Tilapias have good growth rate under wide variety of natural and prepared diets and they are resistant to disease. However, one of the major limitations in the culture and production of tilapia is the prolific breeding nature leading to overpopulation of culture system (Balarin and Hatton,1979; Mair and Little,1991). Apart from these, tilapia has early onset of sexual maturation leading to suppression of growth causing reduction in yields. Solutions in addressing these problems include the production of all male population through hormonal treatment or manual sexing, polyculture with carnivorous species, high stocking population, androgenesis induction using ultra violet ray, culture in cages, and production of monosex hybrids (Balarin and Hatton, 1979; Mair and Little, 1991; Olufeagba, 1992). Apart from these, the production of monosex fingerlings through sex reversal is another major method in solving the problem of prolific breeding in tilapia (Guerrero and Guerrero, 1988; Desprez et al., 1995). Male tilapia is preferred over the female ones because of its uniform size and fast growth. According to Vera and Mair (1994), all male tilapia could be produced through oral administration of sex hormones which is employed to control the sexual development of this species and produce monosex fish.

Testosterone is a naturally occurring hormone in the human body, and is necessary for normal functioning. However, according to Arky (1997), at elevated levels there can be toxic and carcinogenic effects because they are growth promoters. According to Macintosh and Little (1995), estrogens, testosterones, and progesterone which are similar in their chemistry to MT, are used in varying amounts and combinations to treat menopausal symptoms. Apart from this, MT closely resembles the naturally-produced hormone testosterone and therefore, it is used widely in human medicine as a hormone supplement and in agriculture to enhance weight gain in many live stocks. It promotes both muscle growth and male sexual characters. It is also used in human medicine to treat women with breast cancer or breast pain. This study therefore is aimed at the production of all male O. niloticus through sex reversal using 17 α-methyltestosterone and evaluating their performance along with normal mixed sex population.

Materials and Methods
Fresh hatchlings were collected from the mouth of the females maintained in concrete outdoor tanks and move indoor for sex reversal treatment in well aerated 60 cm x 30 cm x 30 cm glass aquaria tanks. Three sets of 100 gms coppens^® powdered feed were weighed.

The first was mixed with 30 µg of 17 α- methyltesto-sterone (Sigma E-4876) which was dissolved in 95% ethanol, hand mixed in a plastic bowl and oven-dried at 65℃ for an hour. The first control feed was only mixed with 95% ethanol and also oven-dried at 65℃ for an hour (Ethanol-feed, control 2, MT was not added). After drying, the feeds were re-mixed and made to powder. The third set of feed that served as the control for the two preparations was not mixed with anything (Control 1). Feeds were stored at 5℃. Water quality parameters such as temperature, dissolved oxygen and pH were monitored fortnightly. After twenty eight days, the fry were transferred into outdoor tanks and fed with 25% crude protein formulated diet. The sex reversed and controls were reared up to sexual maturity in 1 m x 1 m x 1 m outdoor concrete tanks. The tanks were constantly observed for spawning. The sexes of fish were identified through the genital papilla.

Results and Discussion
The survival of hatchlings is shown in Table 1. The percentage survival of all male tilapia in outdoor tanks (93.3%) after six months of rearing in concrete tanks was not significantly different from the control (P>0.05) (Table 2). The percentage survival was 100% in the control and 93.3% in treated group (Table 2). There is no significant difference (P>0.05) in the survival of the treated group and the two control. The average weight of MT treated fish (114.3 gm) after six months of rearing in outdoor concrete tanks was higher than in the two controls, ethanol treated feed (60.7 gm) and control treatment (70 gm) (Plate 1; Figure 1). The average weight of MT treated males is 114.3 gm compare with 70 gm for males in the control without MT treatment. The implication of this is that, apart from reversing the sex of fry, MT treatment also has anabolic activity leading to increase in size. Similar observation has been reported in Coho Salmon fed with 17 α methyltestosterone at the rate of 10 µg/kg of food for 8 weeks. On the contrary, Guppies growth was suppressed by feeding ethynyltestoste rone to fry from birth up to 56 days. It is clear that 30 µg/g MT treated feed is adequate in producing all male as the only “female” in the MT treated group has it ovary undeveloped. The average water quality parameters were all found to be within the desirable optimum range (Temperature=27.2, pH=7.3, Conductivity = 256 and dissolved oxygen=7.1). The water quality parameters during the experimental period demonstrated desirable levels suitable for MT sex reversal.

Plate 1 Photomacrograph of ethanol treated control and all male tilapia (MT-treated)

Table 1 Mean performance of MT treated and control O. niloticus in indoor rearing system

Table 2 Mean survival rate of control and MT-treated fish reared in concrete tanks

Figure 1 A Average length of control and MT treated fish in concrete tanks

Figure 1 B Average weight of control and MT treated fish in concrete tanks

Based on the results of this study, the use of 17 α methyltestosterone (MT) at 30 µg/g diet for 28 days starting immediately after yolk absorption is perfectly ideal under tropical conditions for the production of all male O. niloticus. According to Macintosh (1995), the amount of MT consumed by fry for the 28 days of treatment is very low compared to human testosterone production level. This should allay any fear of negative influence on humans through consumption of sex reversed male using MT. At the time when the feeding with MT stops, the average size of the fry was 0.2 (Table 1). Assuming a food conversion ratio of 1:1, then the quantity of hormone per fish is not more than 0.5g X 0.2/1000 gm = 0.0001 mg. Furthermore, tilapia excrete ingested hormone so rapidly and has been reported that MT level falls below 1% of body weight within a hundred hours of withdrawing the treatment. More importantly, fry are still further raised for five months to attain marketable size before selling them for consumption, when MT residue in the body tissue must have reached a zero level.

References
Arky R., 1997, Physicians' Desk Reference, 51 ed., Montvale, NJ, p.752

Balarin T.D., and Hatton P.J., 1979, Tilapia A guide to their biology and culture in Africa. University of Stirling

Desprez D., Melard C., and Philippart J.C., 1995, Production of a high percentage of male offspring with 17 α-ethynylestradiol sex-reversed Oreochromis aureus. II. Comparative reproductive biology of females and F2 pseudofemales and large-scale production of male progeny. Aquaculture, 130: 35-41
http://dx.doi.org/10.1016/0044-8486(94)00312-C

Macintosh D. J., 1995, Risk associated with using methyltestosterone in tilapia farming, http://media.sustainablefish.org/mt_wp.pdf

Macintosh D.J., and Little D.C., 1995, Nile Tilapia (Oreochromis niloticus).  pp.277-320. In: N.R. Bromage, and R.J. Roberts (eds.). Broodstock Management and Egg and Larval Quality. Blackwell Science, Oxford, UK, p.432
Mair G.C., and Little D.C., 1991, Population control in farmed tilapia. NAGA, 14: 8-13

Olufeagba S.O., 1992, Hybridization trial in three species of Tilapia (Tilapia zillii, Oreochromis niloticus, and Sarotherodon galilaeus). M. Sc. Thesis. Wildlife and Fisheries Management Department, University of Ibadan

Vera Cruz E.M., and Mair G.C., 1994, Conditions for effective androgen sex reversal in Oreochromis niloticus (L.). Aquaculture, 122:237-248
http://dx.doi.org/10.1016/0044-8486(94)90513-4