Use of sewage in split doses to enhance water productivity for fish culture

The problem of sewage disposal has received great attention worldwide. The raw sewage contains a variety of high inorganic and organic matters that affect natural water environment. To mitigate such problem, sewage may be recycled through aquaculture practice. Sewage recycling in aquaculture enhances water productivity through nutrients input. Proper loading of sewage ensures viable aquaculture; otherwise, fish mortality occurs due to poor water quality. To optimize sewage application, two different experiments were conducted, each with four treatments. In both experiments, three fish species namely rohu (Labeo rohita Hamilton, 1822), mrigal (Cirrhinus mrigala Hamilton, 1822), and bata (Labeo bata Hamilton, 1822) were tested in triplicate in FRP (Fibre-Reinforced Plastic) tanks. Different sewage concentrations (0, 25%, 50% and 75%) used in first experiment were prepared by mixing freshwater, showing Biochemical Oxygen Demand (BOD) 2.0 ± 0.4 mg/L, 10.8 ± 1.4 mg/L, 19.6 ± 1.5 mg/L and 41.6 ± 2.58 mg/L, respectively. After 30 days rearing, results showed ≥75% fish survival in sewage concentrations up to 50% with BOD level 19.6 ± 1.5 mg/L. Less than 50% fish survived in 75% sewage concentration, with BOD level as 41.6 ± 2.58 mg/L. The second experiment was conducted for 90 days considering 50% sewage concentration as basal dose with BOD level as 19.6 mg/L as an acceptable limit for fish survival. Split doses of sewage were applied in T1, T2 and T3 treatments fortnightly, weekly and semi-weekly intervals, while single dose was used in C (control) treatment. Application of split doses resulted better hydro-biological changes, including nutrients recovery, in T1, T2 and T3 than that of single dose in control. Fish growth plotted with net primary productivity (NPP), phytoplankton and zooplankton densities exhibited positive correlations in T2 (12 doses) and T3 (24 doses), considered as optimal doses to ensure better water productivity for desirable fish production than sewage with single dose or limited doses (6 doses).