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 northern 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 long term 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, while both copepods and cladocerans showed higher abundances (inshore) during July 1996. Pelagic fish community was dominated by R. argentea, and the three co habiting 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 maintained at high concentrations despite heavy predation.