FAMILY: Cichlidae
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NAME: Oreochromis niloticus, Oreochromis aureus - Tilapia - All male production |
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Data provided courtesy of Aquaculture/Fisheries Center, University of Arkansas at Pine Bluff - (Dr. Peter Perschbacher) |
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INTRODUCTION
Tilapias are now (2011) the second largest cultured fish after carps with a total annual production of about 3 million metric tons in 2010. Tilapias are successfully cultured today because of their desirable qualities as food fish and their ease of culture. Among tilapias, members of the genus Oreochromis, such as O.niloticus and O.aureus are favored in aquaculture because of their performance under culture conditions with O.niloticus being the most preferred because of its excellent growth rate. O.niloticus accounts for over 80% of the tilapia production on a pure species basis. MIXED SEX TILAPIA CULTURE AND ITS DEMERITS Most species of tilapia under captivity will reach sexual maturity within 6–8 months after hatching at a size often not more than 100 g and under favorable conditions, they will continue to reproduce. The early and uncontrolled reproduction in culture systems especially ponds, directs a significant amount of energy in reproductive development and activities reducing the energy available for growth. The uncontrolled recruitment also increases the demand for food and other resources, resulting in stunted growth. Oreochromis species being female mouth brooders, means that the females hardly eat during mouth brooding and only have shorts periods of eating between two reproductive cycles. This also affects their growth and thus females are always smaller than males. Several techniques have been employed to reduce the early and frequent spawning of tilapias like growing them at very high densities (about 300/m3), using predator fish snake largemouth bass, Nile perch, Asian catfish to eat the newly produced fry and all male production. For large scale commercial tilapia culture, all males (monosex) populations are preferred because they grow faster METHODS OF PRODUCING ALL MALE POPULATIONS HAND SEXING: Juvenile males and females of about 30g or larger can be differentiated by looking at their genital morphology. Males have a single opening on the urinogenital papillae while females have two. The method is labour intensive, require skill especially when the size is small and theoretically about 50% of the seed (females) is wasted. HYBRIDIZATION: Females of O.niloticus and O. mossambicus are homogametic (XX) while the males of O. aureus, O. Urolepis hornorum are homogametic (ZZ). The differences in the sex determination mechanisms between the different tilapia species can result in all male (XZ) populations if species of different mechanisms are crossed. The disadvantage with this technique is that the two parental species are kept separate and also there is no growth improvement after hybridization. SEX REVERSAL: Tilapia that are genetically females are made to function phenotypically as males For sex reversal to be effective, it must begin before gonadal differentiation and the differentiation of gonads takes place during the very early few days post hatch which is 6-20 d in O.mossambicus and 30-33 in O.niloticus post hatch. Harvested fry is graded through a 3.2 mm mesh grader to eliminate fry greater than 14 mm. The hormone used mostly for sex reversal is17 α-Methyltestosterone (MT) and the hormone impregnated feed is made by adding 30-60 mg of the hormone per kilo of about 40% proteinous highly palatable feed. Since MT is organic, it’s dissolved in organic solvent like alcohol (95%) to act as carriers (about 200 ml/kg of feed) for MT. The hormone impregnated feed is air dried out of direct sunlight. First feeding fry can eat about 15-20% body weight and weekly sampling is done to assess growth and also adjust the feed. About 1.5 µg/g of fish/d is needed to be consumed to obtain 100% all males. The percentage of males will differ depending on temperature, hormone dosage rate used, size at which sex reversal starts as the contribution of natural foods to the diets of fry (hapas). The percentage of males is normally evaluated by a gonadal squash using acetocarmine stain and viewed under the microscope. The fish should be about 2-5g for the gonads to be visible. GENETIC MANIPULATION: YY male tilapia can be produced to act as broodstock for all male Nile tilapia production. The technique involves first feminizing genetically male tilapia with ethynylestradiol and diethylstilbestrol (100 mg/kg of feed) and then crossing the genetically male but phenotypic female tilapia with normal male tilapia followed by progeny testing to identify genetic males that function as females (XY). These females (XY) when crossed with normal males (XY) will give about 25% YY males which are also identified by progeny testing. All male production by genetic manipulation involves a lot of skill, infrastructure and the results are not always consistent. Sex reversal and genetic manipulation (YY) males do not always yield 100% males because sex determination in tilapia is not only controlled by sex chromosomes and hormones as some studies have revealed that temperature plays a role in sex determination plus some genes in autosomes.
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