Abstract:

The diet, spatial-temporal distribution and abundance patterns of the pelagic cyprinid,
Rastrineobola argentea (Pellegrin) and zooplankton were investigated in conjuction
with limnological conditions in nothern Lake Victoria, between September 1994 and
October 1996. Samples were collected from the shallow inshore and deep open
waters. Fish and zooplankton were sampled with a small mid-water trawl net of 5 mm
stretch mesh size and a Schindler trap respectively from the surface, mid-waters and
near-bottom vertical positions. Concomitant measurements of temperature, dissolved
oxygen (DO), light attenuation, Secchi depths and chlorophyll-a were made. Water
column structure exhibited seasonal alternation of thermal stratification (October-
March) and mixing March-July), synchronous in inshore and offshore waters. Oxygen
depletion (up to < 1.0 mg O2I1) was common in the hypolimnion during stratification,
in contrast with high DO throughout the water column during turnover. Vertical
distribution of fish and zooplankton revealed higher concentrations above the oxycline
during stratification while a near homogeneous dispersion occurred during mixing.
Low zooplankton-high fish densities inshore contrasted with high zooplankton-low fish
densities offshore. Low water transparency-high extinction coefficients inshore
contrasted with high water transparecy-low extinction coefficients offshore.
Phytobiomass was nearly twice as high inshore compared to offshore. Zooplankton
community was dominated by cyclopoid copepods, with minor proportions of
diaptomids, cladocerans, rotifers etc. Higher, relative proportions of diaptomids and
cladocerans, in the historical (1961) sample, however, prompted suggestions of longterm
changes in zooplankton community structure. Most zooplankters attained highest
numerical abundance during the annual turnover, (July), declining thereafter to lower
levels in subsequent months. Zooplankton size distribution in December 1995
indicated lower relative abundance of large copepods inshore compared to offshore,
Mwcbaza - Ndawula 10
while both copepods and cladocerans showed higher abundances (inshore) during July
1996. Pelagic fish community, was dominated by R. argentea, and the three cohabiting
taxa exhibited different seasonal patterns of abundance. R. argentea fed
heavily on cyclopoids, with minor supplements from diaptomids, and cladocerans.
Proportions of diaptomids increased with fish size while nauplii showed the reverse
pattern. Comparision of propoportions of diaptomids and cladocerans in the diet of
historical (1966) and modem (December 1995) fish diet suggests substantial decline in
their relative contributions. No major and consistent variations in diet composition
were observed on different sampling dates, day and night samples or different vertical
positions in the water column. Dominance of the diet by cyclopoids seemed to be
largely due to their much greater pelagic abundance, higher species diversity and wide
size range of food particles. Sustained prominence of cyclopoids in the environment
may be partly explained by high productivity under warm tropical conditions where a
taxon may be be mantained at high concentrations despite heavy predation.