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 ha
y1
Ž .
Brummett 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 . Ž
. 1 NG s Napier grass Pennisetum purpureum , a common weed readily consumed
Ž .
y1 y1
by T. rendalli Chikafumbwa et al., 1991 , applied at a rate of 100 kg dm ha day
Ž .
Chikafumbwa 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
. application rates provided 17 kg of nitrogen and 1.2 kg of phosphorus ha
week .
Ž . Ž
2 NP s inorganic fertilizer applied in the form of diammonium phosphate DAP; .
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 m
2
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 NR s no removal of offspring were returned to their
Ž respective ponds, while those from the other two replicates were removed PR s partial
. removal of offspring . This procedure was repeated weekly for the rest of the study.
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 Ž
. Ž
. NG or inorganic NP inputs over 364 days.
3. Results
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