Effects of Androstenedione and Methyltestosterone on Oreochromis Niloticus Fry Treated

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

1 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 androgen- treated 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).

2 Table 1. Mean weights and survival rates of O. niloticus frya following sex reversal treatment in hapas for 21 days.

Treatment Mean Wt. % Survival (g)

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

a 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. % Survival (g)

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.

3 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 AD- treated 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 androgen- treated O. niloticus fingerlings reared in net cages for 75 days

Treatment Mean Wt. % Survival % Males (g)

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

4 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 androgen- treated 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.

References

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