Effects of fish cage culture on the water quality in Napoleon Gulf, Northern Lake Victoria
Cage fish culture is a fast growing industry in Uganda due to low fish production and the need for increased food security and employment. There are, however, growing concerns that proliferation of fish cages in Lake Victoria will contribute to water quality deterioration. This study therefore focused on providing more information about the effects of cage fish farming on the water quality in the Napoleon Gulf Northern Lake Victoria, using benthic macro-invertebrates. Four sampling sites were set up: site 1 (reference site) was located at about 1km from the fish cages, site 2 and 3 were located in the fish cage zone and site 4 was located opposite site 1 at about 400 m from the fish cages. Water column parameters (i.e. temperature, dissolved oxygen, electrical conductivity, pH, water transparency, phytoplankton biomass and concentrations of dissolved nutrients i.e. Ammonia-N, Nitrite-N, Nitrate-N and Soluble Reactive Phosphorus) and benthic macro-invertebrates community parameters (i.e. numerical abundance, Shannon-Weaver diversity Index and modified Hilsenhoff Biotic Index) were analyzed twice every month from October 2012 to February 2013. Parameter values from cage sites (site 2 and 3) were statistically compared to those from none cage sites (site 1 and 4). Data on water temperature, DO, electrical conductivity, pH, water transparency and total depth were obtained in situ using a portable multi-parameter probe (model HQ40d). Nutrient concentrations, phytoplankton biomass, benthic macro-invertebrate abundance and species diversity were determined in the laboratory from water and sediment samples respectively. Measurements and samples relative to the water column were taken in replicates at both the surface and the bottom. No significant differences were observed in physico-chemical parameters, phytoplankton biomass and benthic macro-invertebrates‟ numerical abundance between cage and none cage sites. Benthic macro-invertebrate species diversity differed significantly between the reference site and cage site 2 (P<0.05), being lower at cage site 2. Cage site 2 had the highest value (8.6) of HBI but no significant differences were detected between cage sites and the reference (P=0.9). Pollution tolerant Chironomus sp. and Melodies tuberculator were significantly higher at the cage sites than the none-cage sites (P<0.05). These results suggest that cage fish farming significantly changed water quality as manifested in the observed reduction of species diversity and increased abundance of pollution tolerant benthic macro-invertebrates‟ species. The study recommends the need for continuous monitoring and the establishment of management strategies for the cage aquaculture sector that prevent or manage future effects on water quality.