Effect of stocking density on growth performance, and survival of Nile Tilapia (Oreochromis niloticus) in cage culture system in Lake Victoria, Kenya

Abstract

The Nile tilapia (Oreochromis niltoticus) constitutes an important animal protein source in the Great African Lakes population but wild supplies are rapidly declining. Cage aquaculture of O. niloticus is increasingly being applied in the lakes to bridge the fish supply-demand deficit. Despite the increasing use of fish cages in the Kadimu bay of Lake Victoria, Kenya, there are hardly any studies documenting acceptable stocking densities for sustainable production or research around cages necessary to inform policy on aquaculture production in the Lake. There is need to determine desirable stocking densities for the cages that provides optimal production and returns without compromising environmental quality of the waters. This study was conducted from February to September 2022 to determine the optimal stocking density and evaluate the influence of stocking density on growth performance and survival of O. niltoticus in floating cages at Kadimu Bay of Lake Victoria, Kenya. O. niloticus fingerlings with initial mean weight of 5.5 ± 1.72g, were stocked at densities of 50, 75, 100, 125 and 150 fish m^-3 in 12m³ cages replicated three times per treatment. The fish were fed thrice per day with a commercially formulated diet and measured fortnightly for growth for a period of eight months. Growth performance was measured as mean weight gain (g) considering the difference between final mean weight of fish at the end of the experiment and the initial mean weight of fish before experimentation, while % survival was measured as the proportion of fish at the end of the experiment divided by the number of fish stocked multiply by 100. 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 g and 527.4 ± 13.80 g, respectively). The least mean weight gain was observed at stocking densities of D125 & D150 (248.3 ± 10.64 g and 253.0 ± 10.04 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) and was significantly lower (p<0.05) than the D50 and D75 treatments. The survival rate was highest (91% to 96%) for fish stocked at densities of D100 and D50 and lowest (79%, 84% and 85%) fish stocked at densities of D150, D75 and D125 respectively. Fish production (kg), was highest for fish stocked at D75 (32.9 ± 7.82 kg) and lowest for fish stocked at D125 (26.9 ± 5.78 kg). The specific growth rate (SGR, % per day) was lowest at D125 and D150 (1.6 ± 0.47 and 1.6 ± 0.30) respectively and highest at D50 and D75 (1.9 ± 0.23 and 1.9 ± 0.21), while feed conversion ratio (FCR) was lowest at D50 (1.2 ± 0.02) and highest at D150 (2.9 ± 2.01). Results of economic analysis showed the lowest Cost-benefit ratio in D50 (0.48) and highest ratio in D150 (1.16). In conclusion, stocking fish at density of D50 is more economically profitable than the other density treatments. Fish production was highest at D75 but the cost- benefit ratio was best for fish stocked at D50, suggesting D50 to be the most suitable stocking density for cage fish farmers in the study area in sharp contrast to the current practice of stocking 100 fish per m³. However, the cost-benefit analysis is based on a single production cycle and will need to be re-evaluated in subsequent production cycles. It is recommended for fish farmers to stock fish at a density of 50 fish m^-3 in the study area as opposed to the current stocking of 100 fish m^-3 for sustainable fish production.