World Aquaculture 39 for the research on the basis of their water exchange systems. The first category was a river connected pond (RCP), where water exchange in occurred directly from the river. The second category was the canal connected pond (CCP), where water exchange occurred from canals, while the third category was the pond connected pond (PCP), where water was exchanged with another culture pond. Experiments conducted using growout ponds. There were inlet and outlet facilities used for receiving and discharging the water. The entire network of the present program comprised one year of sampling at a frequency of twice per month in conjunction with the full moon and new moons. The sampling times were selected using BIWTA (Bangladesh Inland Water Transport Authority) tide tables. Water samples were collected from selected ponds throughout the the month sstudy period. First, intake water was collected from the ponds. Then the inflow water samples were collected during spring tide, just at the time of entering the river, canal or pond. Water flowed into the culture ponds through the inlet. Outflow samples were collected during neap tide, from the outlet at the time of discharge. To select the sampling time, the tide table of the BIWTA was consulted for the Bakkhali river reference point. Water quality analyses involved determination of total nitrogen level from the values obtained for ammonia nitrogen (NH4-N), nitrite nitrogen (NO2-N), nitrate nitrogen (NO3-N). Phosphate-phosphorus (PO4-P) levels were also determined (APHA 1965). Farmers commonly consider a number of variables when they evaluate the health of the shrimp in the culture system. The variables include survival rate, mortality, growth rate, size variation, food conversion ratio (FCR) and appearance of the shrimp (Brock and Main 1994). Survival rate was calculated by using the formula described by Solbe (1954). Absolute growth rate and specific growth rate in terms of length and weight were determined from the simple arithmetic calculation advocated by Sedgwick (1979). The FCR was calculated by the formula used by Castell and Tiews (1980) FCE or feed conversion efficiency is the reciprocal of the FCR converted to a percentage value. Results Coastal shrimp aquaculture in Bangladesh is still traditional or improved traditional. Farmers used low amounts of lime and fertilizer and the applications of feed is rare to absent. The present study indicates that improving traditional systems of shrimp culture did not provide any appreciable loading of nutrients to the surrounding coastal environment During the present investigation, it was found that intake water always carried higher nutrient levels than inlet and outlet or discharged water in the three experimental ponds. In the canal connected pond, average total nitrogen (TN) concentrations of intake, inlet or supplied water and outlet and discharge water were 2.98, 2.62 and 2.67 mg/L, respectively. Total phosphorus (TP) levels were 0.623, 0.59 and 0. 60 mg/L in intake, inlet and outlet water, respectively. The nutrient concentration of water discharged from the ponds increased during the rainy season and reached a maxFig. 1. Map of the study area. Matamuhur River, which is the main water source for shrimp fsrming in Cahkris upazila.
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