EFFECTS OF ANDROSTENEDIONE AND METHYLTESTOSTERONE
ON Oreochromis niloticus FRY TREATED FOR SEX REVERSAL
IN OUTDOOR NET ENCLOSURES
R.D. Guerrero III
Philippine Council for Aquatic and Marine Research
and Development, Los Baños, Laguna, Philippines
L.A. Guerrero
Aquatic Biosystems, Bay, Laguna, Philippines
Abstract
The effects of androstenedione and methyltestosterone orally adminsitered to sexually
undifferentiated fry of O. niloticus at 0.30, and 50 mg/kg diet for 21 days in outdoor
net enclosures were tested. The results showed that while androstenedione-treated fish had
significantly higher male percentages than that of the control, higher percentages of males
were achieved with methyltestosterone compared with androstenedione. The possible
anabolic effect of methyltestosterone on O. niloticus was noted after 30 days of culture
from sex reversal treatment. Androstenedione appeared to have a negative effect on fish
survival. No apparent differences in the growth and survival of the control and androgen
treated fish were observed 105 days after treatment.
Introduction
The Nile tilapia, Oreochromis niloticus, is an important foodfish cultured in many
developing countries. In the Philippines, the Nile tilapia, an introduced species, is
considered second in importance only to the native milkfish (Chanos chanos).
The efficient culture of the Nile tilapia particularly in ponds requires population
control of the fish because of its early maturing and frequent spawning characteristics. An
effective and practical method applied commercially in the Philippines and other countries
for producing all-male tilapia populations is induced sex reversal (Guerrero, 1996; Popma
and Green, 1990). The method involves the use of a synthetic androgen like
methlytestosterone which is orally administered to sexually undifferentiated tilapia fry at a
given dosage and duration. The sex reversal treatment of fry using outdoor tanks and net
enclosures is now common practice.
While the use of methyltestosterone for induced sex reversal of tilapia is acceptable in
the Philippines, Taiwan, Thailand and Israel, its application for food animal production is not
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allowed in France, the United Kingdom and the United States. There are questions on the
possible negative impact of the excreted metabolic products of the chemical on the
environment (Baroiller and Toguyeni, 1995).
To overcome the constraint on the use of methyltestosterone, the natural androgen,
androstenedione (11 -hydroxyandrostenedione) was used by Baroiller and Toguyeni (1995)
for the sex reversal of O. niloticus. They found the treatment of the fry in aquaria with 10-35
mg/kg of androstenedione in the diet for 21 days as effective as methyltestosterone at the
same levels for producing all-male fish.
This study was conducted to compare the effects of androstenedione and
methyltestosterone on O. niloticus fry treated for induced sex reversal in outdoor net
enclosures.
Materials and Methods
Three tests were done for the study. In the first test, 5,000 swim-up fry of O.
niloticus (Egyptian strain) were collected from a breeding pond and stocked in five 2 x 1 x 1
m fine-mesh net enclosure (hapas) at 1,000 fry/hapa in a 200-m2 earthen pond with 0.5 m
water depth. The androstenedione and methyltestosterone were obtained from Sigma
Chemical Company (USA) and mixed with the fry diet containing 35% crude protein
following the method of Guerrero (1975). The fry in the hapas were fed to satiation
separately with each of the following diets: O (control), 30 androstenedione (AD), 30
methyltestosterone (MT), 50 AD and 50 MT mg/kg diet. The mean weights and survival of
the fry in each hapa were measured following the sex reversal treatment period of 21 days.
In the second test, the postfry in the first study were restocked in 2-mm mesh size net
cages (2 x 1 x 1 m) at 250/cage with three cages (replications) for each of the control and
androgen-treated groups. The fish were fed with a normal diet (no androgen) containing
30% crude protein at the rate of 6-9% of fish body weight given in two feedings each day and
adjusted on a weekly basis. After a 30-day culture period, the mean weights and survival of
the fish in each hapa were measured.
In the third test, the fish from the second test were further reared in 2 x 1 x 1 m net
cages with 5-mm mesh size at 100/cage and three cages each for the control and androgentreated groups. The fish were fed at the rate of 3-5% given in two feedings each day and
adjusted on a biweekly basis. After 75 days of culture, the mean weights, survival and
percentage of males of fish in each cage were determined. Sexing of fish was done by
examining the urogenital papillae of all the fish in each cage.
Results
The results of the sex reversal treatment in the first test showed no differences in the
mean weights of the control and MT-treated fish. The mean weight of the AD-30-treated fish
appeared to be lower then that of the control while that for the AD-50-treated fish was
higher. The survival rates of fish treated with AD-30 and MT-30 were higher compared with
that of the control while those for the AD-50 and MT-50-treated fish were lower (Table 1).
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Table 1. Mean weights and survival rates of O. niloticus frya following sex reversal
treatment in hapas for 21 days.
Treatment
a
Mean Wt.
(g)
% Survival
Control
0.19
82.3
AD-30
0.17
91.2
AD-50
0.23
78.5
MT-30
0.19
87.7
MT-50
0.19
77.6
Initial mean weight = 0.01 g
The results of the second test showed that the mean weights of the MT-treated fish
were greater than that of the control while those of the AD-treated fish were similar to that of
the control. Lower survival rates for the AD-treated fish were obtained compared with those
of the control and MT-treated fish (Table 2).
Table 2. Mean weights and survival rates of O. niloticus postfry in net cages after 30
days of rearing.
Treatment
Mean Wt.
(g)
% Survival
Control
3.0
95
AD-30
3.5
85
AD-50
3.0
70
MT-30
5.0
91
MT-50
6.5
93
Note: Figures are means of three replicates.
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The results of the third test showed no apparent differences in the mean weights and
survival rates of the control and androgen-treated fish after 105 days of culture. The
androgen-treated fish, however, had higher percentages of males compared with that of the
control. The percentages of males of the MT-treated fish were higher than those of the ADtreated fish. More males were found in the AD-50 treated fish than in the AD-30 treated
batch (Table 3).
Table 3. Mean weights, survival rates and percentages of males of control and androgentreated O. niloticus fingerlings reared in net cages for 75 days
Treatment
Mean Wt.
(g)
% Survival
% Males
Control
17.7
100
59
AD-30
16.2
100
74
AD-50
15.0
98
81
MT-30
15.8
100
96
MT-50
18.0
100
96
Note: Figures are means of three replicates.
Discussion
The androgen treatments had no apparent effects on the growth and survival of O.
niloticus fry during the sex reversal treatment of 21 days in outdoor hapas. While the mean
weights of fry treated with androstenedione were the lowest and highest, these could have
been due to other factors other than androgen effect. The growth and survival of fry during
sex reversal treatment are dependent on factors such as stocking density, feeding,
temperature and other environmental conditions (Bocek et al., 1992). Jo et al., (1995) found
fry of O. niloticus treated with MT at 5-25 mg/kg diet to be significantly heavier than the
control after the sex reversal period. They suggested that MT had an anabolic effect in the
fish. However, Vera Cruz et. al., (1994) did not find any significant effect on growth and
survival of O. niloticus fry during the treatment period with MT at 40 mg/kg diet. From the
varying results, it appears that lower levels of the androgen may have a positive effect on the
growth and survival of the fry during the treatment period compared with higher levels. Our
data indicate this with respect to survival of the treated fry.
The higher mean weights of the MT-treated fish compared with that of the control
after 30 days from sex reversal treatment can be attributed to the anabolic effect of the
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androgen (Jo et al., 1995). Hanson et al. (1983) reported on the improved food conversion
efficiency of sex-reversed fry of O. niloticus with methyltestosterone.
There is a dearth of information on the use of androstenedione for sex reversal of
tilapia. Baroiller and Toguyeni (1995) did not indicate the comparative effects of
methyltestosterone and androstenedione on the growth of O. niloticus fry. The results of our
study indicate that androstenedione appears to have little or no anabolic effect on O. niloticus
fry at the levels tested. The low survival of AD-50 -treated postfry compared with that of the
control seems to indicate the negative effect of the higher level of androstenedione on
survival of O. niloticus postfry.
There appears to be little or no effect of the androgens on the growth and survival O.
niloticus fingerlings reared in the net cages after 105 days from sex reversal treatment. This
suggests that the anabolic effect of exogenous androgens is probably limited to a certain
period following sex reversal treatment. The higher percentages of males of the androgentreated fish which are significantly different from that of the control (chi-square test) show
that androstenedione and methyltestosterone are effective in masculinizing O.niloticus.
However, the higher percentages of males obtained with MT compared with those using AD
strongly indicate the greater potency of the synthetic androgen under the conditions of our
study. Synthetic androgens are generally more potent than natural androgens for
masculinizing fish (Yamamoto, 1969). The potency of methyltestosterone is attributed to the
methyl group which makes it more resistant to excretion (Donaldson et al., 1979).
Acknowledgement
The authors gratefully acknowledge the support provided by the CIRAD-EMVT of
France for the conduct of the study. The suggestions and encouragement of Drs. Jerome
Lazard and Jean Francocis Baroiller are highly appreciated.
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