water temperature is one of the predominant environmental elements affecting the seasonality and frequency of breeding. For breeding activity to commence water temperatures need to be at least 20 °C, the ideal temperature for maximal spawning being 28°–30°C. Thus in coastal equatorial and tropical regions spawning may occur throughout the year. At higher altitudes and latitudes, however, the breeding season may be reduced to a few months with peak spawning occurring for only 2–3 months.
Tilapias are capable of breeding successfully in brackish and seawater, though the seed yields are low. Generally, at salinities of greater than 30 ppt, spawning is inhibited and the inter-spawning interval lengthened. It is thought that the poor spawning at high salinities is associated with osmotic stress and dehydration of eggs.
For successful spawning a 12 hour day:12 hour night photoregime is best for gonadal development and spawning. During a daylight hour period of less than 6–8 hours gonadal growth may cease.
Choice of species
Even though there are many potential tilapia species, only about ten are cultured to any extent (Table 10.4). The selection of species may, however, be influenced by various local and specific considerations.
SPECIES OF TILAPIA USED FOR AQUACULTURE
(most widely cultured species)
The growth vigour of tilapia species is an important consideration in species selection. In this respect O. niloticus is widely used and its hybrid with O. aureus is also favoured because of its hardiness and high growth rate.
The choice of species may be dictated by local environmental factors. To overcome low winter temperatures or wide diurnal temperature ranges, cold-tolerant species such as O. aureus or notably the hybrid O. niloticus (F) × O. aureus (M) may be chosen. These fingerlings can tolerate winter temperatures as low as 8°– 9°C.
In some areas only brackish or seawater may be available for fish culture. Under these conditions high salinity tolerance would be an advantage. In ponds salinities may rise to as high as 60 ppt as a result of evaporation. Although most tilapia species can tolerate salinities up to 25–29 ppt, their growth and reproduction may be inhibited. Recent studies in Kuwait have shown that O. spilurus grows best at salinities of 37–39 ppt.
In some localities there may be constraints on energy inputs such as artificial diets and inorganic fertilizers. Under these conditions macrophyte and algal feeders such as T. zillii and T. rendalli may be chosen. If seeds are being produced for weed control, T. zillii would be ideal.
To improve the management of ponds, etc., some farmers may want to use the technique of hybridization to produce “all male” seed. For successful hybridization the parents used must be genetically pure. Crosses such as O. niloticus (F) × O. aureus (M) or O. niloticus (F) × O. hornorum (M) are used.
The market acceptability of tilapias may also need to be considered. In some countries and rural areas small fish are acceptable to the consumer and therefore species such as O. mossambicus or O. spilurus which grow well up to about 50 g may be suitable. In some areas dark-skinned fish may fetch lower prices than lighter coloured species. To obtain a higher price for his/her harvest an operator may then choose to use “red” tilapias.
The nutritional status of broodfish is one of the most important factors affecting fry production.
The nutrition of fry destined to be used as broodstock is as important as its later nutrition during breeding. In immature fish a high-protein well-balanced diet is essential during the laying down of germinal tissue which contains the future egg stock.
Similarly, egg development during breeding requires high quality and quantities of proteins and lipids. If food is restricted during this period the proportion of eggs finally maturing and ovulating will be reduced, resulting in lowered fry production. Further, in mouth-brooding species females deprive themselves of food during oral rearing and only feed for the short periods between the end of oral rearing and the next spawning cycle. Therefore, during the periods of feeding, abundant food of high quality needs to be provided to replenish lost energy and supply nutrients for the next spawning.
The quality and quantity of broodstock diet will vary with the type of seed production system. To reduce costs and provide a well-balanced diet, ponds may be fertilized to increase natural food production. Broodfish diet may then be supplemented with diets containing 20–30% protein.
If clear water systems are used a complete well-balanced diet must be provided. Whilst the precise dietary requirements of broodfish are not known, good fry yields are obtained with diets containing 35–45% protein. The quality of the protein is also important. Fish meal is the best source of high quality protein but is expensive and often not available.
Fecundity of broodfish
A major problem facing tilapia broodstock managers is the relatively low fecundity of tilapias compared to other cultured species.
For example, whilst 100 000 carp eggs may easily be obtained from a single carp, more than 100 spawning tilapia females will be required to produce the same number of eggs. Therefore, for the mass propagation of tilapia, a large number of brooders is needed.
Stocking of broodfish
To optimize fry yields the correct balance between stocking density, sex ratio and broodstock size must be achieved. The interaction of these factors becomes critical in confined breeding facilities such as small ponds (less than 0.1 ha), hapas, tanks, etc.
Under sub-optimal conditions, crowding will result in:
an increase in male aggression, and males will find it increasingly difficult to maintain nest sites;
a decrease in the duration of courtship and an increase in disturbance during spawning resulting in partial spawning and poor fertility rates of egg clutches;
a reduction in the proportion of females actively breeding;
an increased rate of cannibalism of fry by non-spawning females and males.
As shown in Figure 10.4 at any given sex ratio the effects of crowding are minimal at low densities (7/m2) but production is limited by the availability of females for spawning. As the number of females is increased (10–12/m2) an optimum balance is reached between stocking density and availability of females resulting in maximum seed production. At very high densities, however, crowding reduces seed yield dramatically. Similar effects are also obtained if a high sex ratio is used. For example at 6 females:1 male, a lower proportion of females may spawn and cannibalism and egg-robbing by non-spawners may increase.
As fish grow, the males require larger territories to establish their nests and aggression between males and females increases. Therefore for a given sex ratio the seed yields will also be affected by broodstock size and stocking density. For example, in one experiment broodfish of various sizes were stocked in 0.45 ha ponds at 3 females:1 male. When 60 g brooders were used at 2 640/ha fry yields were 8.2 fry/m2/month. If larger broodstock were used, say 170 g, then the same level of productivity was achieved only if the density was reduced from 2 640 to 880/ha. Therefore, as the spawning season progresses and brood-stock increase in size their density should be reduced to maintain high seed yields. This will also allow for a more favourable sharing of available food.
Fig. 10.4 Influence of broodstock stocking density on O. mossambicus fry production. Broodfish of 100–150 g stocked at various sex ratios (female:male) in 4.3 m2 tanks. Fry collected daily (data adapted from Uchida and King)
Breeding activity can be a good guide to determine optimum stocking density. Under ideal conditions a high proportion (40–60%) of females will spawn. On this basis, broodstock of about 150–200 g stocked at 800–900 individuals per hectare at a sex ratio of 3 females:1 male should yield good results.
Higher yields of 10 fry/m2/month may be obtained at similar densities (994 individuals/ha) by using larger broodfish (360 g), even though a smaller proportion may be actively spawning.
In larger ponds (0.5–1 ha), sex ratios of 3:1 were found to be high and better seed yields were obtained by lowering the sex ratio to 1:1.
The mouth-brooders belonging to the genus Oreochromis are the most widely cultured tilapia species (Table 10.4). At present their seed production relies on natural spawning in various systems. Different systems, however, manipulate the reproductive cycle in various ways through their management. In the simplest method, females are allowed to rear their clutch for the full duration of oral rearing. In some instances broodfish are robbed of eggs and/or sac-fry which are reared separately (Fig. 10.5).
The seed production systems most commonly used are described in the following sections. They are briefly compared in Table 10.5.
Seed production in earth ponds
Unmanaged traditional ponds. The traditional earth pond is probably still the most widely used method for seed production. These ponds are often claylined and can vary in size from 0.01 to 1 ha. Into these ponds are added a small quantity of tilapia which grow and reproduce naturally. The seed may be left with the parent fish until the entire pond is harvested either seasonally or annually. Broodfish are selected from the harvest and restocked into similar ponds for the next production cycle. Alternatively the seed may be harvested with throw nets or bag seine nets, graded and restocked in fattening or on-growing ponds.
The management inputs of such propagation systems are negligible. Often these types of ponds are not drainable or not drained and therefore fresh nutrients cannot enter the pond and harmful metabolites which may halt breeding cannot be flushed out. Energy inputs in the form of organic and inorganic fertilizers, vegetation and supplementary feeding are rarely added. Consequently, the quantity of seed produced may be very variable and rarely exceeds 1–2 fry/m2/month or 5 fry/female/month. Therefore such systems would require very extensive areas of ponds to produce large quantities of fry.
Fig. 10.5 Methods of manipulating the natural life cycle of mouth-brooding tilapias
A COMPARISON OF MOST COMMON SEED PRODUCTION
SYSTEMS FOR MOUTH-BROODERS
|System||Average Seed Production||Comments|
|- unmanaged||1–2||5||Extensive; minimum management, technical and energy inputs|
|30–400||Semi-intensive; moderate management inputs; energy input in form of organic and inorganic fertilizers and supplementary feeds; use of larger broodstock (1-1-5 kg) and frequent harvesting (20 days) result in higher seed yields|
|(in ponds, lakes, tanks)||150–800||300–400||Intensive; cheap and versatile; high management and feed inputs; labour intensive; nets prone to fouling|
|SEMI-AUTOMATED FRY COLLECTION TANKS|
|Rectangular and circular breeding arenas||100–200||100–150||Semi-intensive; expensive to construct; high management and technical inputs|
|- daily fry collection||400–1 300||200–300||Intensive; high feed,|
|- egg/fry collection||2 000–3 000||1 000–1 500||technical and management input; capital intensive|
Managed spawning ponds. In recent years great increases have been made in seed production through improved pond design and management. The spawning ponds tend to vary in size from 0.05 to 1 ha with each seed producer having his own combination of management practices for broodstock, pond preparation and fertilization, feeding and fry collection frequency and methods.
Pond drainage was the initial improvement leading to an immediate improvement in fry production. Ponds were drained by monks or drainpipes at the lowest point of the pond which allowed most of the seed to be collected. In addition, drainage also helped to flush away harmful metabolites. Refilling with fresh water enriched the pond water with nutrients which helped to sustain subsequent breeding activity.
To sustain higher reproductive activity and therefore higher seed production the quality and quantity of broodfish diet had to be improved. This was achieved in two ways: pond fertilization by inorganic and/or organic fertilizers and supplementary feeding with leaves, rice bran, oil seed cake or commercial diets containing about 20% protein. The combination of methods depends very much on the availability of fertilizers and feeds and their cost.
In the Philippines, for example, small ponds of about 500 m2 are commonly used. Ponds are first prepared by adding 2 000 kg/ha of air-dried chicken manure and 100 kg/ha of inorganic fertilizers (N-P-K:16-20-0). This is followed by weekly applications of chicken manure and inorganic fertilizer at a rate equivalent to 3 000 kg/ha/month and 100 kg/ha/month, respectively. Ponds are stocked with O. niloticus broodfish of 200 g average size at rates between 150 and 400 kg/ha. No supplementary feeding is provided. Fry are harvested with a 6 mm bag seine net first after 60 days of stocking and thereafter at 30-day intervals. Even with the best management this system yielded only 6–10 fry/m2/month or 35–100 fry/female/month. Similarly, in Thailand trials in 200 m2 ponds stocked with 200 g O. niloticus yielded 8 fry/m2/month or 16–17 fry/female/month.
In Côte d'Ivoire, larger females (700 g) are stocked at 0.5/m2 in 0.06 ha ponds at 4–5 females:1 male. The fish are intensively fed for one month after stocking on a 30% protein diet. The broodstock are then removed with a cast net and the fry left to grow-on. Such a propagation system yielded 4 fry/m2/month or 10 fry/female/month.
In addition to pond fertilization and supplementary feeding, seed production efficiency has been improved by spawning greater numbers of larger broodfish in larger ponds, increasing fry harvesting frequency and using differing methods of fry collection.
Harvesting with seine nets often leads to disturbance of spawning, and loss of the vulnerable eggs and hatchlings in pond mud and nets. Some hatchery operators therefore prefer to allow the brooders to naturally release the fry which are then collected on a daily basis with hand nets from along the edges of the ponds.
Ponds of special design. In Brazil, to improve harvesting efficiency and minimize handling of seed, spawning-nursery ponds of special design are being used to produce tilapia fry. The ponds consist of an upper spawning area (400 m2) which drains by gravity into a lower nursery area (200 m2) via a series of graded screens. To harvest fry the boards controlling outflow in the spawning area are removed. Broodstock are held back by a screen in the spawning area and fingerlings and fry enter the nursery. A set of secondary graders holds back fingerlings whilst allowing fry to enter the lower nursery area. The spawning pond can then be prepared for the next spawning cycle.
These ponds are more expensive to construct and require the use of concrete. Managing these ponds will however be easier. Fewer people are required to harvest the fry and survival of fry should be improved.
In Israel, ponds ranging from 0.1 to 1 ha are stocked with about 400 g brooders (O. niloticus (F) and O. aureus (M) at a sex ratio of 1:1. Fry are harvested about once a month. The water level is reduced and the pond is first seine-netted with a 6 cm mesh net to remove broodstock which are then held in temporary housing or simply held under the water inflow. The pond is then seinenetted with a 1 cm mesh net to harvest the fry. The system yields about 15 fry/m2/month or 160 fry/female/month.
By increasing the broodstock size to 1–1.5 kg and harvesting every 17–19 days, fry production has been increased to 45 fry/m2/month or 380 fry/female/ month. To achieve this level of seed production the following pond management procedure was used.
The ponds of about 1 ha were first constructed such that the bottom sloped gently to the monk overflow. An area of about 500 m2 around the monk was covered with a 2.5 cm mesh net. Ponds were filled and stocked with 1–1.5 kg broodfish which were fed on commercial pellets containing 20% protein. Every 17–19 days the ponds were drained. The net was raised to concentrate the broodstock. Females were inspected individually and the eggs and fry collected for artificial incubation or restocking. The broodfish were transferred to temporary housing and the remainder of fry in the ponds collected with a fine mosquito seine net. The ponds were then completely drained and sprayed with rotenone to kill any unwanted fish. Ponds were filled within 24 hours and brooders restocked the next day and the process repeated.
Seed production in net enclosures
In some farming situations tilapia seed production in ponds may not be possible for various reasons. Often land for pond development may not be available due to competition from other farming sectors (cotton, coffee, etc.) or lack of water.
Under these situations net enclosures suspended in nutritionally rich shallow water areas of lakes and lagoons have proved an effective alternative method of seed production. These net enclosures, traditionally known as hapas, are also described as cages and pens. Net enclosures which were originally used for many years in Indian carp propagation are now widely used for tilapia in many countries, notably in the Philippines, Taiwan and Thailand.
Basic design and operation of net enclosures. Hapas are essentially net enclosures made from fine meshed (mosquito netting) nylon or cotton net to prevent escape of very small fry. Collecting is easier and predators are kept out. Hapas can be made into any manageable size, ranging from about 1 to 40 m2 with a depth of 1–2 m.
The nets can be hung from either a floating framework made of bamboo, wood, steel or drums, or a fixed pole frame anchored into the bottom of the water body. Usually a half to three-quarters of the net is submerged in the water.
Advantages and disadvantages. Net enclosures are proving very popular. The main advantages are their simplicity. They are cheap and easy to construct. The design can be easily modified to use locally available materials without loss of fry production efficiency. The sizes of cages can be easily adapted to suit available water bodies. Design modification to hapas to improve fry collection and husbandry can be easily and cheaply carried out. Since hapas are small they can be easily managed by one or a few people. These attractions have also encouraged land-based fry producers to use net enclosures in ponds and tanks.
Because broodfish in these enclosures are usually held at higher densities than in ponds, the hapas are often suspended in shallow lakes, lagoons or ponds rich in natural food. These water bodies often need to be enriched with organic and inorganic fertilizers to improve and sustain the natural food production. However, supplementary feeding in the form of leaves, rice bran, chicken feed or commercial fish pellets may also be necessary.
Since large numbers of hapas may be required to produce large quantities of seed, good management is essential. Nets may need to be removed bi-monthly or monthly for cleaning. They may be sun-dried to remove fouling by debris and algae or scrubbed whilst in the water.
Types of net enclosures. The complexity of hapa design is variable. The simplest type of hapa consists of a fine nylon or cotton netting (mosquito netting) stretched across a four-poled bamboo frame (Fig. 10.6a). The bamboo poles or stakes are set firmly into the substrate to form the frame. The bag net is tied to the four poles and held open in shape with four anchors made of stones or water-filled containers. Generally these simple hapas are used in water depths of about 1 m with a half to three-quarters of the nets submerged under the water. Broodfish ranging in size from 50 to 150 g are introduced into the net enclosures at a sex ratio of 2–5 females:1 male and allowed to breed naturally. Swimming fry may be harvested daily with dip nets, or monthly by lifting the net and crowding the fish in a corner. The seed, which may consist of eggs, larvae and early fry can then be easily separated from the broodstock for collection.
Various improvements can be made on this simple design. In deeper water bodies a catwalk built around the hapas facilitates fry collection, and general husbandry and double net enclosures can be used to improve the efficiency of fry collection.
In the Philippines, such a double-netted hapa has been tested with O. niloticus (Fig. 10.6b). The inner net (10 × 2 × 1.5 m) made of 30 mm mesh was held within a fine meshed outer net (12 × 4 × 2.5 m). The hapas were held on bamboo pole frames driven into the mud bottom of the lake and the nets held 1–2 m above the lake bottom. Adjacent cages were served by common bamboo catwalks. Brooders averaging 50–100 g were introduced into the inner enclosure at a sex ratio of 3 females:1 male and fed on rice bran (12% crude protein) and on commercial pellets (20% crude protein). To escape predation by parents, the released fry swim through the large meshed inner net into the relative safety of the fine meshed outer net enclosure. They can be collected on a daily basis using a dip net. Also the inner net with the broodstock can be raised to facilitate fry collection.
Fig. 10.6 Basic types of net enclosures used for tilapia fry production: mosquito mesh used for outer nets (1.6 mm) and for inner nets (25–30 mm)
These double-walled enclosures are more expensive. Also since the broodstock cannot gain access to graze on the fine netting this outer netting is easily fouled with debris and algal growth. This reduces water flow through the cage, thus reducing oxygen levels and food availability within the hapa. Consequently breeding activity and subsequent fry production may decline.
To minimize this problem the design of net enclosures can be modified. The fine netted enclosure can be divided into a spawning and fry collection area by a vertical wall of 30 mm netting (Fig. 10.6c). By this method brooders have access to graze on part of the fine outer mesh, thereby improving the water flow, and food production within the net enclosure.
Seed production in net enclosures. Seed production in hapas is highly variable and, in addition to temperature and levels of supplementary feeding, depends highly on stocking density, sex ratio and management of fry collection.
In net enclosures seed production varies from 150 fry/m2/month or 50 fry/female/month to more than 880 fry/m2/month or 300–400 fry/female/month. In one study by harvesting fry every 14 days, seed production in 3.3 m2 hapas stocked with brooders at a sex ratio of 2 females: 1 male reached 2 000 fry/m2/ month or 600 fry/female/month.
Studies indicate that no further advantage in seed production is gained by stocking at a density greater than 4–5/m2 and a sex ratio greater than 3 females:1 male.
In Mexico similar productivity has been obtained by using submerged cages. These cages are covered on all sides except the floor with 30 mm rigid mesh. Finer mosquito netting is used for the floor. To the floor of the cage are fixed six buckets which serve as nesting sites or shelters for brooders. The cages can be made to any manageable size for handling, but usually 2–3 × 1.5 × 1 m cages are preferred. Usually up to six of these cages are either floated or held by horizontal poles in a large fine-meshed pen.
Each cage is stocked with 100–200 g brooders at a sex ratio of up to 3 females:1 male and fed on a commercial diet (20% protein). Every 2 or 4 weeks each cage within the pen is lifted and tilted to free the seed into the outer net and the brood cage is transferred to another pen. When all cages are empty the seed can be easily collected for on-growing. Alternatively, a pole can be introduced under the outer net to raise a section of the net to isolate a cage which can then be emptied and the seed collected.
Seed production in tanks
Manual seed production tanks. These systems consist of easily managed, small, rectangular or circular tanks. These systems are ideal in areas of water shortage or cooler climates where water may need to be heated and partially or completely recirculated.
Tanks may be constructed of various materials, e.g., plastic or rubber sheeting, wood, fibreglass, fibreglass-reinforced concrete, concrete, asbestos and steel. Circular tanks are more expensive and require greater skilled labour to construct than rectangular tanks. Plastic-lined wooden frames (marine-ply) and plastic swimming pools (2–6 × 0.5 m) make good, less expensive substitutes. Such tanks are fitted with simple inflows and outflows, using gravity-fed water.
In these systems high seed yields rely on frequent manual harvesting of seed from the spawning tanks by dip nets and inspection of females. If early fry only are to be harvested they need to be collected daily using a dip net. Alternatively the water level in tanks may be lowered every 10–14 days and all the seed, which may consist of eggs, hatchlings, early fry, collected. By virtue of this management, the overall efficiency of seed production will also depend on the need for artificial incubation systems to nurture the vulnerable eggs and hatchlings. In clear-water systems careful observation and monitoring of brood-fish can also improve seed yields. If broodfish are tagged, less productive females can be identified and removed. In addition to saving on food the overall seed yield can be improved. Seed yields in tank systems may be drastically reduced by poor management. In small confined areas fry cannot escape cannibalism from parents and larger fry. For example it has been shown at Auburn University that 7.3 m2 plastic ponds stocked with O. aureus brooders at 1.5/m2 yield 689 fry/m2/month or 560 fry/ female/month if harvested weekly. Decreasing harvesting frequency to 3 weeks reduces production by 23% and harvesting at 3 months reduces the yields by 97%.
In addition to well managed plastic pools, high seed yields may be obtained in rectangular tanks. In Côte d'Ivoire 3 × 18 m tanks (30–40 cm deep) stocked with either 200–400 g O. niloticus at 4–7/m2 or 150–200 g S. melanotheron at 14.5/m2 and fed a 25% protein diet yielded between 700 and 1 300 fry/m2/month (daily harvesting).
In Belgium, 4 m2 fibreglass tanks using recirculated heated water produce about 360 fry/m2/month (O. niloticus).
At Stirling University a combination of circular fibreglass tank recirculation systems and artificial incubation of eggs has increased seed yields further. Tagged O. niloticus broodfish (100–200 g) are stocked in 2 m diameter tanks at 2.5/m2 at a sex ratio of 3 females:1 male and fed at 1% body weight/day on a 40% protein diet. By removing eggs from the female after spawning for artificial incubation and collection of swim-up fry, seed yields of between 2 000 and 3 000 fry/m2/month or 1 000–1 500 fry/female/ month can be achieved. High productivity is maintained by rotating broodstock; spent females are removed to resting tanks and replaced with fattened females. Overall female productivity is improved because of the shortening of the inter-spawning interval due to egg-robbing.
Semi-automated seed production tanks. To improve seed production from mouthbrooders, some hatchery operators have designed special tanks to exploit the natural behaviour of broodfish and fry. Such systems are constructed on the assumption that newly released fry move to shallow, warmer areas. Therefore by placing one-way traps along the edges of tanks, fry can be automatically trapped and easily collected.
The basic design of such a “breeding arena” is shown in Figure 10.7. Its main features are:
a central breeding area (A) called the arena in which males are contained and set up their territories;
a separate brooding area (B) where females can retreat to escape male aggression and rear their clutch in seclusion from other females in the hides provided;
a shallow region (B') within the brooding area to encourage the female to release her fry and help concentrate the fry;
a drainable channel (C) to collect the fry, and a harvesting trap (D).
Fig. 10.7 Plan of circular concrete breeding arena (Kenya design) used for the production of fry of mouth-brooding tilapias. (I) General layout, (II) Cross section, (III) One-way fry traps between brooding ring (B) and fry collecting channel (C). Arrows denote movement of fry. A, spawning arena; B, brooding ring; C, fry collecting channel; D, fry harvesting bay; E, vertical grill to retain males in spawning arena; F, on-growing fry raceway (Balarin and Haller, 1982)
By virtue of its design, this system can only be used for maternal mouth-brooders. Larger males (400–500 g) and smaller females (250 g) are introduced into the arena (A). The larger males cannot pass through the vertical bars of the grills set in the arena walls and so cannot leave the central arena. Here the males jostle amongst themselves and set up territories. It is not necessary to provide any substrate for successful spawning.
The smaller females move freely through the grills to visit the nesting sites, select males, court and spawn their eggs. After spawning the females generally leave the arena via the grills into the safety of the middle brooding ring (B). Here they rear their clutches undisturbed in the “hides” provided.
The outer wall of the brooding ring rises gently to form a shallow lip (B') which is covered with about 1 cm of water. This shallow area allows for the natural behaviour of a female brooder to release her clutch in shallow warmer waters. The released fry hopefully swim with the water flow to the lip and into the outermost fry collecting channel (C). To retain the fry in the collecting channel the lip of the brooding ring has “V” shaped traps. The fry channel drains into a collecting bay (D) from which the fry are harvested or diverted to a series of adjacent raceways (F) for further rearing.
In Kenya, such arenas stocked with O. aureus and O. niloticus brooders at 1.7–2.0/m2 and a sex ratio of 6 females:1 male produced about 180–200 fry/m2/ month or 120 fry/female/month. In Zambia, a similar arena stocked with O. macrochir at 1.8/m2 and 7 females:1 male produced 135 fry/m2/month or 90 fry/ female/month.