Credits: Biovision-Infonet
Biological characteristics of tilapia
Tilapia is the generic name of a group of cichlids endemic to Africa. The important aquaculture genera in Kenya are Oreochromis, and Tilapia. All tilapia species are nest builders; fertilized eggs are guarded in the nest by a brood parent. Sarotherodon and Oreochromis are mouth brooders; eggs are fertilized in the nest but parents incubate them in their mouths including several days after hatching. For the Oreochromis, only females practice mouth brooding, while in Sarotherodon, either the male or both male and female mouth brood.
(c) Mbugua Mwangi, Kenya
Tilapia farming involves the culture of following species:
i. Oreochromis niloticus
ii. Oreochromis mossambicus
iii. Oreochromis aureus
iv. Oreochromis spilurus
v. Oreochromis andersonii
vi. Tilapia zillii.
vii. Tilapia rendalli
Feeding Habits
Tilapias are heterogeneous in their feeding. They and have ability to consume and efficiently assimilate a wide variety of foods. Various species are omnivorous; others are phytoplanktonous while others are macrophyte feeders.
- Omnivorous are: O. mossambicus, O. niloticus, O. spilurus, O.andersonii and O. aureus.
- Phytoplankton feeders: O. leucostictus, O. macrochir, O. esculentus, O. alcalicus grahami, and S. galilaeus
- Macrophytes (feed on larger plants) feeders: T. rendallii and T. zillii.
Maturation
In natural water bodies, tilapias mature in about two to three years. Under culture they tend to mature early. Sexual features distinguishing males from females are clear when fish mature (about 15 cm in Tilapia zillii and 10 cm in Oreochromis niloticus). Males have two orifices situated near the ventral (anal) fin, one is the urinogenital aperture and the other is the anus. The females have three orifices, the genital opening the anus and a urinary orifice (but difficult to visualize with the naked eyes). Separation of males and females can be made easier by applying dye (India ink, indigo, etc.) to the papilla with a cotton swab to outline the male and female openings.
Fecundity
Fecundity refers to the number of eggs produced by a fish in a spawn. This applies well for monocyclic species, that is, once a year breeders. Tilapias are polycyclic (many times breeders) and their ovary may contain eggs at different stages of maturity.
In substrate brooding (nest building) tilapias, fecundity is much higher than mouth brooders. Other characteristics that differentiate substrate brooders (Tilapia) and mouth brooders (Oreochromis) are:
| Characters | Tilapia | Oreochromis |
| Fecundity | high | low |
| Egg diameter (mm) | 1-1.5 | up to 5.0 |
| Yolk percentage | less than 25% | up to 45% |
| Yolk colour | pale yellow | orange |
| Size of fry at feeding | 5-6 mm | 9-10 mm |
| Courtship | prolonged (monogamous) | brief (polygamous) |
| Juvenile mortality | high | low |
| Longevity | up to 7 years | over 9 years |
Environmental requirements
Optimal Temperature
Temperature affects fish distribution, survival and growth, rate of development, reproduction and even susceptibility to diseases. Various species and strains of tilapia differ in tolerance to low temperatures, but growth is generally limited at water temperatures below 16degC and most become severely stressed at 13degC. Death occurs from 12degC with few surviving temperatures below 10degC. Most will not feed or grow at water temperatures below 15degC and will not spawn below 20degC. The normal water temperature should be between 20 to 30degC. Metabolic rate rises at higher temperatures which lead to death.
Optimal Dissolved Oxygen (DO)
Tilapias are able to tolerate low levels of ambient oxygen. Usually, well fertilized ponds will have low levels of oxygen early in the morning. Night activities are dominated by respiration and decomposition which reduce DO. Larger fish are less tolerant than juveniles. This could be due to the difference in their metabolic demand. The optimal DO for tilapia culture is 4 mg/litre (50%) and should not go below 2.3 mg\litre.
Salinity
The Nile tilapia is the least saline tolerant of the commercially important species, but grows well at salinities up to 15 ppt. The Blue tilapia grows well in brackish water up to 20 ppt salinity, and the Mozambique tilapia grows well at salinities near or at full strength seawater
pH
Tilapia can survive in pH ranging from 5 to 10 but do best in a pH range of 6 to 9.
Ammonia
Massive tilapia mortality will occur within a few days when the fish are suddenly exposed to water with unionized ammonia concentrations greater than 2 mg/l Prolonged exposure (several weeks) to un-ionized ammonia concentration greater than 1 mg/l causes deaths, especially among fry and juveniles in water with low DO concentration.
Nitrite
Nitrite is toxic to many fish and chloride ions reduce the toxicity. Tilapia are more tolerant to nitrite than many cultured freshwater fish. In general, for freshwater culture the nitrite concentration should be kept below 27 mg/l.
TILAPIA FARMING
Pond culture is the most popular method of growing tilapia. They are grown in fertilized ponds where the fish utilize natural foods from ponds. Management practices of the systems ranges from extensive; using only organic or inorganic fertilizers, to intensive systems, using high-protein feed, aeration and water exchange. The major problem to overcome in this system is the prolific breeding of the fish that occur in ponds under mixed sex culture. This breeding if not controlled results to overcrowding in the ponds. The end result is stunted growth yielding small size fish (less than 100 g) which may not be of market value. In mixed-sex populations, the biomass of juveniles can make up to 70 percent of the total harvest weight. Therefore strategies for producing tilapia in ponds should aim at controlling spawning and recruitment. For easy management and economical operation in Kenya, grow out ponds should be about 1 to 2 metres deep and at least 300 sq metres for semi-intensive production of tilapia. A harvesting sump in the pond behind the drainage outlet is needed to concentrate the fish in the final stage of drainage. The pond should be drained completely and be allowed to dry to eradicate any fry or fingerlings that may interfere with the next production cycle. This will also kill some parasites, frogs’ egg and other unwanted organisms that may be in the ponds.