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
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