Phosphorus-based carrying capacity, and trophic state index of cage fish sites in the Kadimu Bay, Lake Victoria, Kenya

Fish production in the Kadimu bay of Lake Victoria, Kenya through cage culture has the potential to increase the Phosphorus load to the aquatic ecosystem through fish feed waste. This increasing phosphorus levels in water may cause pollution leading to poor water quality that may be detrimental to fish farmers. This study focused on the calculation of the quantity of phosphorus the cage sites can accommodate and evaluation of water quality state by estimating the trophic state index for cage sites. Sampling for physicochemical parameters and nutrient load was conducted at five cage sites and a control. A replicate of water samples were collected at each site and the sites were selected because they were commercially viable cage fish farms within the Kadimu bay. The GPS of all the sites were recorded with a hand held GPS equipment. Water samples for chlorophyll-a (Chloro-a), total phosphorus (TP) and total nitrogen (TN) were taken to the Laboratory for analysis. The UV Photospectrometer was used for the analyses of TP, TN and Chloro-a. Water transparency at each sampling location were measured with a Secchi disk of approximately 20 cm diameter. The results showed that Kadimu bay had the capacity to accommodate more cages since total-P release from fish waste in to the environment was less than the total-P cage carrying capacity. Estimated Carlson trophic state index (TSI) values suggest light eutrophic state of the cage sites. This suggests a need for a precautionary approach to be adopted for sustainable management of Cage culture in the Kadimu Bay of Lake Victoria. For control and sustainability of fish cage culture within Kadimu bay, it is recommended for fish farmers to monitor physico-chemical parameters of the aquatic environment on a monthly basis, while a competent authority should monitor total-P and total-N on quarterly or annual basis. Keywords: Aquaculture waste, Chlorophyll-a, eutrophication, floating cages, Kadimu Bay, phosphorus, Secchi disk