www.elsevier.nlrlocateraqua-online
Food organism availability and resource partitioning
in organically or inorganically fertilized Tilapia
rendalli ponds
Randall E. Brummett
1( )
International Center for LiÕing Aquatic Resources Management ICLARM , P.O. Box 229, Zomba, Malawi
Accepted 6 August 1999
Abstract
Ž . Ž .
Inputs of either napier grass NG or diammonium phosphate plus urea NP containing similar
Ž y1 y1. Ž y1 y1.
amounts of nitrogen 17 kg N ha week and phosphorus 1.2 kg P ha week were made to Tilapia rendalli ponds from which offspring were either partially removed or left to grow. After
Ž .
364 days, average weight of stocked fish in NG ponds 74.3 g was significantly higher ŽP-0.05 than in NP ponds 57.4 g . Partial removal of offspring reduced net yield and standing. Ž . stock at harvest from 1515 to 1131 kg hay1 and 1679 to 1099 kg hay1, respectively; but it
improved final average weight of stocked fish from 59.2 to 72.3. Reduction of offspring density
Ž .
had no affect on size of fingerlings juveniles )5 g , but it significantly reduced the size of fry Žjuveniles -5 g . Phytoplankton chlorophyll a and rotifer populations were larger in NP ponds. Ž . Ž106.1mg ly1and 3042 organisms ly1, respectively than in NG ponds 23.5. Ž mg ly1and 1075
y1 .
organisms l , respectively . Microcrustacean densities were highly variable, but statistically similar between input and offspring removal treatments, averaging 385 organisms ly1 overall.
Offspring removal had no effect on rotifer populations. Sediments in NG ponds contained a higher percentage of organic matter throughout the study, achieving equilibrium between decomposition and loading of around 8%, compared to 5% in NP ponds. Offspring number did not affect phytoplankton density or organic matter loading in the sediment. Fish growth and stomach content data confirmed that T. rendalli of less than 25 g average weight are basically detritivors while larger T. rendalli target larger plant materials.q2000 Elsevier Science B.V. All rights reserved.
Keywords: Organically fertilized; Inorganically fertilized; Tilapia rendalli
1
Current address: ICLARM, P.O. Box 229, Cairo, Egypt.
0044-8486r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.
Ž .
( ) R.E. BrummettrAquaculture 183 2000 57–71 58
1. Introduction
In Africa, smallholder aquaculture is generally based on inorganic andror organic
Ž .
fertilizers rather than formulated feeds Brummett and Noble, 1995 . In addition to restricted types of inputs, the quantities of the various fertilizing materials available to African smallholders are also limited. To improve productivity within the constraints of this limited resource base, African fish farmers need efficient management strategies and appropriate species combinations. Polyculture and partial harvesting have been proposed
Ž .
for such situations Stickney, 1979 .
Polyculture and partial harvesting rely on complementary andror synergistic food resource partitioning among species or age-classes. However, food webs vary widely
Ž .
over time and among individual ponds Boyd, 1979; Brummett and Mattson, 1996 . This Ž might be due partly to differences in the types of materials used as pond inputs Qin et
. Ž
al., 1995 and might account for recent reports Brummett and Alon, 1994; Hassan et al., .
1997 that monocultures can out-perform polycultures both in terms of yield and efficiency.
Tilapia rendalli, a commonly cultured cichlid in central and eastern Africa, under-goes an ontogenetic shift in diet from omnivory to macrophytophagy at 10–11 cm
Ž .
during the juvenile to adult transition Fryer and Iles, 1972; Brummett, 1995 , although
Ž .
they remain opportunistic throughout their life Munro, 1967; Caulton, 1976 . This dietary shift might permit improvements in production through partial harvesting or the creation of an effective intergenerational polyculture.
Ž .
This research was conducted to: 1 compare the numbers and types of fish-food organisms generated by organic and inorganic fertilizers in order to identify key
Ž .
components of the small pond ecosystem and, 2 examine the efficiency of nutrient use and resource partitioning by T. rendalli populations as a preliminary step in designing more efficient management systems for this important species.
2. Materials and methods
2 Ž .
Eight 200 m ponds 60 cm average depth at the Malawi National Aquaculture
Ž .
Center NAC were each stocked on 11 December 1995 with 400 mixed sex T. rendalli fingerlings of 8.2"0.43 g average weight. The number to be stocked was based on a previous observation that optimal production of T. rendalli from ponds supplied with low-quality inputs is achieved at stocking rates between 20,000 and 30,000 hay1 ŽBrummett and Noble, 1995 ..
The eight ponds were randomly assigned to one of two input regimes. These regimes were similar in terms of nitrogen and phosphorus content and were based on typical dry
Ž . Ž .
matter dm input rates used by Malawian smallholders listed below .
Ž .1 NGsNapier grass Pennisetum purpureum , a common weed readily consumedŽ .
Ž . y1 y1
by T. rendalli Chikafumbwa et al., 1991 , applied at a rate of 100 kg dm ha day ŽChikafumbwa and Costa-Pierce, 1991 . Grass was chopped into approximately 10 cm. lengths and broadcast over the pond surface daily. According to proximate analysis of
Ž .
the grass conducted by the University of Malawi’s Industrial Consultancy Unit , these
y1 y1.
Ž .2 NPsinorganic fertilizer applied in the form of diammonium phosphate DAP;Ž
. Ž .
18-46-0 and urea 40-0-0 to match the nitrogen and phosphorus supplied by the NP applications in Treatment 1. Thus, each pond received weekly applications of 0.12 kg
Ž y1. Ž y1.
DAP 6 kg ha and 0.8 kg urea 40 kg ha . The fertilizer was split into two Ž
half-weekly doses, dissolved in water and broadcast over the pond Teichert-Coddington .
et al., 1992 .
Ž y1.
Each pond was limed with 100 kg of agricultural lime 5000 kg ha 2 weeks prior
Ž .
to filling 3 weeks prior to stocking . During the course of the study, dissolved oxygen and temperature were measured daily at sunrise with an Otterbine Sentry III dissolved
Ž .
oxygen meter. Chlorophyll a and total ammonia nitrogen TAN were measured weekly
Ž .
at 0800 h according to standard methods APHA, 1989 . In order to calculate the percentage of unionized ammonia, pH was measured weekly at the same time as TAN with a Cole Parmer Model 39000-50 pH wand.
Fish were sampled every 30–60 days with a seine net. Forty adult fish from each pond were individually weighed and then returned to their respective ponds.
Zooplankton were sampled weekly. From each pond, 100 l of water from the middle of the water column was collected from 10 sites. To collect a mixed sample of the water column, a 10-l pail was placed top-down on the water surface, lowered to the middle of the water column and inverted. Each sample was then poured through a 100 mm aperture nylon plankton net. Concentrated organisms were collected in a 90 ml cen-trifuge tube from which three sub-samples were collected with a 1.0 ml eye-dropper. Zooplankton were fixed with 0.1 ml of 10% formalin and enumerated in a Sedgwick-Rafter counting chamber at 40=. As T. rendalli select zooplankton based primarily on
Ž .
size Lazzaro, 1991 , these were keyed to subclass and grouped according to size. Cladocerans were -0.4 mm, 0.4–1.2 mm or )1.2 mm. Copepods were less than or greater than 1.2 mm. Rotifera were -0.4 mm, )0.4 mm or colonial. Nauplii and occasional forms were recorded separately.
Ž Pond sediment samples were collected four times over the course of the study days
.
0, 120, 240 and 360 using a sampler constructed from a 5-cm diameter plastic cup. A hole was cut into the closed end of the cup to permit the escape of water. The cup was pushed down into the sediments and a piece of hard plastic slipped through a slot cut in the side to isolate a 3-cm deep core. The pond was divided into a 50=4 m2 grid. Sampling sites were determined at the beginning of the study by a random number generator. On each sampling date, samples were collected and pooled from 10 separate sites within each pond. Sediments were then dried at 1058C, pulverized, sub-sampled,
Ž .
weighed and burned in a muffle furnace at 3508C for 8 h Ayub and Boyd, 1994 . Percent organic matter was estimated by subtracting the weight of ash from the dry matter.
Ž .
On 16 May, after 150 days of culture and 2 days after sampling of adult fish , offspring were sampled for their average weight by seining each pond with a 5-mm
Ž . Ž .
mesh net. Over 300 fingerlings juveniles )5 g and fry juveniles -5 g from each pond were collected, counted and batch-weighed. All captured offspring from two
Ž .
replicates of each treatment NRsno removal of offspring were returned to their Ž
respective ponds, while those from the other two replicates were removed PRspartial .
( ) R.E. BrummettrAquaculture 183 2000 57–71 60
After an additional 214 days, all ponds were drained and harvested. Fry and fingerlings were separated, counted and batch-weighed. Adult fish were individually measured and weighed.
From each treatment, 10 fishes from every 5 g size class between 1 and 70 g were killed and their stomachs preserved in 10% formalin for examination of diet. Stomach contents were subsequently dissected and the frequency of constituent food items enumerated in three sub-samples. Plant fragments were differentiated from detritus on
Ž
the basis of color, shape and cell structure plants consumed directly are greener, and have less surface and marginal distortion, and more intact cells than detrital plant
. Ž .
materials as described by Caulton 1976 . Differentiation of plankton captured from the water column or the detritus was likewise based on subjective indicators such as physical integrity. Stomach contents were grouped as detritus, higher plants or plankton and reported as numerical percentages of total count of particles in the stomach content. Because of the uncertainties inherent in the interpretation of this sort of detrital and
Ž
microphageous fish diet characterization Hyslop, 1980; Bowen, 1983; Ahlgren and .
Bowen, 1992 , these data were not subjected to statistical analysis.
Ž .
For zooplankton counts, data were square-root transformed Zar, 1974 prior to
Ž .
analysis. Data from the first 150 days completely randomized design were compared with Student’s t-test. The 2=2 factorial arrangement resulting from the division of replicates after day 150 was analyzed with two-way ANOVA and an F test of
Ž .
significance Zar, 1974 .
Ž .
Fig. 1. Temperature and dissolved oxygen DO concentration in T. rendalli ponds receiving either organic
3. Results
Ž .
Dissolved oxygen and water temperature Fig. 1 were similar between treatments ŽP-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 ly1 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 ly1 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 ly1 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 ŽLin et al., 1997 , but there were no statistically significant differences among treatments. ŽP-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 .
()
R.E.
Brummett
r
Aquaculture
183
2000
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71
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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 Number Mean Number Mean Number Mean kg ha kg ha
Ž . Ž . Ž .
weight g weight g weight g
NGNR Avg"SD 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 Avg"SD 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 Avg"SD 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 Avg"SD 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 Avg"SD 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 Avg"SD 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 Avg"SD 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 Avg"SD 400 8.15 289a 72.3b 115.5a 6.9a 462b 0.6b 1131b 1099b
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 hay1 and 1679 to 1099 kg hay1, 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.
( ) R.E. BrummettrAquaculture 183 2000 57–71 64
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 Avg"SD 75.60 35.82 68.25 56.76
72.43 32.50 80.83 69.23
y1
Ž .
Cladocera number l Avg"SD 29.17 16.50 88.77 41.09
69.45 30.00 251.65 65.11
y1
Ž .
Nauplii number l Avg"SD 296.27 316.38 275.20 249.14
254.94 363.04 313.37 390.32
y1
Ž .
Total Crustacea number l Avg"SD 401.03a 368.70a 432.22a 338.55a
331.00 392.16 489.39 453.75
y1
Ž .
Rotifera number l Avg"SD 1253.55a 896.22a 2666.65b 3419.28b
1499.13 869.23 2749.55 4075.64
y1
Ž .
Chlorophyll a mg Avg"SD 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 .
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 .
( ) R.E. BrummettrAquaculture 183 2000 57–71 66
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
ŽP-0.05 . Input x fingerling removal interaction components of variance were insignificant P. Ž -0.05 ..
Treatment Day 0 Day 120 Day 240 Day 360
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
Žg fish . Detritus Higher Plankton Dry matter Detritus Higher Plankton Dry matter
y1 y1
Ž .% plants %Ž . Ž .% Žmg fish . Ž .% plants %Ž . Ž .% Žmg fish .
1–5 45.9 0 54.1 34 13.7 0 86.3 190
6–10 73.8 0 26.2 152 11.8 0 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 0 1301
21–25 69.0 31.0 0 222 80.7 19.3 0 1243
26–30 52.0 48.0 0 610 82.0 18.0 0 764
31–35 50.0 50.0 0 328 63.3 36.7 0 491
36–40 36.6 63.4 0 344 66.1 33.9 0 343
41–45 15.6 75.0 9.4 568 55.9 44.1 0 255
46–50 17.2 82.8 0 386 57.4 42.6 0 575
)50 4.5 93.0 2.5 544 59.3 40.7 0 477
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
Zooplankton densities were within the ranges reported by other authors for similar
Ž .
systems Ludwig and Tackett, 1991; Barkoh, 1996 . Phytoplankton, estimated by Ž chlorophyll a, was somewhat low but within ranges reported for similar systems Boyd,
.
1990; Diana et al., 1991 . Fish growth was also low, but consistent with findings of
Ž .
other studies in similar systems at the NAC Brummett and Noble, 1995 where experimental controls simulate on-farm, rather than ideal, conditions. These low growth rates are not surprising if one considers the relatively low inputs used in this study and by Malawian smallholders: about one-half the nitrogen and one-seventh the phosphorus
Ž .
recommended by Teichert-Coddington et al. 1992 .
Inorganic fertilizers produced more phytoplankton and rotifers than did organic inputs. The higher short-term availability of nutrients in the inorganic fertilizers may have favored these forms. High variability among microcrustacean populations probably accounts for the lack of statistical significance between treatments.
Organic matter in pond sediments was within the range for fish ponds reported by
Ž .
( ) R.E. BrummettrAquaculture 183 2000 57–71 68
Ž y1 y1.
the pond bottom. The level of NG inputs used 100 kg DM ha day reached an equilibrium with decomposition at an accumulation of approximately 8% organic matter in the sediments compared to about 5% with inorganic inputs.
Ž .
Milstein and Svirsky 1996 found that fish disturbing bottom sediments in search of food tend to lower the rate of organic matter accumulation. This has been attributed to more rapid decomposition in the aerobic water column during repeated resuspension ŽDelince, 1992 . In contrast to those findings, there was no difference in organic matter
´
. loading rate or phytoplankton density between NR and PR ponds, even though the offspring were clearly targeting detritus as a food source. Despite the 86% reduction inŽ .
offspring numbers, it is possible that the effect predicted by Milstein and Svirsky 1996 was swamped by the number of juveniles that remained behind after seining. Alterna-tively, the observation that fry were larger in NP than NG ponds up until weekly pond seining might indicate that this activity sufficiently resuspended sediments to mask other differences between treatments. It is also possible that T. rendalli juveniles only attack the uppermost layer of detritus and thus do not resuspend it to any great extent. An observational study of feeding behavior could resolve this question.
Stomach content data confirm the shift in preferred diet from detritus to grass at
Ž .
approximately the 11 cm total length 20–25 g average weight in this study reported
Ž .
previously Caulton, 1976; Brummett, 1995 . Only from NP ponds was there firm evidence that fish had definitely captured swimming prey such as Euglena spp. and
Ž .
smaller fish fry. Caulton 1976 found little evidence for the consumption of swimming prey by juvenile T. rendalli in natural systems. Much of the plankton in the stomachs of smaller juveniles may have been sedimented. Having a smaller mouth, these fishes could be expected to frequently access only the upper layers of detritus which might be richer in sedimented plankton than lower layers in which these forms would have decayed ŽDelince, 1992 . Also important is the observation that small fry
´
. Ž-1.5 g were larger at. day 150 in the relatively phytoplankton-rich NP ponds. Either there is an earlier ontogenetic shift from plankton to detritus at a size of -10 g, or T. rendalli juveniles only resort to moving prey when the situation demands a change from the normal diet of detritus. Further study will be required to answer this question definitively.After weekly pond seining began, the removal of some offspring reduced competi-tion, probably for oxygen, improving adult growth rates. In contrast, reduction in juvenile numbers had no affect on fingerling growth. This finding may be related to the
Ž .
higher metabolism of large relative to smaller fish reviewed by Hepher, 1988 . Larger fish would thus be expected to experience oxygen stress more frequently than smaller
Ž .
fish and would benefit more from a reduction in competition. Pauly 1984 anticipated such a phenomenon in noting that a two dimensional gill surface area cannot provide consistently uniform amounts of oxygen to a body volume growing in three dimensions. Larger fish thus have proportionally more difficulty extracting oxygen from water and ‘‘run out’’ of oxygen at higher oxygen tensions than smaller fish.
Reduction of fingerling density also substantially reduced the average weight of fry. This may have been due to the partial harvesting having removed juveniles at a generally younger age and smaller size thus drawing down the average for those ponds.
Ž
Also possible is that the reduced numbers of larger, often cannibalistic Broussard et al., .
Adult T. rendalli seemed to be less flexible feeders than juveniles. Most reported work on T. rendalli has found adults of this species to be predominantly
macrophyti-Ž
vores Munro, 1967; Caulton, 1976, 1977; Denny and Bowker, 1978; Skelton, 1993; .
Brummett, 1996 . T. rendalli adults in this study preferred higher plants even when this food item was in short supply in the NP treatments. Apparently, the plants found in adult
Ž .
stomachs from the NP ponds Table 2 came from weeds growing from the pond dikes out into the pond water. Food requirements that could not be met by the marginal grass had to be filled by turning to detritus.
The importance of even minor modifications in food capture and handling structures
Ž .
and behaviors was reviewed by Hyatt 1979 . Behavioral and physiological changes occurring as the fish mature may restrict their ability to capture and digest foods other
Ž .
than plants Bowen, 1987; Ahlgren, 1996; Fugi et al., 1996 . Such behavioral differ-ences in food collection strategy between both generations and sexes has been
demon-Ž .
strated in feral populations of Oreochromis mossambicus by Bruton 1983 and Bowen Ž1984 . For adult T. rendalli in the NP ponds in this study, there was only limited access. to plant materials so they had to avail themselves of detritus. Since organic detritus was not affected by the reduction of fingerling density, one must assume that this detritus was not in short supply and that adults simply do not handle or digest detritus well. Apart from macrophytes, T. rendalli adults have been reported to consume other fishes ŽCaulton, 1977 , insects Munro, 1967 , zooplankton Munro, 1967; Lazzaro, 1991 and. Ž . Ž .
Ž .
epiphytes Denny and Bowker, 1978 . The author could find no published record of adult T. rendalli consuming detritus. Without further study, it is not possible to identify the actual reason, but the fact that growth of fingerlings was not affected by the levels of competition in NR ponds while adult growth was reduced means that adults were relatively unable to access detritus source of food.
Neither resource partitioning among generations nor partial harvesting improved net yield. Under these low-input and fertility conditions, the opportunistic omnivory of T. rendalli may have blurred the lines between age-class feeding preferences and increased
Ž y1.
competition between generations. It seems that the offspring removal rate 196 kg ha was insufficient, which may have been due to the relatively low fecundity of T. rendalli
Ž .
in small ponds Brummett, 1997 .
Even without a complete understanding of the trophic dynamics of the system, these results underline the importance of carefully matching the culture species and pond management strategy to maximize efficiency and productivity. This might not always be
Ž .
( ) R.E. BrummettrAquaculture 183 2000 57–71 70
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