Evaluation of limnological dynamics in Nile tilapia farming tank

Abstract

For fish farming, water is the limiting factor for good productivity. Limnological characteristics determine the health and the productivity of the fish and can indicate possible problems arising in the ecosystem. The present study aimed to verify the nictemeral and vertical dynamics of limnological characteristics in a tilapia farming tank coated with geomembrane and with low water renewal. Collections were carried out in a 24 hour on period of 18 days with an interval of three hours between the water collections. In order to determine the vertical dynamics, water samples were taken at three depths (5, 35, and 70 cm) in the central region of the tank. It was found that the parameters analyzed showed changes in the period. Dissolved oxygen and pH changed sharply in the 24-hour period (p $ <0.05 $), with values of 3.86 mg L$^{-1}$ at 6:00 AM and 18.39 mg L$^{-1}$ at 3:00 PM; and 8.0 at 06:00 AM and 9.65 mg L$^{-1}$ at 6:00 PM, respectively. With respect to the three layers of the tank, the difference between the values of dissolved oxygen in the epilimnion and the metalimnion (above 14 mg L$^{-1}$) is clear, while in the hypolimnion the concentration of this oxygen was 7.57 mg L$^{-1}$ (p $ <0.05 $). Nitrite and phosphorus had similar behavior, with more pronounced values in hypolimnion. There is a strong correlation between temperature and depth of the tank ($r=-0.85, ~ p = 5.86 \ imes 10^{-9}$), depth and dissolved oxygen ($r=-0.77, p = 9.97 \ imes 10^{-6}$), temperature and ammonia ($r=-0.82, ~ p = 6.33 \ imes 10^{-8}$), depth and ammonia ($r=0.79, ~ p = 4.79 \ imes 10^{-7}$), and nitrite and dissolved oxygen ($r=-0.72, ~ p = 2.15 \ imes 10^{-4}$). The principal component analysis showed that there was a separation between the strata epilimnion, metalimnion, and hypolimnion. The response surface analysis showed the interaction between depth, temperature and the variables dissolved oxygen, ammonia, and phosphorus. The result of this study helps to illustrate the dynamics of a tilapia tank covered with geomembrane and low water renewal over a period of 24 hours at different layers of the water body.