Effect of stocking density on the physico-chemical characteristics of pond water and survival rate of Nile tilapia (Oreochromis niloticus) fish in Bahir Dar, Ethiopia

The quality of culture water is an important parameter in fish farming as it is the major factor that affects the survival, growth, reproduction and health of the culture animal. With fish stocking density, the physical and chemical characteristics of pond water changes frequently to the extent that affects the performance of the fish. This study aimed to evaluate the effect of stocking density on the physico-chemical characteristics of pond water and the survival rate of Nile tilapia. The study was conducted at the Fishery Research Center in Bahir Dar, Ethiopia, from February to August 2021. Throughout the culture period, the physical water quality parameters of each treatment were recorded twice a day and the measurements of fish growth and chemical parameters of pond water were taken every month. Data recorded during the experimental period was analyzed using Microsoft Excel and the variation between the different means was compared using the SAS software version 9.4. The result of this experiment indicated the existence of significant differences in most water quality parameters at different densities of fish stocking. There was also a difference in the survival of the experimental fishes between treatments. However, the difference in fish survival was not significant and there was no problem on the welfare of fishes. Temperature, dissolved oxygen (DO), pH, conductivity, and salinity ranged between 22.07 °C to 29.43 °C, 3 mg/L to 8.15 mg/L, 6.4 to 10.39, 178 µS/cm to 241µS/cm, and 0.035 psu to 0.12 psu respectively. Secchi depth and chlorophyll-a (Chl-a) ranged between 22 cm to 39 cm and 0.0153µg/L and 0.0513 µg/L respectively. The differences in DO, pH, conductivity, secchi depth, and chl-a between treatments were significant with stocking density (p < 0.05). The variation in the level of ammonia was highly significant (p < 0.001) between treatments and water quality deteriorated with density. However, the mean ranges of Physico-chemical water quality parameters were at the recommended range for tilapia culture and hence the survival rate of O. niloticus at different stocking densities was higher. For the sake of getting higher production per unit area, it is recommended to stock 7 fishes/m2.