Table 1 Ž
Methods of analysis of chemical and physical water quality of experimental units ponds, water quality .
sampled from a minimum of three locations per pond; cages, one sample per cage Parameters
Method, frequency and timing of sampling Source
Ž .
Temperature and DO meter, measured twice daily at 6:00 a.m.
YSI model 58 Ž
. dissolved oxygen DO
and 3:00 p.m. Ž
. pH
digital pH meter, measured twice weekly at Pye model 290
6:00 a.m. and 3:00 p.m. Ž
. Chlorophyll-a and
acetone extraction and measuring absorbance APHA 1980
phytoplankton biomass by using spectrophotometer. Ph. biomass s
U
chlorophyll-a=67, measured weekly Ž
. Alkalinity
volumetric analysis using phenolphthalein and Boyd and Tucker 1992
methyl indicators and standard sulphuric acid
U
titrant, measured weekly Ž
. Hardness
Eriochrome black-T indicator, standard CaCO Boyd and Tucker 1992
3
U
and standard EDTA titrant, measured weekly
U
Ž .
Concentration of phenate method, measured twice monthly
APHA 1980 Ž
. ammonia NH –N
3
Ž .
Concentration of cadmium reduction procedure, measured twice
APHA 1980
U
Ž .
nitrate NO –N monthly
3
U
Ž .
Concentration of nitrate diazotizing method, measured twice monthly
APHA 1980 Ž
. NO –N
2
Ž .
Dissolved ascorbic acid procedure, measured twice
Boyd and Tucker 1992
U
orthorphosphorous monthly
Ž .
Total phosphorous persulphate digestion method, measured twice
Boyd and Tucker 1992
U
monthly
U
Water samples are taken at 7:00 a.m.
Feed conversion rates were calculated as the weight of feed given divided by biomass of fish harvested.
Comparison of growth rate of the treatments was analyzed for both experiments using factorial analysis for which a significance level of P - 0.05 was set. When significant
differences between means was detected, a LSD range test was used to compare mean Ž
. values P - 0.05 .
3. Results
3.1. Grow-out of oÕerwintered fry and new-season fry in ponds Both strain and use of hormonal sex reversal affected individual final size of fish
Ž .
stocked as overwintered seed in ponds P - 0.05 . The GIFT strain grew fastest
Ž
y1
. 2.1 0.13 g day
and attained a final size 11.1 larger than the Thai and Viet strains which reached a similar size. Considering all strains, MT-treatment resulted in a
final size of fish 10.7 larger than mixed-sex fish. However, a strain by strain comparison illustrated that benefits of MT-treatment were minor, except for in the Viet
strain. Survival in all treatments ranged from 70.1 to 82.5 and was not significantly
Table 2 Ž .
Ž . Data of stocking and harvest parameters of overwintered I and new-season II seed of three strains of Nile
Ž tilapia raised communally as either monosex male or mixed-sex groups in earthen ponds mean valuess.e.,
. replicates s 3
Strains At stocking
At harvesting Daily
Survival Ž .
weight gain Number
Mean weight Number
Mean weight
y1
Ž .
g day Ž .
Ž . g
g Experiment 1 oÕerwintered seed — 184 days
ab a
Thai mixed-sex 435
10.70.7 305
350.028.6 1.80.15
70.1
bc a
GIFT mixed-sex 435
10.50.8 358
375.335.7 2.00.19
82.3
a a
Viet mixed-sex 435
10.50.9 330
338.737.6 1.80.20
75.8
ab a
Thai monosex 435
10.70.7 320
353.123.5 1.90.12
73.6
c a
GIFT monosex 435
10.30.8 316
400.026.3 2.10.13
72.9
c a
Viet monosex 435
9.91.2 337
380.328.7 2.00.15
77.5 Experiment 2 new-season seed — 112 days
a a
Thai mixed-sex 435
9.00.4 339
199.520.4 1.70.18
77.3
ab a
GIFT mixed-sex 435
9.20.4 322
201.321.1 1.70.17
74.0
a a
Viet mixed-sex 435
9.30.4 342
197.022.5 1.70.16
78.6
b a
Thai monosex 435
9.10.5 310
211.916.9 1.80.16
72.4
a a
GIFT monosex 435
9.00.3 339
194.220.4 1.70.16
77.9
a a
Viet monosex 435
9.10.3 333
192.019.3 1.60.17
76.6
Ž .
different Table 2 . Factorial analysis indicated that new-season fry stocked in ponds showed no significant differences in final size that were related to strain or hormone
Table 3 Ž .
Ž . Data of stocking and harvest parameters of overwintered I and new-season II seed of three strains of Nile
Ž tilapia raised communally as either monosex male or mixed-sex groups in cages
mean valuess.e., .
replicates s 3 Strains
At stocking At harvesting
DWG Survival
y1
Ž .
Ž . g day
Number Mean weight
Number Mean weight
Ž . Ž .
g g
Experiment 1 oÕerwintered seed — 184 days
a a
Thai mixed-sex 90
12.70.8 86
373.034.1 1.90.19
95.6
ab a
GIFT mixed-sex 90
11.30.6 87
391.433.3 2.10.18
96.7
a a
Viet mixed-sex 90
12.40.5 85
337.133.5 1.80.18
94.4
ab a
Thai monosex 90
11.60.5 87
373.326.7 1.90.14
96.7
b a
GIFT monosex 90
12.00.8 86
429.727.5 2.30.15
95.6
a a
Viet monosex 90
12.30.8 86
363.533.3 2.00.18
95.6 Experiment 2 new-season seed — 112 days
ab a
Thai mixed-sex 90
9.50.4 90
102.010.6 0.80.09
100.0
b a
GIFT mixed-sex 90
9.50.4 89
112.511.2 0.90.09
97.8
a a
Viet mixed-sex 90
9.60.4 90
97.69.7 0.80.09
100.0
ab a
Thai monosex 90
9.70.5 89
103.59.4 0.80.07
98.9
ab a
GIFT monosex 90
9.80.5 90
105.110.9 0.80.09
100.0
a a
Viet monosex 90
9.70.4 90
98.99.5 0.80.08
100.0
treatment. A paired comparison indicated that the final size of monosex Thai strain was Ž
. larger than mixed-sex P - 0.05 however.
The survival rate of the three strains ranged from 72.4 to 78.6 showing no significant difference between strains.
3.2. Grow-out of oÕerwintered fry and new-season fry in cages. Overwintered fry raised in cages showed a similar trend in effects of strain and
hormone treatment on performance as fish stocked in ponds. GIFT fish achieved a Ž
. significantly P - 0.05 larger final size compared to Thai and Viet strain. A compari-
son of mixed-sex fish revealed no differences between strain however. A paired
Table 4 Ž
Monthly average of physical and chemical water parameters in experimental ponds and cages mean
. valuess.e. at RIA-1 in period from May to November 1997
Parameters Number of
Experimental ponds Experimental cages
samples per Mean of
Range of Mean of
Range of experimental parameters
parameters parameters
parameters unit
Experiment 1 Ž
. Temperature 8C
344 29.40.8
24.6–32.0 29.60.8
24.8–31.3
y1
Ž .
DO mg O l 344
4.40.7 3.1–9.6
4.50.6 2.4–6.2
2
pH 52
7.7–9.1 7.5–9.1
Ž .
Transparency cm 52
21.35.4 15.6–26.3
21.93.3 15.6–25.0
y1
Ž .
Orthorphosphorus mg l 12
0.020.02 0.01–0.05
0.020.02 0.01–0.06
y1
Ž .
Total phosphorus mg l 12
0.070.06 0.03–0.22
0.070.07 0.03–0.09
y1
Ž .
Nitrite mg l 12
0.070.07 0.01–0.29
0.130.13 0.01–0.2
y1
Ž .
Nitrate mg l 12
0.40.2 0.04–0.94
0.60.3 0.1–0.8
y1
Ž .
Ammonia mg l 12
1.20.5 0.3–1.67
1.40.6 0.6–1.60
y1
Ž .
Hardness mg CaCO l 24
56.88.0 33.3–70.0
66.98.6 61.9–68.5
3 y1
Ž .
Alkalinity mg CaCO l 24
62.27.4 44.6–82.7
68.87.6 62.9–82.7
3
Phytoplankton biomass 24
2512.2 716.2–4774.3 2537.1
1838.5–3223.4
y3
Ž . Ž
. s chlorophyll-a=67
mg m Experiment 2
Ž .
Temperature 8C 220
28.11.0 24.4–31.3
29.00.9 24.4–31.6
y1
Ž .
DO mg O l 220
3.80.5 4.1–4.3
4.20.4 3.1–4.5
2
pH 32
7.5–7.9 7.90.3
7.5–7.7 Ž
. Transparency cm
32 21.94.9
17.5–34.4 22.24.3
11.5–26.6
y1
Ž .
Orthorphosphorus mg l 8
0.050.03 0.01–0.09
0.020.01 0.01–0.05
y1
Ž .
Total phosphorus mg l 8
0.090.05 0.02–0.14
0.090.02 0.03–0.21
y1
Ž .
Nitrite mg l 8
0.070.06 0.02–0.14
0.020.02 0.02–0.11
y1
Ž .
Nitrate mg l 8
0.30.2 0.2–0.5
0.60.4 0.16–0.92
y1
Ž .
Ammonia mg l 8
1.10.8 0.5–1.9
1.20.6 0.6–1.7
y1
Ž .
Hardness mg CaCO l 16
57.115.7 24.5–92.8
62.79.7 42.7–82.5
3 y1
Ž .
Alkalinity mg CaCO l 16
59.511.4 40.1–67.3
65.910.5 51.8–68.9
3
Phytoplankton biomass 16
1899.1 671.3–5372.7 2149.1
895.8–2237.8
y3
Ž .Ž
. s chlorophyll-a=67 mg m
comparison of each strain indicated monosex reached a larger final size than mixed-sex fish, but that hormone treatment resulted in a smaller difference in final size than in
pond-reared fish. The mean DWG of fish in all treatments ranged from 1.8 0.18 to Ž
. 2.3 0.15 g Table 3 . The survival of mixed-sex and monosex stocks in the three
strains ranged between 94.4 and 96.7 and was not significantly different. New-season fry stocked in cages grew poorly compared to fish stocked in ponds
Ž
y1
. 0.8–0.9 g day
. There was no effect of hormone treatment and the effect of strain was less distinct. The survival of fish in all cages was very high however, ranging from
97.8 to 100. 3.3. Sexual maturation of fish
Fry were found in ponds stocked with mixed-sex, overwintered fish from the fourth month onwards. Mouth brooding females were also detected in cages stocked with
mixed-sex fish from the fourth month onwards. In ponds stocked with new-season mixed-sex fingerlings, there were no fry observed during the period of experiment, and
no mouth brooding females found at harvest.
3.4. Water quality parameters Ž
The physical and chemical parameters of pond water during both experiments Table .
4 were maintained within the tolerance range of Nile tilapia, as reported by several Ž
. authors Jiazhao, 1991; Popma and Lovshin, 1996 . Levels of net primary productivity
Ž .
did decline from August to November Table 4 explaining the relatively poor perfor- mance of fish in E2. The new-season fry, reared in cages grew particularly slowly
despite similar water quality to ponds stocked at the same time.
4. Discussion
