ECOLOGICAL CARRYING CAPACITY AND GROWTH PERFORMANCE OF NILE TILAPIA (Oreochromis niloticus) IN CAGE AQUACULTURE WITHIN KADIMO BAY, LAKE VICTORIA, KENYA

Abstract

Fish production in the wild is decreasing globally due to a number of factors including overfishing, pollution, invasive species, and climate change effects. In Kenya, fisheries contribute less than 1% to the national GDP with an annual production of about 400, 000 mt against a demand of about 600,000 mt. Aquaculture production through innovative approaches such fish cage farming, has the potential to bridge the demand deficit. Despite the high potential for cage fish farming in Kenyan water bodies, there have been few studies focused on the effects of fish cages on water quality and trophic status, the nutrient carrying capacity of cage sites, and the appropriate stocking densities for cages in the water bodies. This study therefore was aimed to bridge these data gaps in order to facilitate sustainable management of the increasing fish cage farming of the Nile tilapia (Oreochromis niloticus) in Lake Victoria. Sampling for physico-chemical and biological variables, including nutrient load, was conducted from January to October 2021, at five fish cage sites and a control site within the Kadimo Bay,Lake Victoria, Kenya. The Carlson's Trophic State Index (CTSI) was used to classify the trophic state of the cage sites in the bay, and TN: TP ratio used to determine nutrient limitation in the bay. Fish cage optimum stocking density studies were carried in the bay from February to September 2022. Oreochromis niloticus fingerlings with initial mean (±SD) weight of 5.5 ± 1.72 g, were stocked at densities of 50, 75, 100, 125 and 150 fish m3 in replicate cages and growth and water quality changes monitored. The TP assimilation capacity and fish production potentials for the five cage sites within the bay were determined using a mass-balanced model. Results showed higher electrical conductivity (112.84 ± 1.94 μS cm-1) at cage sites compared to a Control site (97.53 ± 4.17 μS cm-1), similar variations were observed for nitrates and chlorophyll-a. However, 15 physico-chemical variables (DO, Temp., pH, TDS, Turb., TSS,POM, SRP, NO2-, NO3-, TN, TP, NH3, NH4+, SiO4 4-) did not vary significantly between the cage and control sites. The bay was evaluated as being in a light eutrophic state. Nitrogen as opposed to Phosphorus, was indicated to be the limiting nutrient for primary production in the bay. Growth performance results showed that fish stocked at lower densities (D50 & D75) had the highest growth performance in terms of mean weight gain (545.0 ± 15.81 and 527.4 ± 13.80 g, respectively). The Control treatment (D100), which is the normal stocking density used by cage fish farmers, showed intermediate mean weight gain (348.2 ± 11.48 g) which was significantly lower (p < 0.05) than for the D50 and D75 treatments. The feed conversion ratio (FCR) was lowest at D50 (1.2 ± 0.02) and highest at D150 (2.9 ± 2.01). Carrying capacity results, showed for all the five cage sites within the bay, the TP assimilation capacity was exceeded by the TP released by the fish cages. Additionally, the maximum estimated fish production capacities were much less than the current fish production levels for all the sites. Overall, although the results of this study showed cage aquaculture is not a current challenge to the water quality of the bay, regular monitoring is recommended to inform sustainable aquaculture development in the bay and the lake. It is recommended for fish farmers to stock fish at lower densities of 50 fish m3 in order to maximize sustainable economic and environmental benefits of the cage culture system. Policies governing aquaculture production in the lake should be reviewed or enacted in order to include evidence-based information on environmental quality, sustainable production levels, and nutrient carrying capacity of the lake.