International Journal of Aquaculture, 2013, Vol.3, No.7, 30
-
34
http://ija.sophiapublisher.com
33
culture system. For example, Chakraborty et al. (2011)
documented the faster growth of all male tilapia than
females and mixed-sex. This might be attributed to
sex-specific growth ability, female mouth brooding
behavior or the efficient feeding habits of males. In a
mouth brooding fish like
O. niloticus
females fast
during the early stages and probably throughout the
brooding period which causes inconsistent feeding
and subsequently affects the body condition (Tadesse,
1988;
Demeke, 1994). Pandian and Sheela (1995) and
Green et al. (1997) further reported similar result,
where all male tilapia showed superior growth rate over
the females and mixed-sex which is in agreement
with the results of the present study. They attributed
this to the fact that energy is not utilized for
reproduction and there exist no competition with
younger fish in all male tilapia culture.
3
Conclusions and Recommendations
The results of the present study revealed that the
growth performance between male mono-sex and
mixed-sex
Oreochromis niloticus
reared under the
same culture condition was significantly different,
where the male mono-sex fish grow faster and
attained a larger size (2 times) than mixed-sex.
Therefore, culturing all male tilapia will enable fish
farmers to produce twice a year. However, it is
very difficult to get 100% all male tilapia using
manual sexing. Therefore, we strongly recommend
action oriented on-farm training for fish farmers
before implementing any stocking program to
enhance the accuracy of manual sexing of small size
fish (<40 g).
4
Materials and methods
4.1
Study area
National Fisheries and Other Living Aquatic Resources
Research Center-Sebeta (08°54′N; 38°38′E) is located
in south-west Ethiopia some 24 km from the capital
Addis Ababa, at an altitude of 2 220 m a.s.l.
4.2
Sampling
Physico-chemical variables: Dissolved oxygen (DO),
pH, specific conductivity and water temperature
were measured on monthly basis using a multi-probe
(
Model HQ40d, HACH Instruments).
Total length
(
TL) and total weight (TW) of 50% of the stocked
fish were measured from each experimental pond
every month throughout the study period. Fish were
fed with wheat bran between 10:00 h and 16:00 h at
a daily feeding rate of 5% of fish body weight, and
the daily ration was then calculated and adjusted
regularly according to the body weight gain of the
fish every month. Dead fish were removed and
recorded to evaluate the survival rate of the stocks.
Subsequently, the total fish length and total weight
were measured to the nearest 0.5 cm and 0.1 g,
respectively. Fulton’s condition factor (k), an indicator of
the “well-being” of an individual fish was calculated
using the following formula (Lagler, 1956):
100
3
Length
Weight
K
The daily growth rate, total weight gain and survival
rate were calculated following the equations given
below (Ridha, 2006):
DGR (g/day) = Final weight (g) – Initial weight
(
g)/culture period; Weight gain (g) = Final weight (g)
–
Initial weight (g); Survival rate (%) = (Number of
fish harvested/Number of fish stocked)
×
100;
FCF =
TW/TL
3
×
100.
where TW is total body weight in g, TL total length in cm.
4.3
Culture techniques
Growth performance of mixed-sex and male mono-sex
were evaluated for eight months in earthen pond.
Ponds were prepared by draining and sun drying for
two weeks. Wire mesh enclosures were installed on
the water inlets and outlets of each pond to prevent
entry of unwanted fish and escape of experimental
fish. Lime was spread over the pond bottoms at the
rate of 250 kg/ha and the ponds were filled to 30 cm
deep with water to increase the alkalinity of the
ponds. After 5 days the level of water in all ponds
was raised to 1 m.
Oreochromis niloticus
fingerlings
collected from Lake Babogaya were stocked and
acclimatized at Sebeta earthen ponds for a period of
two weeks. Prior to stocking sexing was done
manually by visual inspection of the external
urogenital pores with the aid of magnifying hand
lens. Each pond was stocked at a stocking density of
2
fish/m
2
.
The average initial weight at stocking was
39.35
g and 43.60 g for mixed and male mono-sex,
respectively. The relatively higher initial body weight
of fish in the male mono-sex group was intentional to
reduce percentage of errors in identifying sexes of
Oreochromis niloticus
at smaller size.