Tilapia

Tilapias (Cichlidae) are natives of Africa. They have been introduced into a large number of tropical and subtropical countries around the world since the 1960s (Pillay, 1990). It is also commercially known as Mango fish or Nilotica. Tilapias have been called as the “everyman’s fish” (Pullin, 1985). Nile tilapia (Oreochromis niloticus) is one of the first fish species that was cultured in the world (Pompa and Masser, 1999). Tilapia was first introduced in Bangladesh in 1954. About 80 species of tilapia have been described out of which 10 species are reported to be used for culture (Macintosh and Little, 1995). Until the late 1970's the tilapias, were all classified into a single genus, Tilapia, however most taxonomists now classify them into three genera, Tilapia, Saratherodon and Oreochromis  according to their breeding behaviour. The existing strain Nile tilapia (Oreochromis niloticus L.) was first introduced into this country by the United Nations International Children Emergency Fund (UNICEF) in 1974, and later by the Bangladesh Fisheries Research Institute (BFRI) from Thailand (Gupta et al. 1992). It is a species of high economic value and is widely introduced outside its natural range; probably next to Oreochromis mossambicus, it is the most commonly cultured cichlid.

Another promising Genetically Improved Farmed Tilapia strain known as GIFT (Eknath et al. 1993) has recently been introduced in July 1994 from the Philippines. The GIFT  strain was  developed  by  the  International Center  for Living Aquatic Resources Management  (ICLARM)  through  several generations  of  selection  from a base population  involving  eight different strains  of Nile  tilapia, Oreochromis niloticus (Eknath et al. 1993). It performs 60% better growth and 50% better survival than the commercially available strains of tilapia (Hussain et al. 2000). GIFT have generally performed as well or better, and in some countries much better, than existing farmed tilapia. GIFT and GIFT-derived strains have proven to be good genetic material for continued selective breeding.

The GIFT strain has, meanwhile, proved to be very suitable fish for aquaculture in Bangladesh. The desirable characteristics of this genetically improved strain are as follows:  High yielding, Excellent breeder, Efficient converter of organic and agricultural wastes in to high quality protein, Resistant to disease, Very hardy, Tolerant to over crowding conditions, Able to grow in either fresh or brackish water.

On the other hand, NEW GIFU tilapia is the 11^th strain of Oreochromis niloticus was introduced in Bangladesh from China, which is invented by Dr. Li Sifa, professor, Shanghai Fisheries University, China. The NEW GIFU tilapia strain was introduced in Bangladesh by Bangladesh Fisheries Resources Institute from China. In China, experiments showed that NEW GIFU tilapia strain grew more than twice as fast as the local commercial strain.

Tilapias have distributed to so many different types of water, to so many different types of culture systems in the world that they have been even labeled as the “aquatic chicken” (Maclean, 1984). Tilapias are fast growers, prolific breeders and can thrive in many unfavourable culture situations. They have good resistance to poor water quality and disease, tolerance of a wide range of environmental conditions, ability to convert efficiently the organic and domestic waste into high quality protein, rapid growth rate and tasty flavour (Ballarin and Hallar, 1982). Tilapias can tolerate DO concentration of 1mg/l and can survive by using atmospheric oxygen when dawn DO concentration dropped to less than 1mg/l (Chervinski, 1982). Tilapia has the ability to survive under extremely low dissolved oxygen rich surface water layer by reducing activity (Chervinski, 1982).  Because of this, tilapias are deemed as the source of animal protein in the diet of many people of the world. Quite a few years have lapsed since tilapias entered into Bangladesh but not many of our people could know the biology of this group of fish. They prolifically reproduce in the culture system and very quickly compete for food and shelter. At this stage, the culture system is seen to be full of fishes of different sizes. This happens because when the population in the culture system is very dense, the maturity age and size both get reduced; in other words, denser is the population, more is the reproduction. At this stage, the fishes start dying because of starvation on one hand and on the other hand, they start eating the young ones due to cannibalism that develops in the adults. However, due to these factors, very soon the population starts diminishing from the culture system and at a later stage, completely vanishes away. People observing such a phenomenon call this fish as a miracle fish, a wonder fish.

The precautious maturity and frequent spawning in tilapias result in over dense, stunted population in the culture system. Hence, control of reproduction is necessary so that the stocking population could be maintained at stable condition. On the other hand, if commercial culture programme of tilapias is undertaken, larger numbers of broods have to be maintained for the production of millions of fry, since the fecundity of cultured tilapias is low. Most of the cultured tilapias spawn in monthly intervals or so. However, the spawning frequency can be doubled if the eggs are taken out from the female’s mouth and artificially hatched in incubators. The problem of over population due to uncontrolled reproduction in the culture system can be overcome by adoption of monosex culture i.e., to culture of fishes of one sex only. In tilapias, the males grow faster than the females. Thus, the culture of males in tilapia profitable. Separation of male fry from the mixed brood is not possible because identification of sex in tilapia at the early stage is not possible. Thought it is possible at a latter stage, substantial risk of wrong identification exists. Efforts thus went on in different ways for the production of monosex male seed of tilapia for commercial culture. Some inter-specific crosses in tilapias produce nearly 100% male hybrids. Several such crosses have been used to produce male hybrid seen in many countries of the world, especially in Israel and Taiwan. Unfortunately, as such crosses do not always produce 100% male hybrids, the prospect of the treating the progeny with androgenic steroid hormones to induce sex change in females. This method appeared to be the main method for successful large scale production of male fry in tilapia. With this method, the amount of hormone which is used in the feed does not have any residual effect to pose a threat to the health of consumers.

The choice of conversion of sexes (either all males or all females) depends on growth performance characteristics of individual sexes of fish species. For instances, in tilapia males grow faster than female masculinization using androgen hormones (Shelton et al. 1978; Guerrero, 1979 and Guerrero, 1988) and in case of salmonids and cyprinids, where females grow faster than males, feminization using estrogen hormone (Shelton, 1987) have become a popular practice.

Tilapias have great potential in Bangladesh and they are going to be the prime culture species in near future for freshwater and brackish water ecosystem. It is expected that about 50% of total aquaculture production can be met up by tilapia farming. Among the Asian countries, very soon Bangladesh will be one of the leading countries in tilapia production.

Tilapia are shaped much like sunfish or crappie but can be easily recognized by an interrupted lateral line characteristic of the Cichlid family of fishes. They are laterally compressed and deep-bodied with long dorsal fins. The forward portion of the dorsal fin is heavily spined. Spines are also found in the pelvis and anal fins. There are usually wide vertical bars down the sides of fry, fingerlings, and sometimes adults.

Besides phytoplankton, tilapia will also eat zooplankton, detritus, aquatic plants, insects and even small fish fry.  Commercial pellet, waste food and almost any other type of feed given, with perhaps the exception of meat, is also eagerly devoured.  Very little investment is, therefore, required in their nutrition.

Male tilapias are usually larger than females of the same age. The male has two body openings situated just forward of the anal fins, of which one is the anus. The other is the opening of the urethra, at the end of the genital papilla (an oval-shaped lobe just rearward of the anus), from which milt (sperm) and urine are discharged.

The female has three body openings, of which one is the anus. The genital papilla of the female has two openings. They are the urethra, which is hardly visible to the naked eye and the opening of the oviduct (a crescent-shaped slit), from which eggs are released.

These features are more visible and identifiable when the fish have grown to 10–20cm in length and 100–150g in weight. Mature Nile tilapia can also be distinguished by their coloration under the jaw- reddish in males and greyish in females.

Tilapia are more tolerant than most commonly farmed freshwater fish to high salinity, high water temperature, low dissolved oxygen, and high ammonia concentrations. 

All tilapias are tolerant to brackish water. 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. Therefore, the Mozambique tilapia is preferred for saltwater culture. Some lines of the Mozambique tilapia reportedly have spawned in full strength seawater, but its reproductive performance begins to decline at salinities above 10 to 15 ppt. The Blue and Nile tilapias can reproduce in salinities up to 10 to 15 ppt, but perform better at salinities below 5 ppt. Fry numbers decline substantially at 10 ppt salinity.

The intolerance of tilapia to low temperatures is a serious constraint for commercial culture in temperate regions. The lower lethal temperature for most species is 50 to 52^o F for a few days, but the Blue tilapia tolerates temperatures to about 48^o F. 

Tilapia generally stop feeding when water temperature falls below 63^o F. Reproduction is best at water temperatures higher than 80^oF and does not occur below 68o F. In subtropical regions with a cool season, the number of fry produced will decrease when daily water temperature averages less than 75^oF. Optimal water temperature for tilapia growth is about 85 to 88^oF. Growth at this optimal temperature is typically three times greater than at 72^o F. 

Tilapia survive routine dawn dissolved oxygen (DO) concentrations of less than 0.3 mg/L, considerably below the tolerance limits for most other cultured fish. In research studies Nile tilapia grew better when aerators were used to prevent morning DO concentrations from falling below 0.7 to 0.8 mg/L (compared with un aerated control ponds). Growth was not further improved if additional aeration kept DO concentrations above 2.0 to 2.5 mg/L. Although tilapia can survive acute low DO concentrations for several hours, tilapia ponds should be managed to maintain DO concentrations above 1 mg/L. Metabolism, growth and, possibly, disease resistance are depressed when DO falls below this level for prolonged periods. 

In general, tilapia can survive in pH ranging from 5 to 10 but do best in a pH range of 6 to 9.

Massive mortality of tilapia occurs within a few days when fish are suddenly transferred to water with unionized ammonia concentrations greater than 2 mg/L. However, when gradually acclimated to sub-lethal levels, approximately half the fish will survive 3 or 4 days at unionized ammonia concentrations as high as 3 mg/L. Prolonged exposure (several weeks) to un-ionized ammonia concentration greater than 1 mg/L causes losses, especially among fry and juveniles in water with low DO concentration. The first mortalities from prolonged exposure may begin at concentrations as low as 0.2 mg/L. Un-ionized ammonia begins to depress food consumption at concentrations as low as 0.08 mg/L. 

Nitrite is toxic to many fish because it makes the hemoglobin less capable of transporting oxygen; chloride ions reduce the toxicity. Tilapias are more tolerant of nitrite than many cultured freshwater fish. When dissolved oxygen concentration was high (6 mg/L) and chloride concentration was low (22 mg/L), the nitrite concentration at which 50 percent of the fish died in 4 days was 89 mg/L as nitrite. In general, for freshwater culture the nitrite concentration should be kept below 27 mg/L as nitrite.

Oreochromis species  usually  seek  out  shallow  areas  and  group  together  for  breeding. Males develop bright colouration and set up territories in which they build their nest: they hollow out a small area on the pond bottom approximately 10–15cm in diameter. They display courtship behaviour and lure females to the nesting site. The nest is used as temporary site for courtship. If the female is receptive to the courting male, she will be induced to spawn.

In the nest, eggs are laid and fertilized by the male, who discharges sperm over the eggs. The female collects the fertilized eggs in her mouth, and mouth-broods the eggs for around 6–10 days. After hatching, the newly hatched fry continue to shelter in her mouth for another 4–7 days. The fry begin to swim freely in schools, but may return to the mouth of the mother when threatened. Females do not feed while they are incubating the eggs or mouth-brooding the newly hatched fry.

Male tilapia can mate with several females (polygyny) and females may mate with several males (polyandry). In ponds, Nile tilapia becomes sexually mature at three to five months of age (150–200g weight). As  soon  as  sexual maturity  is  attained, most  female  tilapia  are  able  to  undergo  successive spawning  to produce a new brood every 4  to 6 weeks. Temperature plays an important role here.  If water temperature remains at 22°C and above, tilapia will spawn throughout the year. Temperature in the range of 25–30°C is considered ideal.

The number of eggs per clutch increases as the female fish grows. On average each clutch of eggs will produce 100–500 fry. As female fish get older (more than one year old) they produce less fry compared with when they were younger. The best size of female for breeding is 150–300g. On average a 200g breeder would produce 200–500 fry per month. Fry production will also depend on the condition and health of the breeders. In ponds or any confined conditions, egg size and clutch size may vary.

Kohinoor et al. (2003) studied the breeding biology and monosex male seed production of GIFT strain of Nile Tilapia (Oreochromis niloticus) in hatchery under different conditions in Bangladesh. But comparison of egg production, hatching rate, fry survival rate and percentage of monosex fry production between GIFT tilapia strain and NEW GIFU tilapia strain have never been reported. So the present investigation was undertaken to find out the comparison of egg production, hatching rate, fry survival rate and percentage of monosex fry production between GIFT tilapia strain and NEW GIFU tilapia strain in Bangladesh.   

From the above discussion it is evident that mono-sex male tilapia is fast growing and increased total production. Therefore, the present study was undertaken to know the performance of mono-sex tilapia production in a hatchery.