International Journal of Aquaculture, 2013, Vol.3, No.7, 30
-
34
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32
Figure 1 Growth curve of live body weight for male mono-sex
and mixed-sexd
Oreochromis niloticus
during the study period
(
mean ± SD)
Data on economic performance of
Oreochromis niloticus
for both treatments are depicted in Figure 2. Net revenues
over variable costs were lower in the mixed-sex than
in the male mono-sex group. However, both
treatments showed positive net return for 6, 7 and 8
culturing months. Apparently, there was a difference
in attaining optimum net return between male
mono-sex and mixed-sex group (Figure 2).
2
Discussions
2.1
Water quality
Performance of fish in aquaculture is extremely dependent
on the water quality. For fish to effectively feed,
grow, excrete wastes and reproduce in a water body,
its aquatic environment has to be optimum and conducive.
Figure 2 Net returns from culturing
Oreochromis
niloticus
as
male mono-sex and mixed-sex at different culture periods, 6
th
,
7
th
and 8
th
months
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Both physical and chemical aspects of the aquatic
environment must be suitable to the requirements of
the cultured fish species. Based on the data presented
herein, the key water quality parameters: temperature,
dissolved oxygen, pH and conductivity measured
during the study period were all within the optimum
range for rearing tilapia (Boyd and Tucker, 1992; Xu
et al., 2005; Azaza et al., 2008). The percentage
dissolved oxygen level barely fell below 100%, with
slight super-saturation occurring in both experimental
ponds. Specific conductivity varied strongly among
the experimental ponds, however the values were within
the range recommended for fish ponds (Boyd, 1997;
Hargreaves and Heussel, 2000). As referenced in
table 3, Hanley (2005) gave environmental requirements
of
Oreochromis niloticus
.
Table 3 Some environmental physiology of
Oreochromis niloticus
Variables
Optimumrange for tilapia species
Description
Temperature (
℃
)
25.0-32.0
< 22
℃
:
Reproduction stops; < 16
℃
Feeding stops
Salinity (
‰
)
0-20.0
< 10
‰
:
Fecundity decreases, however some will reproduce
pH
0.5-8.0
Strongly influenced by photosynthesis & hardness
Dissolved oxygen (mg/L)
3.0-9.0
Highly influenced by temperature and partial pressure of other gases
Ammonia-N (mg/L)
0.0-0.5
Influenced by pH and temperature
Hydrogen sulfide (mg/L) < 0.01
Highly dependent on dissolved oxygen
Note: Source: Hanley, 2005
2.2
Fish growth
Although, the growth performance of
Oreochromis
niloticus
is highly influenced by genetics, quality
and quantity of food, stock management and
environmental factors (Gjedrem, 1997; El-Sayed, 1999),
sex-specific differences in the growth of
O.niloticus
is apparent (Green et al., 1997; Schreiber et al., 1998;
Bwanika et al., 2007). The results of the present
study revealed that the growth performance between
all male and mixed-sex
Oreochromis niloticus
reared
for 240 days under the same culture condition was
significantly different, where the male mono-sex fish
attained a larger final individual size which is nearly
twice the mixed-sex group. Several investigators
have studied the sex-specific growth difference of
Oreochromis niloticus
under semi-intensive pond