Abstract:

Growing of fish in cages is currently practiced in Uganda and was first introduced in northern
Lake Victoria in 2010. An environment monitoring study was undertaken at Source of the Nile,
a private cage fish farm, in Napoleon gulf, northern Lake Victoria. In-situ measurements of key
environmental (temperature, dissolved oxygen, pH and conductivity) and biological (algae,
zooplankton, macro-benthos) variables were made at three transects: Transect 1- the site with
fish cages (WC); transect 2- upstream of the fish cages (USC-control) and Transect 3- downstream
of the cages (DSC). Upstream and Downstream sites were located approximately 1.0 km from the
fish cages. Environment parameters varied spatially and temporally but were generally within
safe ranges for freshwater habitats. Higher concentrations of SRP (0.015-0.112 Mg/L) occurred at
USC during February, September and at DSC in November; NO2
-N (0.217- 0.042 mg/L) at USC
and DSC in February and November; NH4
-N (0.0054- 0.065 Mg/L) at WC and DSC in February,
May and November. Algal bio-volumes were significantly higher at WC (F (2,780)=4.619; P=0.010).
Zooplankton species numbers were consistently lower at WC with a significant difference compared
to the control site (P=0.032). Macro-benthos abundance was consistently higher at the site with
cages where mollusks and low-oxygen and pollution-tolerant chironomids were the dominant
group. Higher algal biomass, concentration of low-oxygen/pollution-tolerant macro-benthos and
depressed zooplankton diversity at WC suggested impacts from the fish cages on aquatic biota