(Peters)
1. Distinctive characteristics of early developmental stages
(a) Eggs and larvae/alevins (not from the mangroves)
(Details of spawning behaviour are provided in salient ecological information of this section).
The eggs are immediately taken by the maternal mouth brooder after fertilization for protective incubation in the mouth. The eggs neither float nor adhere to any object – a characteristic feature of bearers as opposed to guarders. The fertilized eggs are elliptical in shape with a maximum diameter (length-wise) of 2.5 mm. The yolk is yellow in colour. The eggs hatch in the mouth within 60 hours at a temperature of 28°C. The young fish measure about 4 mm in length immediately after hatching. The yolk-bearing stage lasts for 3-5 days. The smallest fish with a completely absorbed yolk measures around 8 mm in length. The alevins (post larvae) stay for another 2 to 3 days in the buccal cavity of the mother then emerge.
(b) Early juveniles and juveniles
Oreochromis mossambicus 1
Early juveniles possess a black ovate blotch at the base of the first few dorsal rays which disappears as the specimens reach the juvenile and adult stages. Faded vertical bands (around 6 in number) are present on the body. Sometimes dark pigmentation is present at the nape and caudal peduncle.
2. Salient biological characteristics (after Popper and Lichatowich, 1975; Dussart, 1963; Noakes and Balon, 1982; Caulton and Hill, 1975; Jubb and Jubb, 1967; Philippart and Ruwet, 1982; Pullin and Lowe-McConnell, 1982; Miranova, 1969; Chen, 1976)
This species grows to a maximum size of 390 mm in length. The females and males have distinct colouring and are easily differentiated in adults (males with a dark body colour and reddish fins). Also, females possess 3 orifices in the genitalia while the males have only two. Moreover, the males grow bigger and faster than females.
Their size at first maturity is as small as 45 mm in length and 6 grams in weight for dwarfed stock in the aquaria. As for the natural population, the size at first maturity varies widely from 90-165 mm in length in different geographic locations. The age at first maturity is 2-3 months. Thereafter, this species has the potential to spawn once every 6 days under favourable conditions, but normally spawns from between 22 and 40 days. However, the spawning frequency is greatly influenced by climatic and other biotic and abiotic factors in the system. On average, a female fish spawns 6-11 times per year. During each spawning, the females lay between 75 and 1000 eggs. It is a maternal mouth brooder (the females take care of their eggs/young ones through buccal incubation).
The fry of less than 10 mm were reported to be a zooplankton feeder. Young fish between 10 and 60 mm feed on phytoplankton, epiphytic diatoms, filamentous algae and zooplankton. Adults feed on phytoplankton, periphyton filamentous algae (both planktonic and benthic) and at times on fresh and decayed aquatic angiosperms and gymnosperms in natural waters. In cases of scarcity in vegetable food, animal matters (including worms, insects, crustaceans and fish larvae) and detritus/bacteria are taken by this species as food. In general, various food items were reported from different geographic locations and in different stages of the life cycle. This species is bold enough to grab the dirt from the feet of people bathing from rural temple tanks. The feeding intensity of this species is reported to be at its maximum between 12 noon and 4 pm (Man and Hodgkiss, 1977; Munro, 1967; Naik, 1973; Weatherley and Cogger, 1977; Bowen, 1982).
3. Salient ecological information (after Popper and Lichatowich, 1975; Dussart, 1963; Noakes and Balon, 1982; Caulton and Hill, 1975; Jubb and Jubb, 1967; Philippart and Ruwet, 1982; Pullin and Lowe-McConnell, 1982; Miranova, 1969; Chen, 1976)
It is a native species from the fresh waters of the East coast of Africa between South Africa and Somalia. It was introduced outside Africa for the first time during 1929, throughout the tropics and sub-tropics and even in some temperate regions. It survived and established well in many parts of the world, especially in Southeast Asian countries. At present, the southern most limit of the occurrence of the introduced population is 33°42’S. In the northern hemisphere, it is being maintained in temperate regions shifting the stock to warm water springs during critical periods. In India, it was introduced during 1952 from Thailand and Sri Lanka and it invaded many parts of the inland water bodies and certain brackish water systems.
In the mangroves, early juveniles and juveniles occur in the upper reaches. The adults seem to avoid mangroves probably because of the soft muddy bottom (the adults are noted for their nest-building habit in the substrate and their territorial behaviour). The maintenance of such behavioural characteristics may not be possible in the dynamic mangrove ecosystems.
This hardy species is noted for its adaptability to survive and grow in widely fluctuating environmental variables, and when ideal conditions prevail it flourishes well and establishes itself in diverse water bodies in different geographic locations. It performs extremely well in fresh water and adapts up to 120 ppt salinity (Pantastico and Baldia, 1979). Certain races of O. mossambicus are reported to perform reproduction even in a water salinity of 49 ppt, while certain other races of this species do not breed in waters above 30 ppt salinity (Popper and Lichatowich, 1975; Philippart and Ruwet, 1982; Philippart and Ruwet, 1982) Fortunately, it has not yet invaded the sea (as opposed to certain tilapines which have).
This Mozambique Tilapia is eurythermal, which tolerates waters with temperatures ranging from 8-42°C and performs well between 20°C and 35°C (spawning is also unaffected in this ideal range). It exhibits maximum swimming performance at 32°C and can adapt to 20 m in depth when the temperature of the water is 30°C and to only 7 m in depth when the temperature is 15°C (Caulton and Hill, 1975). It can establish in water bodies located up to 1000 m in altitude from the mean sea level, provided the water temperature does not drop below 5°C. Although this species tolerates 8°C for a brief period, prolonged exposure to water bodies below 14°C leads to heavy mortality.
It can withstand water bodies with extremes of acidic conditions (pH as low as 5) and alkaline nature (pH as high as 11). It is also noted for its tolerance to extreme drought conditions (it was recorded 0.5 m below the dry river bed in Zimbabwe – by surviving in mere damp subsurface moisture: (vide Discussion part of Kuo, 1969).
In natural waters, males of O. mossambicus build their nest in the littoral terrace to attract females and exhibit territorial behaviour. In the nesting zone (arena), a female fish makes a brief visit, stops in one of the nest basins and lays one batch of eggs. The male belonging to that particular nest basin immediately fertilizes the eggs and the fertilized eggs are taken by the females for incubation, all within one minute. The females then move from the arena to the brooding area (where vegetation occurs, as a shelter for the brooding female and young ones).
Males of O. mossambicus were also reported to take part in guarding the young ones (which is an exceptional case among cichlids) (Liao, 1990). The male and female parents guard the young ones (eggs and alevins) for a period of 22 days (Liao, 1990; Bruton and Boltt, 1975; Russock and Schein, 1977). On the contrary, successive polygyny and polyandry were also reported in this species (Ruwet, 1963). In captivity, O. mossambicus young were produced even in floating cages and also in the aquaria. In certain cases, mouth brooding took place even in high density (200 fish/m^3) cage culture (Bardach et al., 1972 and Guerrero, 1975). This is some of the evidence indicating the flexibility of this species to alter or adjust its typical reproductive behaviour under modified situations and conditions without affecting its noted reproductive capabilities.
4. Capture fisheries and utilization
Caught in cast-nets, gill-nets and with hand lines. Hand picking and capturing the Tilapias with scoop nets during the fag end of the season (when the water is at its minimum in the ponds and tanks) is a common sight in rural areas of many South Asian inland water bodies. Although the flesh is tasty this species fetches a relatively poor price (when compared with other fresh water fish like carps) in certain Indian markets, most probably because of its smaller size (as a result of uncontrolled reproduction), dark pigmented skin and strong odour and flavour when not served as a hot dish.
From the socio-economic point of view, in many South Indian regions the produce of this species reaches the rural and urban poor. Although nutritionally on a par with other quality fishes, because of food habits of certain communities and upper and middle class people, this species is considered as non-prestigious to be taken (this is another reason for its lesser market value, and at present hybrid red-Tilapias are trying to break this barrier in the markets).
5. Culture fisheries
Over 75 species of cichlids occur globally, of which, in about one third of the species, attempts were made at different times and levels to test-verify their worthiness in aquaculture. Even amidst these selected species of cichlids with culture potentiality, only half a dozen species are very promising, of which Oreochromis mossambicus is one among them.
According to levels of management, the production in culture ponds varies widely. An extensive culture system meant for O. mossambicus when receiving no feed or fertilizers can sustain only 840 kg fish/ha, while under high levels of fertilization it can sustain 2,466 kg/ha and with bath fertilization and feed the carrying capacity can be increased to 6,165 kg/ha The corresponding calculated density was 1,475-2,950 fish, 2,950-5,900 fish and 14,750-44,200 fish per hectare respectively under the above mentioned three management levels (Van der Lingen, 1959 and Hepher and Pruginin, 1982).
In sewage-fed fish farming, this species showed production to the tune of 7 tonnes/ha per year without any feed and chemical fertilizer input. In the eutrophic and volcanic soil/crater lakes of the Philippines, extensive commercial cage culture of this species showed a maximum net production of 7 kg/m^3 in 4-6 months (with a range of 1.5-7 kg/m^3) (Coche, 1982).
The semi-intensive cage culture of O. mossambicus in Laguna de Bay, the Philippines, produced a net production of 2.3 kg/m^3 in a 3 month period when supplemented feed containing rice bran (70%) and chopped snail (30%) as ingredients, and with 41% protein were provided (Pantastico and Baldia, 1979). Intensive culture of O. mossambicus in cages yielded 2.9 kg/m^3 net fish production in a 3 month culture period with a poor FCR of 4 (Whitfield and Blaber, 1979). In eutrophic lakes, cage culture systems in an extensive style would be useful.
Cage culture experiments conducted with O. mossambicus by providing feed made from rice bran (77%) and fish meal (23%) exhibited an FCR value of 2.5 while the feed made from rice bran (60%), fish meal (20%) and Leucaena leucocephala (20%) showed a poor FCR of 4.
Because of its assured periodic recruitment of young ones to the stock through parental care and prolific breeding, the stocking density in the culture system is altered drastically – resulting in overcrowding, which in turn leads to the production of small sized fish with a poor market price. To overcome this, monosex (males alone) culture is attempted by manual sorting. Androgenic steroid hormones (mostly 17 alpha methyl testosterone and ethynyl testosterone) are also administered through feed and through clip treatment for attaining all male Tilapias. Chemosterilants (metepa and tetramine) are also used to suppress brood formation.
Sterile fish are also produced through chromosome manipulation (by means of androgenesis/gynogenesis/triploidy/tetraploidy) by preventing the separation of a second polar body from the ovum during meiosis and or by preventing the first mitotic division of the zygote through heat shock/cold shock/pressure shock/chemical shock, by inactivating the genetic material either in the sperm or in the ova through irradiation and by mating the tetraploid male or female with a diploid partner – either individually or in different permutation combinations of the above mentioned procedures. (vide Vaas and Hofstede, 1952; Hickling, 1963; Clemens and Inslee, 1968; Al Daham, 1970; Guerrero and Guerrero, 1975; Guerrero, 1979; Guerrero, 1982; Pandian and Varadaraj, 1987; Pandian and Muthukrishnan, 1988; Pandian and Muthukrishnan, 1990 for further details).
Carnivorous fish or predator fish are often used to control the recruitment of Tilapia in the culture systems. They include: Elops machnata , Lates calcarifer , Megalops cyprinoides and Microperus salmoides (Fortes, 1979 and Swingle, 1960). Among them, the O. mossambicus/ L. calcarifer combination seems to be favourable and very beneficial.
The high cost of construction and maintenance of floating cages, the current cost of fish meal (a minimum of US$ 0.4 per kg as of January 1993), the high cost of androgenic steroid hormones (besides their well established harmful side effects in human beings) for the production of all male offsprings, and exorbitant expenditure involved in establishing sophisticated biotechnological tools to produce sterile Tilapia through chromosomal manipulation on the one hand, and poor sale prices of O. mossambicus (as low as US$ 0.12 per kg on a wet weight basis in certain developing South Asian countries) on the other hand, will make the intensive cage culture systems economically non-viable at this stage and will further question the protein production in real terms.
Therefore, to bring viability to the intensive cage culture system, O. mossambicus was crossed with other species of tilapines to produce various types of hybrids of the following, which, with favourable culture traits, added acceptability (because of better colouring other than the original awesome black pigmentation) and an attractive sale price.
Females of O. mossambicus when hybridized with males of O. hornorum produced all male hybrids (Hickling, 1960). On the other hand, the males of O. mossambicus when crossed with females of O. spilurus niger produced all female hybrids (Whitehead, 1960).
The females of O. mossambicus when crossed with males of O. niloticus produced all female O. niloticus x O. mossambicus hybrids in F1 generation (Chen, 1976). This hybrid showed better growth than the O. mossambicus x O. niloticus hybrid, pure O. mossambicus and pure O. niloticus (Kuo, 1969).
During 1969, the O. niloticus x O. mossambicus hybrid was produced in Taiwan (Lucana fish culture station) by using a normal coloured O. niloticus and a mutant reddish-orange coloured O. mossambicus . The reddish-orange coloured hybrid (one fourth in F1 generation) obtained did not show as good a growth rate as the normal hybrid but the gap was reduced by selective breeding. Because of its appealing colour, larger size and better growth rate due to heterosis the ‘reddish-orange Tilapia hybrid’ was named ‘blessed fish’.
Contradictory vIews exist about the red/reddish orange/golden male strain used to produce red Tilapia due to various reasons as follows:
a) Trade secrets;
b) the lack of scientific background on hybridization programmes by entrepreneurs/fish farmers;
c) the commercial level production of such hybrids in natural waters/open waters by using cages in the presence of other wild tilapine species;
d) the usage of fertile alleles with a diverse ‘genetic make-up’ and segregating genes available at fish farmers’ disposal (usually derived from polyculture systems of certain developing Southeast Asian countries, where more than one pure line species/hybrid of different filial generations are stocked in a mixed manner with low importance to the purity of genetic materials);
e) the absence of appropriate, enthusiastic, innovative, non-bureaucratic government machinery, with a good long-term vision and balanced judgement to cater for the challenging needs of dynamic entrepreneurs with matching spirit and co-ordination to import pure line tilapines of promising aquaculture characteristics for potential use in hybridization programmes (from the native geographic locations), and
f) the consequent clandestine ‘importing’ of the coloured strains/species (and at times spurious strains) by leading entrepreneurs, even by skipping the quarantine in certain developing countries.
Attempts are being made to introduce the red colour to pure species like O. niloticus in different parts of the world. As a result, in the absence of genetic purity and clear-cut identity, the following names are under usage to denote ‘red Tilapia ’ in the literature. They include:
1. O. mossambicus/ O. niloticus hybrid
2. Oreochromis sp.
3. Red O. niloticus
4. A hybrid of male O. niloticus x female (mutant) O. mossambicus etc.
Red Tilapia could be reared up to 1 kg in size in fresh, brackish and sea water and those cultured in sea water have a taste similar to sea bream and could substitute the sea bream market in Japan at highly remunerative prices. Moreover, the demand for red Tilapia is also increasing steadily in the U.S. market and is marketed as ‘cherry snapper’ in Guam Island (Chen, 1976; Kuo, 1969; Liao, 1990).
Besides ‘old world’ red Tilapia , Oreochromis mossambicus also has its due share in the production of Florida red Tilapia . (Male) Oreochromis mossambicus x (Female) Oreochromis urolepis hornorum hybrid).