40 December 2011 Table 1. Category wise Nutrient recycling ( percent) in the experimental pond Pond types Total Nitrogen (mg/L) Total Phosphorus (mg/L) Intake Outlet Inlet percent of Intake Outlet percent of TN recycling TN recycling Canal connected 2.98 2.67 2.62 1.68 percent 2.98 2.67 1.68 percent River connected 3.264 2.80 2.725 2.30 percent 3.264 2.80 2.30 percent pond connected 2.803 2.43 2.29 5.00 percent 2.803 2.43 5.00 percent Table 2. Nutrient dynamics of canal connected, river connected and pond connected ponds. Subject Canal Connected Pond River Connected Pond Pond Connected Pond N P N P N P N, The rate of 50.94 92.214 43.457 33.685 nutrient input (kg) 78.471 54.896 F. the Flushing rate 80 80 96 96 60 60 U, the volume of 1351698 1351698 1630131.60 1630131.60 342780.9 342780.9 water in pond C. Nutrient 0.0007 0.0004 0.0005 0.0002 0.002 0.001 concentration (g /m3) Water nutrient 0.07 0.25 0.05 0.125 0.02 0.625 concentration Table 3. Growth record of P. monodon in the 3 ponds after rearing 150 days. Types of pond Pond size (m2) S.D/m2 Length Weight Growth Stock Survival No of Biomass Production (mm) (gm) (g/day) (percent) juvenile (kg) (kg/acre) Canal 809,400 5 165 38.45 0.256 4,047,000 45 1,821,150 70,023.21 55.12 connected River 1,072,455 4.5 170 39.08 0.260 4,826,047 50 2,413,023 94,300.93 60.78 connected Pond 283,290 5 158 32.45 0.230 1,416,450 40 566,580 18,385.52 42 connected pond imum during in the month of August. TN concentration in experimental ponds was higher in discharged water than inlet or supply water. Inlet TN was higher than discharged water during the pre-monsoon period (February to May) and reduced in the rainy season (June-August) compared to the outlet. Inlet and outlet water samples had about to same TN concentration during September when the post-monsoon season began at the end of the rainy season. TP concentration was higher in supply water than discharged water in the RCP and PCP but discharged water was higher in the CCP. The TP concentration was also higher in discharged water in inlet or supply water during rainy season. The overall data showed that TN concentration was slightly higher in outlet water than inlet and TP concentration was higher in inlet water. The lower outlet concentration of N and P indicated a net retention of nutrients in the pond ecosystem from the surrounding aquatic system (Tables 1-4). Hambrey et al. (1999) referred to the effluent standards for coastal water in the Asian Pacific countries where the Ministry of Agriculture, India in 1995 mentioned the TN was 2 mg/L and TP was 0.2-0.4 mg/L. The nutrient levels were high in terms of NH3-N (0.50-2.60 mg/L) at the experimental ponds compared to the standard level declared in 1991 by the Department of Environment of Bangladesh (<0.4 mg/L) from March to August, during the peak shrimp culture period. Thus, there is no question that eutrophication occurs during the shrimp culture period. A 60-96 percent flushing rate in the experimental ponds may control eutrophication within the system. The environmental capacity is not being exceeding according to the Vietnam standard (4mg/L TN and 2 mg/L TP). Survival rates obtained in the culture ponds were 45 percent (5.0 /m2), 50 percent (4.5/m2) and 38 percent (5.0/m2) (Continued on page 42)
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