An objective of this study was to find out the minimum dose and time
of 17-alpha methyl testosterone to obtain all male population of Mozambique
tilapia (Oreochromis mossambicus).
Aquaculture now accounts for over 20% of the world fish production for
The consultative group on International Agricultural Research has recently
predicted that within 15 years fish farming and sea ranching could provide
more than 40% of fish for the human diet and more than half of the value
of the global fish catch. The world is in a state of transition from the
hunting and gathering of fish to the production of fish by aquaculture.
In these days one of the most popular choices for aquaculture is tilapia
Early sexual maturity in tilapia culture is a well recognized problem
which resulted, inbreeding in overstocked ponds, reduced production and
farmed stocks of a generally low quality. To overcome these problems and
to develop improved breeding stock of tilapia, fish scientists, economists
and commercial fish producers joined together and decided to produce mono-sex
and preferably all male population of tilapia because the male of tilapia
grows faster than females (Barras and Melard, 1997).
There are different ways such as hormonal sex reversal, manual sexing,
hybridization and super male production to attain, mono sex population.
Sex reversal method has been used as a valuable tool in the elucidation
of sex determining mechanisms in addition to its value in production of
mono sex population for aquaculture. Sex reversal by oral administration
of feed incorporated with Methyl Testosterone (MT) is the most effective
and practical method for the production of all male tilapia. Dosage of
17-alpha Methyl Testosterone (MT) used to produce all male tilapia, vary
wildly. The dosage rates vary from 10-100 mg kg-1 MT of diet
for tilapia (Abucay and Mair, 1997). Sex reversed tilapia showed a better
growth rates than normal because administration of androgen have both
an androgenic and anabolic effect. There are several studies comparing
the growth of sex reversed, near all male populations, to that of a mixed
sex population after hormone treatment showed the improved growth of sex
reversed fish than non-treated because the presence of females reduce
the growth rate due to their slower growth rate or reproduction (Mocintosh
et al., 1985). Sex reversal may also affect the meat quality of tilapia
(Bhandari Nakamura et al., 2006). There are several stasis in regard
of effect of different feed ingredients on meat quality of tilapia but
a scarcity of work is persisted about the effect of sex reversal on meat
quality (proximate body composition) of tilapia. The aim of this study
was, therefore, to find out optimum dose rate of MT treatment for sex
reversal along with its effects on growth performance of tilapia (Oreochromis
mossambicus) in hydrochemistry factors of Iranian waters.
MATERIALS AND METHODS
The experiment was conducted by Marine Biology Group, Department of Marine
Science and Oceanography, Sciences and Researches Branch, Azad Islamic
University, Tehran, Iran.
The sex reversal of tilapia fry was achieved by oral administration of
17 alpha methyl testosterone through feed. MT was obtained from Aborihan
drug company. At first 400 fertilized eggs were obtained from the moth
of several parent females and shifted to circular tank in order to get
the tilapia fry after three days (7 day post fertilization (1 dpf)) of
hatching, the fries were placed into 12 glass tank each having 65 L water
each experimental trial consisted a control group and a group fed with
ethanol treated diet (fish were fed with a diet treated with ethanol).
For sex reversal treatment three different dosage groups (50, 75, 100
mg kg-1 MT of food (each in 2 replicates) were set out, MT
was added to diet with Alcohol (Ethanol 96 °) dry method. And each
trial was stocked with 35 fish.
The fry were fed daily with a commercial diet (dry pellet food, copens
Brand) for 21 days (28 dpf). The important water quality parameters was
fixed and recorded, for e.g., temperature (28 ± 1*), dissolved
oxygen (7.2 ± 0.2 ms L-1) and pH (7.7 ± 0.3).
Fry were maintained by increase in 20% Body Weight (BW) per day and the
important growth parameters such as body weight and total length were
recorded at 7 dpf (start of feeding), 28 dpf (end of feeding) and 42 dpf
(2 weeks after the end of feeding).
The experimental design was tested by RCBD (Randomized Complete Block
Design). At the end of the experiment, the head and tail of the fry were
cut off and the body was fixed in 10% neutral buffered formalin, dehydrated,
embedded in paraffin sectioned at (5 μm thicknesses) and stained
with haematoxylin and eosin for histological examination. The tissues
were observed under microscope 40x powers. Tissues were recorded as testes
(male), ovaries (female), ovotestes or intersex (gonads containing both
ovarian and testicular tissues) and sterilized gonads.
Logistic model regression was used to analyze the data of sex reversal.
These models were used to study such type of experiment where there is
a binary response, Y = 1 (male in this case), Y = 0 (not male), which
may be influenced by explanatory variables. For the statistical analysis
of the means of growth performance parameters Analysis of Variance (ANOVA)
and Tukey test, were carried out. SPSS software was used for statistical
analysis and chi square analysis was done to analyze the sex ratio. The
present research was conducted from 4 May 2007 to 20 August 2008.
RESULTS AND DISCUSSION
Result of the present study showed that each hormone treated group gave
a mean male/female ration that deviate significantly from the normal 1:1
ratio (Chi-square analysis) (Table 1), with significantly
higher growth rate in males than females, while the control group showed
normal 1:1 ration. Non of the experimental trial (for dose rate of MT)
gave 100% male population of O. mossambicus. The results of the
present study showed that all MT receiving treatment showed a significantly
higher male proportion than control. When considering mortality, no significant
different was observed between control group, dose 50 and dose 75 but
mortality in dose 100 mg kg-1 MT was significantly higher than
the other groups. Control group exhibited 51.96% male population and 0.95%
mortality. Application of dose rate of 50 mg kg-1 of MT showed
74.29% male population and 0% mortality. Application of dose rate of 75
mg kg-1 MT of feed resulted in maximum male population (98.09%)
with 1.91% sterilized fish. Mortality in this trial was 0%. Application
of dose rate of 100 mg kg-1 hormone showed 79.38% male population,
20.61% sterilized fish with 7.62% mortality (Table 1).
Mortality and sterilized fish when applying 100 mg kg-1 MT
was significantly higher than other treatment.
Greater than 90% of male population were obtained at a variety of dose
rates, for e.g., Jae-Yoon et al. (1988) obtained 97% of O. niloticus
males when applying dose rate of 10 mg kg-1 MT of diet. Other
researchers have used the higher dose rate to achieve sex reversal.
Oreochromiss sp., Romerio et al. (2000) obtained 98% male
population at dose rate of 60 mg kg-1 MT of feed. The results
of this study showed a significantly lower male proportion (84.3%) for
highest dose rate of MT, 100 mg kg-1 of feed. These results
are in line with the findings of Okoko (1996), who obtained 71.9% males
at the dose rate of 120 mg kg-1 MT of feed. Also other results
showed the over dose such as 100 mg kg-1 MT give the sterilized
Different dose rates of MT, significantly effected the growth of Oreochromiss
mossambicus, all the treatments which received MT, showed more average
(Av) Body Weight (WB) and gain in body weight of Oreochromiss mossambicus,
than the control (Table 2). Dose 7.5 mg MT for 21 days
showed 17.4 g gain in weight followed by dose of 100 mg kg-1
MT and 10.9 g, by dose 50, 10.1 g at the end of the experiment.
||Percentage of Oreochromis mossambicus classified
as male, female and sterilize under control, ethanol group and different
MT sex reversal treatment
|*The Latin character show the result of Tukey analyze,
same character don`t have significantly difference but unlike character
have a significantly difference
|| Observation on average and gain in weight body of Oreochromis
mossambicus under different treatments
|Av: Average, BW: Body Weight
Statistical analysis on gain in body weight, showed a significant difference
among different reversal treatments. Treatment received 75 mg kg-1
MT of feed for 21 days (having the highest male percentage of 98.01%).
Showed the highest gain in body weight, this fish production is 1.3 times
greater than control (received no hormone), these results are in line
with the findings regarding anabolic effect of MT in fish and all male
culture of tilapia by different authors observed faster growth of hormone
treated O. niloticus.
Hanson et al. (1984) reported that 10-60 ppm MT-treatment showed
the best growth than control, these are also in line with Dan and Little
(2000), who compared the culture performance of different strains of O.
niloticus and found that considering all strains, MT treatment resulted
in a final size of fish 10.7% larger than mixed sex fish.