Group Discussion

Interesting About Tilapia

Life cycle and system of tilapia

Tilapia

Tilapia is a genus of fish in the cichlid (pronounced ‘sishlid’) family. These freshwater fish are native to Africa but have been widely introduced to warm waters in many countries. Their natural habitat is lakes but they are now extensively bred and reared in fish farms. 

The cichlids are mostly carnivorous, hunting about for food on the bottom or snapping up organisms floating in the water. Most species of Tilapia, however, feed on algae in plankton, though Tilapia zillii eats the stems and leaves of water plants. When swimming in ‘schools’, the fish usually move almost simultaneously and this tends to keep the school together.

 

 

In their breeding behaviour, the cichlids may be divided into two groups, the substrate spawners and the mouth brooders. The former lay eggs in a pit dug in the bottom of the lake or pond. The eggs are aerated and guarded by the parents. 

The mouth brooders, on the other hand, though they may lay their eggs in pits, then take them into the mouth and keep them there till the young hatch.

The male guards the territory round the pit, driving off intruders by nipping or pushing them (see Fig.1).

If a female enters the territory, the male changes his behaviour pattern to a slow, swimming movement, with body tilted downwards, leading her towards the pit he has dug (Fig. 2). There may follow a courtship pattern, which varies very much with the species, but often consists of quivering movements or swimming round and round in the nest-pit (Fig. 3), while the male butts at the protruding genital tube of the female.

Such an elaborate behaviour pattern ensures that sperms are shed at the moment that eggs are laid, thus increasing the chances of fertilization.In the mouth brooders, the courtship pattern is followed by spawning in which the female lays perhaps hundreds of eggs in the nest and these are immediately taken up into the mouth.

For about ten to twelve days the female carries a mouthful of eggs or young fish (Fig. 4). The breathing movements are restricted and feeding is not attempted (Fig. 5). If the young fish swim out of the mouth, they are snapped up again but after about ten days the mother ejects them in swarms. These swarms form a school and follow the mother. In some species the youngsters will return to the mouth if danger threatens (Fig. 6); the mother ‘calls’ them by swimming slowly backwards.

Tilapia Culture In Ponds:

Pond culture is the most popular method of growing tilapia in the world. They are grown in fertilized ponds where the fish utilize natural foods from ponds.

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 and small size fish (less than 100gms) which may not be of market value. Therefore strategies for producing tilapia in ponds should aim at controlling spawning and recruitment.

• Mixed-sex culture

In mixed-sex culture of tilapia, both males and female are cultured together but harvested before or soon after they reach sexual maturity. The disadvantage in this is that fish is harvested at a smaller size due to the limited growth period.

In this culture practice, fish are usually stocked at low rates to reduce competition for food and promote rapid growth. One month-old, 1-gram fry are stocked at 1 to 3 per square meter into and grown for about 4 to 5 months. In cold areas where the water temperatures are low and therefore slow growth, tilapia might not reach marketable sizes in that period.

Newly-hatched fry should be used all the time because older ones will reach sexual maturity at a smaller, unmarketable size. They could also be mature fish but stunted. Supplemental feeds with 25 to 32 percent protein are generally used. The average harvest weight is about 250 grams, and total production about 0.25 Kgs/sq m for a stocking rate of 1 fish/m². Higher stocking densities can be employed to achieve higher production but must be combined with better management. Expected survival is about 80 percent.

In Kenya two to three crops of fish can be produced annually depending on the water temperature. 

• Mono-sex culture

To overcome the problem resulting from prolific breeding of tilapia, ponds are stocked with males only because the males grow almost twice as fast as females.

Male fingerlings can be obtained by three methods:

• Hybridization
• Sex-reversal and
• Manual sexing.

None of these methods is always 100 percent effective, and a combination of methods is recommended. Hybridization can used to produce better results of males only. The hybrids can then be subjected to hand sexing and/or sex-reversal treatment. Sex-reversal requires obtaining recently hatched fry and rearing them in tanks or hapas where they are subjected to hormone laced feed for about three weeks.

Diet, reproduction, and life cycle

Most tilapias are omnivorous with a preference for soft aquatic vegetation and detritus.The type of parental care helps to distinguish the genera of tilapias. Members of the genera Oreochromis and Sarotherodon are mouthbreeders (Chapman 1992), whereby usually the female carries the fertilized eggs and newly hatched eggs in the mouth, offering incubation and protection (Nelson 2006). 

 Mouthbrooders normally are polygamous (Nelson 2006). After the female has released the eggs and they are fertilized, the females (typically) pick up the eggs from the nests and incubates and protects in their mouths (Chapman 1992). Species of Oreochromis in particular have mouth-brooding where only the females incubate and protect the young in their mouths (Chapman 1992). 

Other tilapias, belonging to the genus Tilapia, are normally substrate brooders, which incubate the eggs in a nest on the lake or pond bottom (Chapman 1992; Nelson 2006). Substrate brooders are usually monogamous and both parents may care for the eggs (Nelson 2006), including fanning water through the nests to incubate the embryos (Chapman 1992). Tilapias are notable for being able to become sexually mature at a small size (eight to ten centimeters or three to four inches) and a young age (two to three months old) (Chapman 1992). They live six to eight years or more, including up to age 11 or 12 (Chapman 1992).

Aquaculture

Raising of tilapias for food consumption is believed to first have taken place about 2,500 years ago (Chapman 1992). Tilapia today has become the third most important fish in aquaculture after carps and salmonids, with production reaching 1,505,804 metric tons in 2002 (Fessehaye 2006).

Because of their large size, rapid growth, and palatability, a number of species of Oreochromis, Sarotherodon, and Tilapia are at the focus of major aquaculture efforts. Like other large fish, they are a good source of protein and a popular target for artisanal and commercial fisheries. Originally, the majority of such fisheries were in Africa, but accidental and deliberate introductions of tilapia into freshwater lakes in Asia have led to outdoor aquaculturing projects in countries with a tropical climate such as Papua New Guinea, the Philippines, and Indonesia. In temperate zone localities, tilapiine farming operations require energy to warm the water to the tropical temperatures these fish require. One method involves warming the water using waste heat from factories and power stations.

Tilapiines are among the easiest and most profitable fish to farm. This is due to their omnivorous diet, mode of reproduction (the fry do not pass through a planktonic phase), tolerance of high stocking density, and rapid growth. In some regions the fish can be put out in the rice fields when rice is planted, and will have grown to edible size (12–15 cm, five to six inches) when the rice is ready for harvest.

Commercial species of tilapia of particular importance include the Java or Mozambique tilapia (Oreochromis mossambicus; synonym Tilapia mossambica), blue tilapai (O. aureaus; syn. Tilapia aurea), Zanzibar or Wami tilaopia (O. hornorum; syn. Tilapia urolepis), Nile tilapia (O. niloticus; syn. Tilapia nilotica), and redbelly tilapia (O. zilli; syn. Tilapia zilli) (Chapman 1992).

Reproduction

In all Oreochromis species the male excavates a nest in the pond bottom (generally in water shallower than 3 feet) and mates with several females. After a short mating ritual the female spawns in the nest (about two to four eggs per gram of brood female), the male fertilizes the eggs, and she then holds and incubates the eggs in her mouth (buccal cavity) until they hatch. Fry remain in the females mouth through yolk sac absorption and often seek refuge in her mouth for several days after they begin to feed.
 
Sexual maturity in tilapia is a function of age, size and environmental conditions. The Mozambique tilapia reaches sexual maturity at a smaller size and younger age than the Nile and Blue tilapias. Tilapia populations in large lakes mature at a later age and larger size than the same species raised in small farm ponds. For example, the Nile tilapia matures at about 10 to 12 months and 3/4 to 1 pound (350 to 500 grams) in several East African lakes. Under good growth conditions this same species will reach sexual maturity in farm ponds at an age of 5 to 6 months and 5 to 7 ounces (150 to 200 grams). 

When growth is slow, sexual maturity in Nile tilapia is delayed a month or two but stunted fish may spawn at a weight of less than 1 ounce (20 grams). Under good growing conditions in ponds, the Mozambique tilapia may reach sexual maturity in as little as 3 months of age, when they seldom weigh more than 2 to 4 ounces (60 to 100 grams). In poorly fertilized ponds sexually mature Mozambique tilapia may be as small as 1/2 ounce (15 grams).
 

Fish farming strategies that prevent overcrowding and stunting include: 

1) cage farming where eggs fall through the mesh to the pond bottom before the female can collect them for brooding

2) polyculture with a predator fish, such as fingerling largemouth bass, at 400 per acre 

3) culture of only males (monosex). 

All-male culture is desirable in ponds not only to prevent overpopulation and stunting but also because males grow about twice as fast as females.Methods of obtaining predominately male fish include: 

1) manually separating the sexes based on visual examination of the genital papilla of juvenile fish (hand-sexing) 

2) hybridizing between two selected species that produce all-male offspring (for example, Nile or Mozambique females crossed with Blue or Zanzibar males) 

3) feeding a male hormone-treated feed to newly hatched fry for 3 to 4 weeks to produce reproductively functional males (sex reversal) 

4) YY male technology (currently under development and not yet a commercial option). 

The sex of a 1-ounce (25-gram) tilapia fingerling can be determined by examining the genital papilla located immediately behind the anus (Fig. 1). In males the genital papilla has only one opening (the urinary pore of the ureter) through which both milt and urine pass. In females the eggs exit through a separate oviduct and only urine passes through the urinary pore. Placing a drop of dye (methylene blue or food coloring) on the genital region helps to highlight the papilla and its openings.