Effect of different doses of organic fertilizer (cow dung) on pond productivity and fish biomass in stillwater ponds

The effects of five (5 000, 10 000, 15 000, 20 000, 24 000 kg ha^?1 year^?1) different doses of organic fertilizer (cow dung) were studied on pond productivity in terms of plankton production and fish biomass in freshwater fish ponds. The grow out period was 60 days. Physico-chemical factors of pond waters were also monitored. With an increase in the fertilizer dose, biochemical oxygen demand (BOD) (1.7 ± 0.1 – 10.35 ± 0.05 mg L^?1), O-PO₄ (0.04 ± 0.0 – 0.77 ± 0.02 mg L^?1) and NH₄-N (0.03 ± 0.02 – 0.32 ± 0.02 mg L^?1) increased significantly (P < 0.05). Alkalinity (79.0 ± 1.6 – 164.0 ± 3.8 mg L^?1) also increased with the increase in fertilizer dose, declining after 60 and 75 days (48.8 ± 1.13 – 67.9 ± 2.1 mg L^?1). NO₃-N was maximum (1.66 ± 0.2 mg L^?1) in the ponds which received cow dung at 15 000 kg ha^?1 year^?1, and declined (0.94 ± 0.5 mg L^?1) at higher doses. Dissolved oxygen (DO) remained significantly high (4.7 mg L^?1) up to the third (15 000 kg ha^?1 year^?1) treatment. Highest plankton population (phytoplankton 17 350.0 ± 1 250.0 no L^?1), zooplankton (373.0 ± 22.0 no L^?1), species diversity (phytoplankton 3.0, zooplankton 2.3), fish biomass (4.45 kg) and specific growth rate (SGR) (2.36 % body weight (BW) d^?1) were also observed in ponds which were treated with fertilizer at 15 000 kg ha^?1 year^?1. However, at higher doses, a decline in these parameters (phytoplankton, 0.0 – 8 810.0 ± 690.0 no L^?1; zooplankton, 0.0 – 205.0 ± 25.0 no L^?1; fish biomass, 2.3 kg; SGR, 1.25 % body weight (BW) d^?1) was observed. Furthermore, with a decrease in the water temperature from 24 °C (on day 60) to 21 °C (on day 75), a decline in nutrient release, plankton population L^?1 and species diversity was observed. Sediment analysis indicated that with an increase in the fertilizer dosage, a significant and progressive increase in the accumulation of organic carbon (0.787 ± 0.006 – 0.935 ± 0.01), total nitrogen (0.877 ± 0.071 – 1.231 ± 0.03), NH₄-N (54.4 ± 0.57 – 68.95 ± 0.81), NO₃-N (78.5 ± 1.21 – 98.5 ± 0.35), total P (140.0 ± 0.50 – 151.0 ± 1.27) and soluble P (7.15 ± 0.18 – 10.1 ± 0.56) took place; similarly, electrical conductivity (EC) values of sediment also increased progressively (from 200.0 ± 7.1–300.0 ± 10.63 μ mhos cm^?1).