Egyptian Journal of Aquatic Research (2012) 38, 59–66

National Institute of Oceanography and Fisheries

Egyptian Journal of Aquatic Research

http://ees.elsevier.com/ejarwww.sciencedirect.com

FULL LENGTH ARTICLE

Monosex production of tilapia, Oreochromis niloticususing different doses of 17a-methyltestosterone with respect

to the degree of sex stability after one year of treatment

Z.A. El-Greisy *, A.E. El-Gamal

National Institute of Oceanography and Fisheries, Kaetbey, Alexandria, Egypt

Received 13 June 2012; accepted 6 August 2012Available online 9 November 2012

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KEYWORDS

Oreochromis niloticus;

Monosex production;

17a-Methyltestosterone;

Androgen treatment

Corresponding author.

-mail address: zeinab_elgrei

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d Fisheries.

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Abstract With the purpose of solving some practical problems concerning the complete masculin-

ization of Oreochromis niloticus by oral administration of 17a-methyltestosterone (17a-MT) with

respect to the dose of the hormone used, different doses of 17a-MT (40, 60 and 80 mg of 17a-MT/kg of feed) were orally administered to sexually undifferentiated fries from the 7th to the

28th day posthatching to produce all male tilapia population.

After 15, 35 and 75 days and 1 year of treatment, growth parameters, survival rate, sex ratio and

gonad histology of the resulting larvae and fish were demonstrated. The results showed that the

highest survival rates of the fry were recorded in the group treated with 60 mg of 17a-MT/kg of feed

compared to the control group, 40 and 80 mg of 17a-MT/kg of feed treated groups. The maximum

sex ratios of males (95% and 97%) were recorded at 60 mg of 17a-MT/kg of diet after 75 days and

one year of treatment respectively.

After the first year, the degree of sex stability was observed significantly shifted towards males.

The histological examination showed that the treatment caused an increase in male gonad recrudes-

cence in most of the treated fish. In presumed genetic females, there are oocytes of young stages,

gonial germ cells and different stages of degenerating oocytes within the period around 35 days

posthatching.

The present results concluded that 17a-MT has a potential effect in directing gonadal sex differ-

entiation of O. niloticus towards males according to the dose involved in the treatment.ª 2012 National Institute of Oceanography and Fisheries. Production and hosting by Elsevier B.V. All

rights reserved.

o.com (Z.A. El-Greisy).

nal Institute of Oceanography

g by Elsevier

Oceanography and Fisheries. Prod

05

Introduction

Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) is likely tobe the most important cultured fish in the 21 century (Ridha,2006). It grows and reproduces in a wide range of environmen-

tal conditions and tolerates stress induced by handling(Tsadik and Bart, 2007). With the purpose of achieving more

uction and hosting by Elsevier B.V. All rights reserved.

60 Z.A. El-Greisy, A.E. El-Gamal

productivity in growing tilapia, Oreochromis niloticus, at theunit time, it is important to produce monosex culture that con-stitutes totally of males (Mair and Little, 1991). Male monosex

tilapia cultures are preferred to females because of the differen-tial growth in favour of males. In males, the metabolic energyis channeled towards growth. They benefit from anabolism

enhancing androgens (Tran-Duy et al., 2008; Angienda et al.,2010). In females, there is a greater reallocation of metabolicenergy towards reproduction.

Although monosex male population can be obtained by di-rect or indirect methods, oral administration of Oreochromisniloticus has been reported to be the most preferred methodin commercial uses (Green and Teichert-coddington, 2000;

Wahby and Shalaby, 2010; Celik et al., 2011). 17a-methyltes-tosterone (17a-MT) is a synthetic male hormone which closelymimics the naturally-produced hormone testosterone. The

most common sex-reversal treatment involves giving a pow-dered fish feed to the first-feeding (and still sexually undifferen-tiated) tilapia fry. This diet contains 30–60 mg 17a-MT/kg of

feed until the 25–60th days posthatching (Macintosh andLittle, 1995). Nile tilapia and the other Oreochromis speciesthat dominate commercial tilapia farming are mouth brooders.

After their eggs are released and fertilized, the female brood-fish carry the eggs orally until they develop into fry (Macintoshand Little, 1995). Treatment with 17a-MT should begin fromthe second or third day after the fry are released from maternal

care. However, other studies recommended the treatment tostart from the seventh day posthatching until 30th day(Nakamura and Iwahashi, 1982).

In the present study, three doses of 17a-MT (40, 60 and 80 mgof 17a-MT/kg of feed) were orally administered to sexuallyundifferentiated fry of Oreochromis niloticus from the 7th to

the 28th dayposthatching to produce allmale tilapia population.The present study aims to investigate the potentiality of dif-

ferent doses of 17a-MT with respect to the determination of the

optimum dose and to demonstrate the degree of sex stability inmonosex Oreochromis niloticus after the first year of treatment.

Materials and methods

Experimental design

Broodfish of Oreochromis niloticus were allowed to breed nat-urally in cement ponds with area of 3 · 6 m, and water heightof 40 cm during June 2011 with a sex ratio of 3 females to

1 male/m3 at El-Serw Station for Fish Researches, NationalInstitute of Oceanography and Fisheries, Egypt. About 400of the newly hatched fry were stocked in a tank (150 L). After

seven days posthatching, when the yolk sacs were absorbed,the fry were divided into four groups in glass aquaria, 50 Leach, with a density of 100 fry for each aquarium. The water

of the aquaria were maintained at pH (7–7.4), dissolvedoxygen (5.5–6.5 mg/l), water temperature was ranged from25 to 28 �C and kept under natural light conditions. After28 days, the fish were transported to the cement ponds until

the 75th day, then transported to the rearing ponds.

Preparing of control and steroid containing diet

A control diet, as described by El-Gamal et al. (2007), con-tained 30% fish meal, 30% solvent-extracted soya bean meal,

18% wheat bran, 13% yellow corn, 6% corn oil, 2% vitaminsand minerals premix and 1% carboxy methyl cellulose.

In addition to the control diet, other three experimental

diets were prepared with the addition of 40, 60 and 80 mg/17a-MT/kg of feed.

They were prepared by the method of spraying the hor-

mone dissolved in 50 ml of 95% ethyl alcohol and mixed wellin fine granules of feed. Glycerin was added at 0.5%/kg by vol-ume to render the harmful effect of the alcohol. The mixture of

feed has been completely dried at room temperature and thensealed in air tight black container and stored in refrigeratorsuntil use to retard bacterial or fungal contamination (Celiket al., 2011).The diets containing 17-aMT were characterized

as follows:

Diet (1): Control (untreated).

Diet (2): control diet + 40 mg of 17a-MT/kg of diet.Diet (3): control diet + 60 mg of 17a-MT/kg of diet.Diet (4): control diet + 80 mg of 17a-MT/kg of diet.

Feeding protocol

The diet of 17a-MT was applied from the 7th to the 28th dayposthatching, after that the feeding of the fish continued on thecontrol diet. The feeding was supplied from two to four timesdaily during the day light hours. At the beginning of feeding,

the diet was given as fine granules. The daily ration of feedranged from 15% to 20% of fish weight until the fry reachesan average length of 15 cm. After that, gradual reduction

down to 10% of fish weight daily until the end of theexperiment.

The mean length, weight and survival rate of the fish in each

treatment were recorded. After one year posthatching, atwhich the fish were completely sexually mature, the percentageof males and females were recorded.

Specifying of sex rates

To find out how sexual development after one year of treat-ment, the sex was determined depending on the secondary sex-

ual characters. In small fish, small pieces of gonads were putbetween two slides and examined under stereo-microscope.

For histological investigation, samples of gonads were col-

lected after 15, 35, 75 days and after one year posthatching.The gonads were fixed in Bouin’s solution for a period of48 h and then transported to 70% alcohol until dehydration

process was completed. Fixation process was followed bydehydration, parafinization and sectioning. After that, the go-nad sections were stained with haematoxilyn and eosin and

then examined under light microscope.

Statistical analysis

In order to calculate the statistical significance between the

growth of the control and those of the groups treated with dif-ferent doses of 17a-MT, a comparison of the different param-eters were described according to Fisher (1950) and Sokal and

Rohlf (1969). T-test was used to find out the statistical signif-icance in terms of growth parameters, survival rates and sexratio.

Monosex production of tilapia, Oreochromis niloticus using different doses of 17a-methyltestosterone 61

Results

Effect of Dietary 17a-methyltestosterone on growth parametersand survival rate of the larvae of Nile tilapia, Oreochromis

niloticus

After 15 days posthatching, the highest values of averageweight (1.97 g) and highest survival rate (93%) of tilapia wererecorded significantly (p < 0.05) in the fry treated with 60 mg

17a-MT/kg diet compared to those of groups treated with 40,80 mg 17a-MT/kg diet or in the untreated control group. Onthe other hand, the lowest values of average weight (0.754 g)and lowest values of survival rate (65%) were recorded in tila-

pia treated with 80 mg 17a-MT/kg diet (p> 0.05) (Table 1).After 35 days posthatching, the highest values of average

weight (3.50 g) and the highest survival rate (94%) of tilapia

were recorded in fry treated with 60 mg 17a-MT /kg diet com-pared to those of fry treated with 40 mg 17a-MT/kg diet,80 mg 17a-MT/kg diet or in the untreated control group. On

the other hand, the lowest values of average weight (2.81 g)

Table 2 Effect of orally-administered 17a-methyltestosterone on gro

35 days of treatment.

Treatment category Growth parameters

Mean length (cm)

Min Max Avg ± SD

Control 3.00 3.80 3.45 ± 0.332

40 mg/kg of diet 4.80 3.90 4.28 ± 0.297

60 mg/kg of diet 5.60 4.30 5.06 ± 0.396

80 mg/kg of diet 4.20 3.30 3.79 ± 0.262

a Significantly different compared to the control group (p < 0.05).

Table 3 Mean weights, survival rates of Oreochromis niloticus reare

of 17a-methyltestosterone.

Treatment category Mean weight (g)

Min Max

Control 12.7 15.8

40 mg/kg of diet 14.9 17.0

60 mg/kg of diet 15.1 18.0

80 mg/kg of diet 13.8 16.5

a Significantly different compared to the control group (p < 0.05).

Table 1 Effect of orally-administered 17a-methyltestosteron on gro

15 days of treatment.

Treatment category Growth parameters

Mean length (cm)

Min Max Avg ± SD

Control 1.20 1.80 1.42 ± 0.200

40 mg/kg of diet 2.70 3.70 2.94 ± 0.377

60 mg/kg of diet 3.50 4.20 3.80 ± 0.221

80 mg/kg of diet 3.10 2.20 2.66 ± 0.341

a Significantly different compared to the control group (p < 0.05).

and lowest values of survival rate (60%) were recorded in tila-pia treated with 80 mg 17a-MT/kg diet as recorded in Table 2.

After 75 days of treatment, the percentages of males were 46,

88, 95 and 65% in control, treated fry with 40, 60 and 80 mg of17a-MT/kg diet respectively. After one year of treatment, thepercentages of males were 30, 85, 97 and 93% in control, treated

fry with 40, 60 and 80 mg of 17a-MT/kg diet respectively(Table 5). It is obvious that methyltestosterone influenced thesex ratio in favour of males. However, the percentage of males

is higher in the treated groups compared to the control.Generally, the highest values of growth parameters and sur-

vival rates of the larvae of Nile tilapia, Oreochromis niloticus,were recorded at the dietary dose of 60 mg of 17a-MT/kg diet

compared to the control, 40 and 80 mg of 17a-MT/kg dietafter 15, 35 and 75 days posthatching (Tables 1–4).

Effect of 17a-methyltestosrerone on sex differentiation of thegonads

Based on histological examination, the ovaries and the

testes of Oreochromis niloticus fry were developed from

wth and survival rate of Nile tilapia, Oreochromis niloticus, after

Survival rate (%)

Mean weight (gm)

Min Max Avg ± SD

1.50 2.00 1.766 ± 0.158 92

2.70 3.30 3.060 ± 0.183 90

3.20 3.90 3.500 ± 0.269a 94

2.50 3.20 2.810 ± 0.199 60

d in cement ponds after 75 days of treatment with different doses

Survival rate (%)

Avg ± SD

14.257 ± 1.018 100

15.871 ± 0.755 100

16.782 ± 0.868a 100

15.040 ± 0.644 70

wth and survival rate of Nile tilapia, Oreochromis niloticus, after

Survival rate (%)

Mean weight (gm)

Min Max Avg ± SD

0.40 0.80 0.500 ± 0.440 88

0.50 1.50 0.975 ± 0.359 79

1.80 2.30 1.970 ± 0.154a 93

0.40 1.30 0.754 ± 0.350 65

62 Z.A. El-Greisy, A.E. El-Gamal

undifferentiated gonadal tissue. The key morphologicalevents are described as follows: (see Table 4).

At 15 days posthatching, the undifferentiated gonads of the

group treated with 40 mg of 17a-MT/kg contained small germ

Figure 1 Histological sections of fry at 15 days posthatching treated w

kg of feed, the gonad appears in pear shaped with gonia and primordia

80 mg of 17a-MT/kg of feed, showing somatic cells (arrow head) and

treated with 60 mg of 17a-methyltestosterone/kg of feed, showing clu

ovarian differentiation in control group at age period from 25 to 31 day

The ovary of treated female with 60 mg of 17a-MT/kg of feed, few oo

400·. (f) Presumptive ovary of treated female with 80 mg of 17a-MT/

Table 4 Mean weight of Oreochromis niloticus in the rearing p

methyltestosterone.

Treatment category Mean weight (g)

Min Max

Control 130 180

40 mg/kg of diet 180 200

60 mg/kg of diet 200 230

80 mg/kg of diet 195 210

a Significantly different compared to the control group (p< 0.05).

cells which were observed under mesonephric duct. The germcells were distinguished easily from the somatic cells by theirdefinite, roundish contour, clear aspect of the cytoplasm and

larger nuclear size of 7–10 lm (Fig. 1a). By that time, no

ith 17a-MT/kg of feed. (a) Gonad treated with 40 mg of 17a-MT/

l germ cells (arrow) (H&E) 100·. (b) Indifferent gonad treated with

primordial germ cells (arrow) (H&E) 400·. (c) Indifferent gonadsters of primordial germ cells after 15 days posthaching. (d) The

s was firstly observed with ovarian cavity (arrow) (H&E) 400·. (e)gonia (arrow head) and large area of stromal cells (arrow) (H&E)

kg of feed (H&E) 400·.

onds after 1 year of treatment with different doses of 17a-

Survival Rate

Avg ± SD

157.50 ± 22.17 100

191.25 ± 8.53 100

215.00 ± 12.90a 100

203.75 ± 7.50 95

Monosex production of tilapia, Oreochromis niloticus using different doses of 17a-methyltestosterone 63

prominent changes were observed in histological aspects andnumber of germ cells of the early gonads. At the same time,the gonads of treated fry with 80 mg of 17a-MT/kg, showed

that they began to shift bilaterally from the dorsal root of mes-entery with small number of somatic cells enclosing primordialgerm cells (Fig. 1b). On the other hand, the gonads of treated

fry with 60 mg of 17a-MT/kg (15 days posthatching), thepaired arrangement of the gonad became more distinctly thanbefore. Active mitotic division of germ cells was seen in most

of the gonads. As a result, germ cells became smaller thanbefore making clusters especially in the anterior region of thegonad (Fig. 1c).

At the period from 25th to 31st day, the first sign of ovarian

differentiation was found. At this period, the gonads fall intotwo groups, the larger gonad contained oogonia with clearovarian cavity and smaller gonad contained cysts of germ cells

as shown in female (Fig. 1d).

Figure 2 Histological sections of fry at age of 21–35 days posthatch

with 40 mg of 17a-MT/kg of feed, showing the ovary contained perin

400·. (b) Presumptive testes treated with 60 mg of 17a-MT/kg of feed,

(H&E) 400·. (c) Presumptive testes treated with 80 mg of 17a-MT/kg

efferent duct appeared towards the outer margin of testes (arrow head

treated with 60 mg of 17a-MT/kg of feed, showing many degenerated o

with 80 mg of 17a-MT/kg of feed, showing many degenerated oocytes

the oocyte appeared in ripe stage (arrow) and few numbers of young

During this period, the ovary of female treated with 60 mgof 17a-MT/kg showed that the ovary contained few oogonia inthe anterior region and a large area of stromal cells that were

dense along the mesentery side (Fig. 1e).The ovary of female treated with 80 mg of 17a-MT/kg

showed that the ovary contained few oogonia and some germ

cells underwent the mitotic division and the ovarian cavity wasclearly observed (Fig. 1f). Ovary of female treated with 40 mgof 17a-MT/kg of feed, contained perinucleolus oocytes filled

up the entire ovary and some ovaries contained lamellae(Fig. 2a). On the other hand, the other type of gonad are foundafter 21–35 days posthatching of treatment with 60 mg of 17a-MT/kg of feed showed that the presumptive testes contained a

large number of germ cells under mitotic division. Further dif-ferentiation or spermatogonial growth was apparent in thepresumptive testes. The testes can be differentiated by having

small slit indicating to the presumptive efferent duct

ing treated with 17a-MT showing: (a) the ovary of female treated

ucleolus oocytes (arrow head) and ovarian cavity (arrow) (H&E)

showing primordial germ cells (arrow) and future of efferent duct

of feed, showing primordial germ cells (arrow) and presumptive

) (H&E) 400·. (d) The ovary of female after 75 days posthatching

ocytes. (e) The ovary of female after 75 days posthatching treated

(H&E) 400·. (f) The ovary of control female after 1 year showing

oocytes (arrow head) (H&E) 100·.

Table 5 Percentage of males and females of Oreochromis

niloticus reared in cement ponds after 75 days and 1 year of

treatment with different doses of 17a-methyltestosterone.

Treatment

category

Sex ratio

After 75 days

of treatment

After 1 year

of treatment

Males

(%)

Females

(%)

Males

(%)

Females

(%)

Control 46 54 30 70

40 mg/kg of diet 88 12 85 15

60 mg/kg of diet 95 5 97 3

80 mg/kg of diet 65 35 93 7

64 Z.A. El-Greisy, A.E. El-Gamal

(Fig. 2b). During the same period, the presumptive testes ofmales after treatment with 80 mg of 17a-MT/kg of feedshowed similar histological features in the testes of fry treated

with 60 mg of 17a-MT/kg of feed. However slow developmentin the testis occurred after treatment with 40 mg of 17a-MT/kgof feed (Fig. 2c).

After 75 days posthatching, the ovary of females treatedwith 60 mg of 17a-MT/kg of feed showed that the large num-ber of young oocytes were deformed and devoid area are free

from oocytes appeared towards the center of the gonad(Fig. 2d). After the ovary treated with 80 mg of 17a-MT/kgof feed at the same period, most of the oocytes were deformedand others were degenerated (Fig. 2e). After one year, the con-

trol female contained the oocytes in advanced stage with fewnumbers of small oocytes (Fig. 2f). During the same period,the testes appeared in mature stage in the group treated with

60 mg of 17a-MT/kg of feed (Fig. 3a). However, after one yearposthatching, the testes contained spermatogenic cells in addi-tion to collected sperms inside the seminiferous tubules

(Fig. 3b).

Figure 3 Histological sections of testes treated with 17a-MT/kg

of feed. (a) After 75 days, the testes of male treated with 60 mg of

17a-MT/kg of feed, showing the testes in mature stage, containing

different spermatogenic cells, sperms aggregated in seminiferous

tubules (arrow) (H&E) 400·. (b) After 1 year, the testes of male

treated with 80 mg of 17a-MT/kg of feed, showing the testes

appeared in spawning stage (arrow) (H&E) 400·.

Sex stability towards all male production

After one year, the mean value of gross weight was higher inthe group of fish previously treated with 60 mg of 17a-MT/kg or 80 mg 17a-MT compared to the other groups. The high-

er value of survival rate was recorded either in control or thetreated doses even 40 mg of 17a-MT (Table 5). The percentageof males was 30, 85, 97 and 93 in control and the groupstreated with 40, 60 and 80 mg of 17a-MT diets respectively.

17a-MT influenced the sex ratio and sex stability of males.However, the percentage of males is higher in the treatedgroups with the hormone compared to the control group.

Discussion

Use of 17a-methyltestosterone hormone to induce sex reversal

in farmed tilapias has become a common practice in manyparts of the world. It is a simple and reliable way to produceall-male tilapia stocks, which consistently grow to a larger uni-

form size than mixed sex or all-female stocks. 17a-Methyltes-tosterone is a synthetically produced anabolic andandrogenic steroid hormone, i.e. it promotes both muscle

growth and the development of male sexual characters. It clo-sely mimics the naturally-produced hormone testosterone(Phelps and Popma, 2000).

A key factor in the success of sex reversal treatments is the

amount of hormone that is actually ingested by each individualfish during its labile period of sexual differentiation. Therefore,in the present study, different concentrations of 17a-MT con-

taining feed were applied so as to observe the effect of thesedifferent doses of 17a-MT during the first 28 days of feedingon sex differentiation.

In the present study, the potentiality of different doses of17a-MT (40, 60 and 80 mg of 17a-MT/kg of feed) was investi-gated and compared to the control group. Monosex tilapiashowed significantly higher weight, length, survival rate com-

pared to the control group (p < 0.05). However, the resultsof our study indicated that certain doses of 17a-MT appearto have a higher anabolic effect on the fry of Oreochromis

niloticus rather than other doses. The higher values of weightof the fish treated with 60 mg of 17a-MT/kg of feed comparedto that of the control, 40, and 80 mg of 17a-MT/kg of diet

after 15, 35 and 75 days of the treatment can be attributedto the anabolic effect of 17a-MT (Jo et al., 1995). In thisrespect, some studies reported that 17a-MT treatment showed

an increase in individual growth of tilapia (Mair et al., 1995;

Monosex production of tilapia, Oreochromis niloticus using different doses of 17a-methyltestosterone 65

Dan and Little, 2000; Little et al., 2003). Other studiesreported that the higher mean weights could be attributed tothe improvement of food conversion efficiency of sex-reversed

fry of Oreochromis niloticus (Chakraborty and Banerjee, 2010).The average of the survival rate increases with the increase

of age of the larvae. However, some studies reported that 17a-methyltestosterone treatment has no effect on survival of tila-pia (Vera Cruz and Mair, 1994; Chakraborty et al., 2011). Thesex ratio also showed an increase in the group treated with

17a-MT. After 75 days of treatment, the percentage of malesincreased to reach the maximum value (95%) at 60 mg of17a-methyltestosterone/kg of feed. After one year, the percent-age of males increased to reach the maximum value (97%) at

the same dose.It has been reported by Barry et al. (2007) and Green and

Teichert-Coddington (2000), that over 95% of the population

was masculinized in 21–28 days when 30-60 mg 17a-MT/kgfeed with 17a-MT that was applied orally to the tilapia larvae(7–12 days of age, 9–11 mm TL and 10–15 mg of total weight).

In the present study as well, the highest sex reversal occurred at60 mg/kg feed dose.

Methyltestosterone suppressed the oogenesis. This inhibi-

tory effect on the development of oocytes is dependent on thedose of methyltestosterone. The ovary is almost occupied by so-matic elements (Wolf et al., 2004). It is important to considerthe dose of the hormone to avoid the problems related to over-

doses. Goudie et al. (1983) reported that excessive doses of hor-mone lead to sterility or paradoxical feminization followingaromatization of androgens to estrogens, although sub-optimal

treatments resulted in intersexes (Popma and Green, 1990).In the present study, even though the administration of

17a-MT was commenced prior to the stage of ovarian differen-

tiation, it could not completely prevent the presumed geneti-cally females to produce at least young ooytes at the earlystages of maturation.

Generally, methyltestosterone influenced the sex ratio in fa-vour of males. Independent on the dose, the percentage ofmales is higher in the treated groups.

Depending on histological observations in the present

study, the first sign of gonadal development is the formationof genital ridge and appearance of primordial germ cells(PGC). They were located between the gut and kidney. Similar

observations were recorded by Nakamura (1984) on two spe-cies of salmonids.

As described previously, the initial ovaries either in control

or in treated groups were easily identified by their well devel-oped perinucleolus oocytes, whereas the testes did not showa clear distinction from undifferentiated gonads. Similar obser-vations were noticed by Sacobie and Benfey (2005).

As described in Tilapia mossambica, treated with methyltes-tosterone at dose of 50 lg/g of diet, some germ cells in the go-nads of possible genetic females underwent oogenesis even

under the influence of androgen, but were found to be degen-erated eventually (Nakamura et al., 1974). High doses of 17a-MT did not release milt in Grouper, Epinephelus suillus. It

caused malformation or even agenesis of the sperm duct sys-tem (Tan-Fermin et al., 1994).

In this respect, long-term administration of high dosages of

methyltestosterone has also been reported to inhibit spermia-tion in immature gonochoristic milk fish (Lee et al., 1986).

In conclusion, the present study establishes base line regard-ing the best dosage of 17a-MT (60 mg/kg of 17a-MT) on sex

reversal of Oreochromis niloticus fry. It revealed that the highervalues of mean length, weight and survival rates were recordedin fish treated with 60 mg of 17a-MT/kg of feed. Further re-

searches on histological and morphological characteristics ofgerm cells of the gonad at that critical period of gonadal devel-opment must be studied to throw the light on how mechanism

of hormone induced sex reversal in fish. Other factors ratherthan the dose of the hormone could be investigated in combina-tion with the hormone dose to increase the percentage of male

produced.

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