A MANUAL FOR COMMERCIAL TILAPIA PRODUCTION

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Tilapia culture has been part of aquaculture history since the late 1940s to early 1950s. Tilapia was described as a "wonder fish" and introduced to most parts of the world as "African perch", "Java tilapia" and "miracle fish". Probably the most well known and widely distributed of the tilapias, Oreochromis mossambicus has been grown worldwide and cultured in Trinidad since the early 1950s. The fish were first stocked in ponds at the Bamboo Grove Experimental Station, Valsayn, but quickly escaped to the Caroni Swamp where they flourished and established themselves as wild stocks. Tilapias were also stocked in previously existing ponds, and ponds that were dug primarily as watering holes for cattle. These ponds were difficult to harvest and the resultant fish production was low and uncertain. These subsistence levels of production gave little profit or economic return to the farmer. There were many constraints to the commercial production of tilapias. Tilapia was stocked in ponds based on a "mixed-sex" system of males and females and the attractive feature of rapid breeding of the fish in ponds soon became a major management problem. The result was overpopulation of the ponds with large numbers of small, stunted fish of little market value. There was also an associated "muddy taste" and off-flavor of the flesh of the fish and insufficient marketing information. Since the available technology and culture practices were not able to resolve these shortcomings, the fish was soon looked upon as a pest.

There are many factors which make tilapias excellent for aquaculture. Tilapias are attractive, easily bred, hardy and resistant to disease, and can live in a wide range of water conditions. They can be stocked at high densities and feed on both prepared rations and natural foods. The flesh is firm and is of excellent eating quality. Tilapias cultured in Trinidad are the "black tilapia", "silver tilapia" and various red hybrids (Figure 1). The biology of the various species and the culture techniques used are similar, but because of the enhanced marketability of the red hybrid it is strongly recommended as the preferred fish for culture.  

Mixed-Sex Culture:

There are several culture techniques used in tilapia production that have changed rapidly over the last fifteen years. Early attempts at tilapia farming used the method of mixed-sex culture where ponds were stocked with both male and female fingerlings. However, mature tilapias in a normal population have a high reproductive capacity and this led to overcrowding and resulted in the production of small, stunted and commercially useless fish. Mixed-sex culture is not recommended for commercial tilapia production. Monosex culture of males is the management practice most widely used to prevent overpopulation of ponds and simultaneously capitalizes on the superior growth rate of males. 

Monosex Culture:

Tilapias are best cultured on a semi-intensive level based on a monosex-culture technique. Fish production ponds are stocked with all-male fingerlings that are reared for final growout and marketing. A number of methods are used to produce male fingerlings for growout in monosex culture. These include manual sexing, hormonal sex reversal, and a combination of genetic manipulation and hormone treatment.  

Hormonal Sex Reversal:

Artificial sex-inversion or sex-reversal of gonadally undifferentiated fry is achieved by feeding very young fish with a specialized diet containing male hormones. This method produces 95-100% males and its technical feasibility depends on precautions taken to reduce potential sources of error. Artificial sexinversion or sex-reversal of fry is the most efficient means of producing male fingerlings for monosex tilapia production in commercial aquaculture systems. 

Identifying The Male:
Adult males can usually be easily identified by their larger size and brighter color. At the fingerling size, 15 to 25 grams, they are much more difficult to identify. The genital papillae of the male are quite pronounced and there is only one opening which is at the tip of the papilla. The other opening is the anus.

Male tilapia have a tapered shape below the anus, whereas the female have a more rounded shape with a triangular indentation in the center. In this picture, the female is on the left and the male is on the right.

Identifying The Female:
Females are smaller than males and less brightly colored. At the fingerling size, the genital papillae are used for identification. There are two (uro-genital) openings, one at the tip of the papilla and the other more pronounced and slit-like. The other opening is the anus. In sexually mature females release of eggs is a confirmation of the sex.

The Brood Phase

The brood or hatchery phase involves the actual production of the young fish or fry. This is achieved in specially selected ponds called brood ponds.Selecting or purchasing red tilapia brood stock for your farm or aquaculture enterprise is probably the most important decision you will have to make concerning your venture. There are number of different hybrid crosses which have resulted in the red tilapia that we know today. In addition to those pure lines, there are strains. Strains are a particular group which belong to the same species but have different or specific features relating to that group. For example, one might have a fast growing strain of tilapia, another a disease-resistant strain, although both strains belong to the same species O. niloticus . However,greater care must be taken with the breeding programme to prevent in-breeding and'throwbacks' to the original unwanted characteristics of the parents.Typical characteristics of good brood fish include:

• Good body conformation traits(small head: body ratio, deep body,relatively thick, good dress percentages)

• Free from pathogens and disease

• Bright even colour

• Good egg production

• From good genetic pool

Brood ponds are stocked with sexually mature male and female fish (brood stock) at low stocking densities of 10,000 to 15,000 per hectare. Sex ratio of three females to one male (3:1) is common but up to a 4:1 ratio can be used. After about two weeks the young fry are observed at the pond edges and are removed with a fine-meshed seine (knot-less nylon 2.5 mm). This is done once or twice per week.Fry at about 1 to 5 grams are then transferred to the nursery ponds.

Production  of sex-reversed populations

Female brood tilapia release young fry, which are gonadally undifferentiated. For effective sex reversal, the fry should be at most 2 days old. The male hormone treatment must be continued until the gonadal tissue of the developing fry has differentiated into testes.Genetic females develop into phenotypic males.The sex-reversed males are functionally male but remain genetically female and the breeding of these males will result in a larger percentage of females in their offspring.

Brood Ponds

Before filling brood or reproduction ponds, 3 cm nylon mesh netting should be draped in the harvest sump. The brood fish are then stocked at a 1:3 male to female ratio respectively. The stocking density of the brood pond should be approximately 10-12,000 per hectare. The spawning cycle lasts approximately 19 to 21 days. At the end of this period the pond is drained and the brood stock are removed by lifting the 3-cm mesh. The mouths of parent females are checked for incubating eggs. Any eggs present are washed out and retained for later incubation in hatchery tanks. The free-swimming fry of a suitable size for sex-reversal remaining in the pond are netted out using a mosquito net.

Figure: Brood Pond

Feed  Preparation

The sex-reversal diet is prepared by an alcohol evaporation method. Ordinary fish ration (a filler) is mixed at a ratio of 40 to 60% with a high protein fish meal substitute of approximately 60% protein, which is used as base. The hormone, alpha-methyl-testosterone is dissolved in a solvent of methanol or ethanol at a rate of 60-70 mg of hormone to 1 litre of solvent. This is added to the feed at a rate of 60-70 mg of hormone to 1kg of feed,hand-mixed, and allowed to dry by air. This is done 3 to 4 days prior to usage. This mixture should not be stored for more than 4 days.

 Figure: feed preparation with hormone

Tank culture and cage culture

There are various types of sex reversal techniques, but two practices have been most successful each with its own advantages and disadvantages. Tank culture uses "clear water"and aeration and is usually done indoors(Figure 7) while cage culture is confined to cages in outdoor earthen ponds.

Tank culture

Fry are brought into the wet laboratory from brood ponds, and sorted using a 2.5 mm mesh.Those passing through the mesh are used for sex-reversal and range from approximately 7-11 mm in length. They are stocked in treatment tanks where they are chemically treated for diseases and parasites, which they may acquire from the outdoor ponds. They are then transferred to the culture tanks by siphon at stocking densities of approximately 5-6,000 per cubic metre.

Figure: Concrete tanks used for sex-reversal of tilapia fry

The fry are maintained in a circulating stream of biologically filtered water at a temperature of 23-26°C and fed at a rate of 20% body weight per day 4 times daily. The sex-reversal cycle is approximately 28-30 days and the feed is gradually reduced to 10% body weight per day at the end of the cycle. Weekly sampling of the fry facilitates determination of feeding rates. Ecto-parasitic infections can cause diseases during the feeding treatment and these have to be carefully monitored and controlled with the use of effective prophylactic agents.During the treatment period, the growth of fry is slow because of the high densities and relative low treatment temperatures maintained. Upon completion of the sex-reversal treatment, fry are stocked in nursery ponds for rapid growth prior to their stocking in production ponds. Fish ration of about 28-32%protein is used for the nursery and production cycle.

Cage  culture

Fry from the reproduction ponds are passed through a 3.1 mm mesh grader to remove fry larger than 14 mm. The smaller fry are stocked in cages of mesh size 1.5 mm and stocked at stocking densities of approximately 2-3,000 per square metre. The fry are fed on the sex-reversal ration 4 times daily starting at 20% biomass per day at the start of the cycle and reducing to 10% at the end. The fry in the cages show more substantial growth than those sex-reversed in`clear' water. The fry are then removed from the cages at the end of the cycle and stocked in nursery ponds and fed on a normal fish food diet. The disadvantage of this technique is its exposure to the elements and possible loss of thousands of "partially sex-reversed fry" in the event of damage or removal of nets or cages.

Figure: Tilapia fry being fed in net-cage with hormone-treated feed

Hatchery or Brood Phase

Brood ponds should have a firm bottom and good drainage. The feeding rate is 1-3 % body weight per day using a supplemental feed ration with crude protein of about 26-35 %.It is a good practice to maintain brood stock separate from your other fish and to prevent inbreeding. This is usually achieved by not allowing brood ponds to continue for long periods. They should be drained and re-stocked every 2 to 3 months when possible.

Nursery Phase

The major problem encountered in nursery ponds is high mortality due to poor handling during stocking and harvesting. By following good fish handling procedures at this stage, unwarranted moralities are reduced. Nursery ponds should be completely drained, allowed to dry out and all water puddles poisoned with a fish toxicant like Rotenone, lime (Ca (OH)2) or household bleach (e.g. Clorox®).

Growout Phase

After 6 months, the growth rate slows significantly and feed conversion becomes less efficient. It is at this point that the fish farmer must make the decision to harvest and sell his fish or risk losing money for a few extra grams.In the final analysis, it is the possible profit that will influence the exact timing of harvest and the marketing of the product.

Harvesting

Harvesting of fish for the market is usually with a seine. Two methods are employed, a partial harvest or total drain-down harvest. Partial harvest involves pulling a seine to remove the larger marketable fish and leaving the smaller ones to grow. A total harvest is achieved by draining the pond and removing all of the fish.This usually follows several seine hauls or a partial harvest. Typically, the period between a partial harvest and a total harvest is about two weeks. Fish should be harvested in the cool of the morning or evening to prevent undue stressing of the fish. Care should also be taken to keep fresh water in the pond, especially at the drain end. Fish should be quickly transported either to a holding area or to processing facilities.Sometime fish are placed in cages for holding just prior to market.

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