Spatial variation in water quality, nutrients and heavy metal bioaccumulation in Nile TILAPIA, oreochromis niloticus (l.) In fish cage sites and open waters of Usenge, Lake Victoria, Kenya

Abstract

Fish cage farming is an aquaculture production system involving the holding of fish in floating net pens and was introduced in Lake Victoria to improve fish production due to diminishing stocks and subsequent increase in demand. The lake has experienced a decrease in water quality recently resulting from increased anthropogenic activities. Emerging fish cage farming in the lake may be contributing to alteration of water physico-chemical parameters, nutrient levels and high concentration of heavy metals in water and bioaccumulation in fish. The information on spatial variations of water quality, nutrients and bioaccumulation of heavy metals in fish cage sites and open waters in Lake Victoria and particularly in Usenge area is a critical environmental issue. This study’s main objective was to determine the spatial variation in water quality, nutrients and heavy metal bioaccumulation in Nile Tilapia (Oreochromis niloticus L.) in fish cage sites and open waters of Lake Victoria in Usenge area. The specific objectives were; to assess the spatial variations in water physico-chemical parameters (temp, turbidity, conductivity, dissolved oxygen and biological oxygen demand) within the fish cages and open waters, investigate the spatial variations of levels of nutrients (nitrates and phosphates) and selected heavy metals (Pb, Fe, Cd, Zn and Cu) within the fish cages and open waters and investigate the level of bioaccumulation of selected heavy metals in gills of O. niloticus fish obtained from the fish cages and open waters in the waters. A quasi experimental design was adopted in which water and fish samples were collected from selected sites within fish cages and open waters and analyzed. Physico-chemical parameters were analysed in situ while the heavy metals were analysed using atomic absorption spectroscopy. One-way analysis of variance (ANOVA) was performed to check the variations and associations within and between variables. Only DO, pH, Fe and Cu (p<0.05) varied significantly between fish cage sites and open waters sites. The levels of pH, DO, conductivity, phosphates, Fe and Cu showed variations in the different directions from the cages where water samples were collected. Both Cu and Fe were higher in fish obtained from the open waters than the fish cage sites and also varied in the different directions from the center of the cage. Turbidity, total nitrates, total phosphate and Cd were beyond acceptable limit for portable water in accordance to WHO and USEPA. The findings revealed that tilapia cage farming studied did not have any significant impact on the lake water quality and nutrients nor did it contribute to heavy metal bioaccumulation in water and in fish in the cages and open waters except turbidity and phosphate levels. The findings of this study are important for policymakers in setting guidelines for effective cage culture system management to protect lake waters and for safe fish human consumption. It is therefore, recommended that regular water quality monitoring be done for appropriate management interventions.