The experimental cage culture was conducted at Shirati bay, Lake Victoria from February to August 2013, to investigate the impacts of the small scale cage culture on the environment. Three locations along the cages, at the intermediate and one in the offshore (control) were sampled for water quality parameters, phytoplankton and macro invertebrates. A notable increase in nutrient concentration was observed after the set of cages among the stations. However DO, pH, and water transparency showed no major changes and was within the recommended ranges. Cyanophytes an indicator of inorganic pollution dominated before and after the set of cages, an increase in phytoplankton numerical abundance was observed after stocking of fish in cages. In addition there was an increase in the invertebrate community especially bivalves and gastropods. In conclusion we found no consistent environmental change caused by cage culture, and therefore it can be allowed in Lake Victoria, Tanzania part, with close monitoring of its impacts.

Assuming that the inshore and offshore waters of Lake Victoria are impacted differently by human activities in its catchment, this study investigated the water quality dynamics of the lake. A total of 29 stations were sampled in 2005-2008 for dissolved oxygen (DO), pH, Secchi transparencies, temperature, turbidity, chlorophyll a, NO 3 , SRSi, TN and TP. There was a decreasing trend of the measured parameters towards offshore sites, except for Secchi transparency and NO 3 , which increased towards the offshore waters. DO concentrations (mean ± SD) varied between 6.97 ± 0.57 mg O 2 l -1 and 5.80 ± 0.72 mg O 2 l -1 in the inshore and offshore waters, respectively. Turbidity values were comparatively higher in the inshore (3.73 ± 2.21 NTU) than the offshore waters (2.19 ± 1.81 NTU). Chlorophyll a concentrations varied between 17.36 ± 6.13 μg l -1 and 8.09 ± 4.38 μg l -1 in the inshore and offshore waters, respectively. Increases of unsustainable human activities in the lake and its catchments, plus increased degradation of wetlands, are among the causes of the observed water quality changes. In order to be fruitful and sustainable, the management of Lake Victoria and its catchment needs to take an ecosystem approach, and to involve all key stakeholders.

Benthic macroinvertebrate community composition was investigated in 2008 at 55 stations at various depths in the nearshore and offshore waters of Lake Victoria, in relation to dissolved oxygen, pH, temperature, chlorophyll a and conductivity. Macroinvertebrate abundance was higher in the shallow nearshore waters than in the deep offshore waters. The communities of the shallow, intermediate and deep environments were composed mainly of molluscs, worms and insects. Gastropods (52.68%) and bivalves (40.18%) constituted the major part of the benthic community. Bellamya unicolor, Melanoides tuberculata and Coelatura spp. were the most abundant molluscs. Various macroinvertebrates showed depth preferences, with gastropods having a strong preference for shallow waters and bivalves a weak preference for deep waters. The present findings contrast with those of most studies done before 1984, but agree with many done after that period. The presence of macrophytes and abundant detritus, a food resource for macroinvertebrates, account for their high abundance in shallow waters.

No abstract

Zooplankton composition and abundance, and its spatial and temporal distribution patterns were investigated from September 2005 to October 2007 at 51 sampling stations at various depths in the near shore, intermediate, and offshore waters of Lake Victoria. Relative to near shore (54.86 ± 32.33 ind. l−1) and far offshore (38.63 ± 11.14 ind. l−1), the intermediate waters (109.56 ± 64.70 ind. l−1) had highest abundance of zooplankton. A total of 43 zooplankton species were encountered: of those 26 species were rotifers, with Brachionus angularis, Brachionus calyciflorus, and Asplanchna sp. as the most abundant species. There were 9 species of cladocerans, with Diaphanosoma excisum and Bosmina longirostris as the most abundant species. There were 7 cyclopoid species with Thermocyclops emini, Tropocyclops tenellus, and Thermocyclops neglectus as the most abundant species. Only one calanoid species, Thermodiaptomus galeboides was encountered. Cyclopoid copepods were the most abundant (76.5%) followed by calanoids (11.3%), rotifers (8%) and cladocerans (4.3%). The present findings contrast with earlier studies from the 1930s and 50s, but agree with many thereafter. There is change of zooplankton composition and abundance with a trend towards small sized species instead of the large. Increased eutrophications, pollution, and predation pressure from an increasing abundance of dagaa, juvenile perches, and reappearing haplochromines, among others, are responsible for the observed spatial and temporal variability in distribution and abundance patterns, and generally, change.