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¹ Aquaculture and Applied Limnology Research Unit, Department of Zoology, University of Kalyani, Kalyani-741 235, West Bengal, India Viewed : 4784 - Downloaded : 3130 The effect of different doses of mixed fertilizer on the response of some biogeochemical cycling bacteria were examined using seven different doses (i.e, 105, 211, 422, 844, 1689, 3378 and 6757 g/tank/week) of mixed fertilizers with a fixed C : N : P ratio of 88.6 : 7.5 : 1. Advanced fry of Catla catla (1.2±0.04 g), Labeo bata (0.99±0.05 g), and Cyprinus carpio (1.3±0.06 g) were introduced at the rate of 16 fish/tank and reared for 120 days. Water samples were collected and monitored for bacterial population (HB, CDB, DNB and PSB), primary productivity, plankton, water quality parameters, and total phosphate in the sediment at ten days intervals. Number of bacteria (HB 15–270 x 10³/ml, CDB 15–237 x 10²/ml, DNB 5–221 x 10²/ml and PSB 3–159 x 10¹/ml) increased directly with the increase in the fertilizer doses and showed maximum rate of bacterial growth at the fertilizer doses 48,000 kg/ha/yr. Three levels of responses to fertilizer doses - poor bacterial density and rate of fertilizer mineralization efficiency (D₁₂ to D₄₈), moderate bacterial density and highest rate of fertilizer mineralization efficiency (>D₄₈ to D₁₉₂) and highest bacterial density but slow rate of fertilizer mineralization efficiency (>D₁₉₂ to D₇₆₈) were observed. The fish growth tended to rise with the increase in fertilizer dose. Maximum fish growth was at D₉₆ (96,000 kg/ha/year) and decline thereafter. Though the bacterial load increased with fertilizer doses, the primary productivity of phytoplankton and fish growth did not increased when fertilizer doses increased. Therefore, it can be concluded that fertilizer dose 48,000 kg/ha/yr is required to manipulate the optimum bacterial population and productivity of the aquaculture pond and application of excessive doses of fertilizer increases production cost and causes environmental pollution. Keywords : mixed fertilizer dose, bacterial growth rate, fertilizer mineralization efficiency, fish growth