3. Results
Ž .
Dissolved oxygen and water temperature Fig. 1 were similar between treatments Ž
. P - 0.05 and remained within acceptable limits for T. rendalli growth throughout the
Ž .
study Wohlfarth and Hulata, 1983 . Dissolved oxygen at sunrise was never less than 1 mg l
y1
in any pond. Temperature over the first 20 weeks averaged above 268C and then declined somewhat according to the Malawian cool season before rising again at week
36. Temperatures never declined below 158C. Total ammonia fluctuated around 0.20 mg l
y1
with occasional peaks of up to 0.44
y1
Ž .
mg l Fig. 2 . At 800 h with an average pH of 6.5, unionized ammonia would not
have exceeded 0.24 mg l
y1
in either input treatment. The bacteria-based foodweb in the NG ponds produced similar amounts of ammonia as the phytoplankton-based foodweb
in the NP ponds. This concentration of ammonia may have reduced growth rates slightly Ž
. Lin et al., 1997 , but there were no statistically significant differences among treatments
Ž .
P - 0.05 . Pond harvest data are shown in Table 1. Survival of stocked fish averaged 72 and
Ž .
did not differ among treatments P - 0.05 . This survival rate is typical for T. rendalli, which is more easily captured by the abundant bird fauna at the NAC than other
Ž .
commonly-cultured tilapiine cichlids Brummett and Chikafumbwa, 1995 . Fish growth data are shown in Fig. 3. Prior to removal of offspring, individual weight
Ž .
of stocked fish was significantly greater P - 0.01 in ponds receiving NG than in ponds
Ž .
Ž .
Fig. 2. pH and total ammonia nitrogen TAN concentration in T. rendalli ponds receiving either organic NG Ž
. or inorganic NP inputs over 364 days.
R.E. Brummett
r Aquaculture
183 2000
57 –
71 62
Table 1 Ž
. Ž
. Ž
. Ž
. Stocking and harvest data for a 364-day comparison of T. rendalli ponds receiving either organic NG or inorganic NP fertilizer with partial PR or no NR
removal of offspring Results of two-way ANOVA are discussed in the text. Analysis of variance was used to compare tabulated means. Within columns within sections, averages with
Ž .
Ž .
different associated letters are significantly different P - 0.05 . Input x fingerling removal interaction components of variance were insignificant P - 0.05 . Stocking
Harvest Number
Mean Adults
Fingerlings Fry
Net yield Stand. stock
y1 y1
Ž . Ž
. Ž
. weight g
kg ha kg ha
Number Mean
Number Mean
Number Mean
Ž . Ž .
Ž . weight g
weight g weight g
NGNR AvgSD
400 8.15
293 67.4
1858 5.3
2538 2.1
1583 1746
0.15 3.0
2.47 107.5
1.22 236.5
0.23 207.5
201.5 NGPR
AvgSD 400
8.15 287
81.3 121
6.8 493
0.6 1245
1219 0.35
0.5 2.62
16.0 0.32
56.0 0.12
17.5 30.8
NPNR AvgSD
400 8.25
282 51.1
1449 8.4
2692 2.2
1447 1611
0.25 17.0
1.41 452.0
0.90 269.0
0.30 23.7
18.7 NPPR
AvgSD 400
8.15 292
63.4 110
7.0 430
0.6 1018
979 0.65
17.5 5.37
11.0 0.53
81.0 0.06
90.8 118.9
NG AvgSD
400 8.15
290a 74.3a
989a 6.0a
1515a 1.3a
1413a 1483a
0.31 4.5
8.29 1006.5
1.36 1197.0
0.88 259.2
350.7 NP
AvgSD 400
8.20 287a
57.2b 780a
7.7a 1561a
1.4a 1232a
1295a 0.57
20.7 7.97
856.7 1.19
1326.0 0.95
259.1 378.3
NR AvgSD
400 8.20
288a 59.2a
1653a 6.8a
2615a 2.1a
1515a 1679a
0.24 15.5
9.68 446.7
2.20 305.8
0.32 187.9
189.3 PR
AvgSD 400
8.15 289a
72.3b 115.5a
6.9a 462b
0.6b 1131b
1099b 0.60
14.6 10.87
17.08 0.52
88.25 0.11
151.1 170.9
Ž .
Fig. 3. Individual growth of stocked T. rendalli over 364 days. At day 150, weekly offspring removal PR Ž
. began in two replicates of each treatment while in two replicates offspring were left to grow NR .
receiving NP, averaging 58.0 4.69 g and 42.8 5.91 g, respectively. At day 150, Ž
. Ž
. offspring were significantly smaller P - 0.01 in NG ponds 0.75 0.04 g than in NP
Ž .
Ž .
ponds 1.25 0.18 g . At harvest
Table 1 , the effect of fertilization regime on Ž
. individual growth of stocked fish remained significant P - 0.01 , but average weight of
Ž .
offspring no longer differed significantly P - 0.05 . Ž
. Comparison of total fry and fingerling counts at harvest Table 1 indicate that partial
fingerling removal reduced offspring density by approximately 86. Partial removal of Ž
. offspring significantly improved growth of the stocked fish P - 0.01 . Average weight
Ž of fingerlings was similar between treatments, but individual weight of fry those
. Ž
. collected at harvest plus those captured during sampling was significantly P - 0.01
reduced from about 2 g down to 0.6 g in ponds from which offspring were being removed. Interaction between input and offspring removal treatments were not signifi-
Ž .
cant P - 0.05 for any growth parameters. Ž
y1
. An average of 3.91 0.38 kg of juveniles 196 kg ha
was taken from those ponds Ž
. Ž
. where offspring were removed each week after day 150 . Net yield
P - 0.05 and Ž
. standing stock at harvest P - 0.01 were significantly decreased from an average of
1515 to 1131 kg ha
y1
and 1679 to 1099 kg ha
y1
, respectively, by periodic offspring Ž
. removal, but were not affected by fertilization strategy Table 1 . Interaction effects
Ž .
between fertilization and offspring removal were not significant P - 0.05 for these
parameters. Average weekly phytoplankton density, as estimated by chlorophyll a concentration
Ž .
Ž .
Boyd, 1979 , was higher in NP than NG ponds P - 0.001 , which difference increased
Table 2 Ž
. Average zooplankton and chlorophyll a densities in T. rendalli ponds receiving either organic NG or
Ž .
Ž .
Ž .
inorganic NP inputs with PR or without NR partial offspring removal Results of two-way ANOVA are discussed in the text. A post hoc Duncan’s NMRT was used to compare
Ž .
tabulated means. Averages within rows for total crustacea, rotifera and chlorophyll a
with different Ž
. associated letters are significantly different P - 0.05 .
NG NR NG PR
NP NR NP PR
y1
Ž .
Copepoda number l AvgSD
75.60 35.82
68.25 56.76
72.43 32.50
80.83 69.23
y1
Ž .
Cladocera number l AvgSD
29.17 16.50
88.77 41.09
69.45 30.00
251.65 65.11
y1
Ž .
Nauplii number l AvgSD
296.27 316.38
275.20 249.14
254.94 363.04
313.37 390.32
y1
Ž .
Total Crustacea number l AvgSD
401.03a 368.70a
432.22a 338.55a
331.00 392.16
489.39 453.75
y1
Ž .
Rotifera number l AvgSD
1253.55a 896.22a
2666.65b 3419.28b
1499.13 869.23
2749.55 4075.64
y1
Ž .
Chlorophyll a mg AvgSD
22.92a 24.11a
92.07b 120.11c
16.73 21.96
59.55 52.65
Ž .
over time Table 2, Fig. 4 as phytoplankton density grew in NP ponds while remaining almost constant in NG ponds. Chlorophyll a concentration did not differ between PR
and NR ponds. Multiple regression of chlorophyll a against total zooplankton and total Ž
. microcrustaceans was not significant P 0.05 .
Ž .
Ž .
Fig. 4. Chlorophyll a concentration in T. rendalli ponds receiving organic NG or inorganic NP .
Ž .
Ž .
Fig. 5. Rotifer density in T. rendalli ponds receiving organic NG or inorganic NP inputs.
Ž .
Fig. 6. Density of microcrustaceans copepods, cladocerans and their nauplii in T. rendalli ponds receiving Ž
. Ž
. either organic NG or inorganic NP inputs.
Average weekly zooplankton density was highly variable over time and between Ž
. Ž
. ponds Table 2 . Rotifers were significantly P - 0.01 more abundant in NP than in NG
Ž .
Ž .
ponds Fig. 5 while microcrustacean copepods and cladocerans densities were not Ž
. Ž
. different P - 0.05 between input treatments Fig. 6 . Within input treatments, neither
Ž .
rotifer nor microcrustacean densities were affected P 0.05 by fingerling removal. Accumulation of organic matter in pond sediments was consistent with input treat-
Ž .
ments. NG pond sediments contained significantly P - 0.05 more organic matter than NP ponds. Over the first 240 days, sediment organic matter in NG ponds increased from
Ž .
4.9 to 9.6 and then declined to a final value of 7.7 Table 3 . Organic matter in NP pond sediments remained fairly constant at between 4.5 and 5.0 throughout the study.
Ž .
There was no significant difference P 0.05 in organic matter among NR and PR
ponds. Examination of fish stomach contents revealed that, while targeting detritus, smaller
Ž .
Ž .
Ž fish - 15 g were capable of capturing other foods Table 4 . Detritus including what
. were apparently sedimented plankton dominated the stomach contents of fingerlings
from NG ponds and there was no evidence of cannibalism. Fingerlings in NP ponds ate a wider range of food items including fry, Pyridinium and Euglena spp. Pieces of grass
formed the major part of the stomach contents of the majority of fish larger than 25 g in NG, while those in NP ponds remained reliant on detritus for a substantial part of their
food throughout the study. In individual fish, the ontogenetic transition was found to occur rather abruptly, with stomachs dominated by either detritus or plants and few
stomachs containing an intermediate diet. Rather, individual variation in the timing of the shift accounted for average stomach contents that appear mixed when averaged
Table 3 Ž .
Ž .
Ž .
Sediment organic matter in T. rendalli ponds receiving either organic NG or inorganic NP inputs and Ž
. Ž
. with PR or without NR partial removal of offspring
Results of two-way ANOVA are discussed in the text. Analysis of variance was used to compare tabulated means. Values within columns within sections with different associated letters are significantly different
Ž .
Ž .
P - 0.05 . Input x fingerling removal interaction components of variance were insignificant P - 0.05 . Treatment
Day 0 Day 120
Day 240 Day 360
NG NR AvgSD
4.6 7.3
9.5 8.0
0.15 0.95
1.6 0.9
NG PR AvgSD
5.2 6.4
7.9 7.4
0.40 0.30
1.25 1.4
NP NR AvgSD
5.3 5.2
4.9 4.8
1.40 0.45
0.25 0.35
NP PR AvgSD
4.8 4.5
5.0 4.8
0.55 0.40
0.60 0.55
NG AvgSD
4.9a 6.8a
8.7a 7.7a
0.44 0.82
0.44 0.44
NP AvgSD
5.0a 4.8b
4.9b 4.8b
1.10 0.54
0.47 0.46
NR AvgSD
4.9a 6.2a
7.2a 6.4a
1.22 1.49
2.99 2.04
PR AvgSD
5.0a 5.5a
6.4a 6.1a
0.61 1.17
2.00 1.96
Table 4 Ž
. Ž
. Stomach content analysis percentage of total count of T. rendalli grown in ponds receiving organic NG or
Ž .
inorganic NP fertilization These data were not subjected to statistical analysis.
Size Class NG
NP
y1
Ž .
g fish Detritus
Higher Plankton
Dry matter Detritus
Higher Plankton
Dry matter
y1 y1
Ž . Ž .
Ž . Ž
. Ž .
Ž . Ž .
Ž .
plants mg fish
plants mg fish
1–5 45.9
54.1 34
13.7 86.3
190 6–10
73.8 26.2
152 11.8
88.2 762
11–15 62.6
35.6 6.7
260 62.4
4.5 33.1
223 16–20
82.8 14.1
3.1 244
89.9 10.1
1301 21–25
69.0 31.0
222 80.7
19.3 1243
26–30 52.0
48.0 610
82.0 18.0
764 31–35
50.0 50.0
328 63.3
36.7 491
36–40 36.6
63.4 344
66.1 33.9
343 41–45
15.6 75.0
9.4 568
55.9 44.1
255 46–50
17.2 82.8
386 57.4
42.6 575
50 4.5
93.0 2.5
544 59.3
40.7 477
Ž .
across the sample populations Table 2 . Below 10 g and above 50 g average weight, variability among individual fish was virtually nil. Most variation occurred between 10
and 40 g, indicating that it is within this range that the dietary transition is occurring.
4. Discussion
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