International Journal of Aquaculture, 2015, Vol.5, No.3 1
-
13
9
Table 8 Economic information for mono-sex male Nile tilapia intensively reared in concrete tanks for 44 weeks at three stocking
density and two water exchange rates
Water exchange 8 L/min.
Water exchange 12 L/min.
Stocking density
Stocking density
Parameters
50 /m
3
75 /m
3
100 /m
3
50 /m
3
75 /m
3
100 /m
3
No. fish stocked / tank (4m
3
)
200
300
400
200
300
400
No. fish harvested
200
300
400
200
300
400
Survival rate (%)
100
100
100
100
100
100
Initial wt. stocked (kg/tank)
1.50
2.25
3.00
1.50
2.25
3.00
Total production (Kg/tank 4m
3
)
44.18
54.96
52.00
43.38
44.70
53.88
Net production (Kg/tank 4m
3
)
42.68
52.71
49.00
41.88
42.45
50.88
Food used (kg/tank 4m3)
98.64
121.12
135.48
98.49
111.90
131.06
Fingerling cost (0.20 LE
*
./fish)
40.00
60.00
80.00
40.00
60.00
80.00
Food cost (1.7 LE. /kg diet)
167.69
205.90
230.32
167.43
190.23
222.80
Total cost (LE.)
207.69
265.90
310.32
207.43
250.23
302.80
Value of harvest (9 LE. /kg)
397.62
494.64
468.00
390.42
402.30
484.92
Net profit (LE.)
189.93
228.74
157.68
182.99
152.07
182.12
Note: LE. = Lever Egyptian (about 0.18 Dollar). Fingerlings cost calculated as 200 L.E./1000 fingerlings
important among them being the use of supplementary
feed, polyculture, and auxiliary aeration during the
night (Sarig 1989). Higher yields were obtained in
specially designed smaller units, 50-1000m3 (Zohar et
al
.
1985; Van Rijn et al. 1986), which differ from
conventional ponds in design. These are made of
concrete or are plastic-lined, and their configuration
allows periodical removal of organic matter from the
bottom. Most of these units are operated in a
semi-closed mode, allowing optimal use of water and
hence, minimal water discharges. Due to their reduced
environmental impact, national and regional
authorities support their development. Pollution
control is, therefore, another factor underlying the
development of intensive systems. Finally, culture of
fingerlings (mainly tilapia) during off-season in heated,
indoor systems is rapidly expanding, and so, heat
conservation can be counted as an additional factor
promoting the use of intensive recirculating systems.
Besides genetics and environmental condition, water
exchange rates and stocking density are important
factors affecting growth and maturation of cultured
fish (Huang and Chiu 1997). As stocking density
increases, competition for food and living space
usually intensifies providing one of the most effective
controls on animal production (Huang and Chiu 1997).
The effects of density may be divided into two
categories: the density dependent and the density-
independent. The stocking density that significantly
negatively affects the growth of fish was considered as
the density dependent category, such as the cases
found for blue gill,
Lepomis macrochirus
Rafinesque
(Wiener and Hanneman, 1982), chinook salmon,
Oncorhynchus tshawytscha
(Martin and Wertherimer
1989), Nile tilapia,
Oreochromis niloticus
(L.)
(Siddiqui et al. 1989). In the present study the similar
case of negative curvilinear relationship was found
between stocking density and growth weight of Nile
tilapia (Figure 4).
The changes in growth of fingerlings are physiological
response to environmental condition (Wootton 1990).
Water quality has complex side effect on high
stocking density. Miao (1992) found that higher
stocking density was accompanied by lower pH and
dissolved oxygen and suggested that the resulting
changes in water quality might play an important role
in affecting growth and survival of fish. Chen et al.
(1997) indicated that metabolic wastes, which are
directly proportional to stocking density, have been
implicated in inhibiting the growth of fish and to be
toxic to fish. Moreover, fish need oxygen for aerobic
generation of energy for body maintenance,
locomotion, feeding and biosynthesis. A minimum
dissolved oxygen level of 3.0 PPM was recommended
during cage culture of tilapia in freshwater (Coche
1982). The water flow system in the present study
provided fairly good water quality consistently
throughout the experimental period at all tanks. There
were no evidences of large physiochemical
fluctuations, occurrence of disease and handling stress,
