38 December 2011 Nutrient recovery and production of tidal water exchange-based coastal shrimp ponds of the southeast coast of Bangladesh Prabal Barua1* and M. Shah Nawaz Chowdhury Bangladesh has placed a high priority on coastal aquaculture, particularly in shrimp culture (Thia-Eng et al. 1989). However, this industry has come at a huge environmental cost to coastal communities and ecosystems (Salequzzaman 2001, Yeh 2002) because it is accompanied by mangrove destruction, loss of fishery communities and biodiversity, pollution of land and water, loss of employment activity and even violation of human rights (Aksornkoae 1989, Salequzzaman 2001). In shrimp culture water exchange is now emerging as a very powerful tool for disease control. Water is partially replenished during spring tide by making use of the tidal energy. If good water quality is available, water exchange can remove excess plankton. Changing more than 20 percent on one occasion often leads to rapid changes in water quality. This can stress the shrimp, causing mortalities or increasing susceptibility to disease (Chanaratchakool et al. 1995). Nutrient load is the levels of nutrients released from the ponds into surrounding waterways over a period of time. Because of the use of inorganic fertilizers and manure, water that passes through agricultural soils often carries a high nutrient load into local creeks. An overabundance of nutrients in water causes algal blooms and decrease dissolved oxygen levels (Hepher 1988). Water also is discharged from some ponds in response to water exchange. Although there is considerable interest in water reuse, or closed cycle production systems, it currently is not technically or economically feasible to conduct most types of aquaculture without discharge. Normally, no more than 25 to 30 percent of the nitrogen and phosphorus applied to ponds in the form of fertilizers and feeds is recovered in fish or shrimp at harvest (Boyd and Tucker 1992). Ponds have a remarkable ability to assimilate nitrogen and phosphorus through physical, chemical and biological processes (Schwartz and Boyd 1994). Nevertheless, ponds often have higher concentrations of nutrients, plankton, suspended solids and oxygen demand than the water bodies into which they discharge (Schwartz and Boyd 1994). In Bangladesh, water exchange in shrimp ponds is carried Coastal shrimp ponds out for four to six days at new and full moon every fortnight. For 3-4 hours, water enters the pond. However, during the rainy season, the heavy rainfall reduces the water intake time to only one hour. The water level in the ponds is drained during ebb tide and replenished during flood tide. Considerable amounts of nutrients entered the shrimp ponds with the inlet water (Islam et al. 1998). Wahab (2003) estimated that in extensive shrimp culture about 70 percent of phosphorus and 30 percent of nitrogen entering with the inlet water becoming trapped in the system. Study Methods Chakaria Upazila, the highest shrimp culture and production area for Cox’s Bazar, was selected for the research. Its geographical coordinates are 21°45’0”N and 92°5’0”E. The selected experimental culture ponds are located in the intertidal area of the Matamuhuri river estuary which drains into Bay of Bengal. The farms were situated on the south side of the Matamuhuri (Bara Muhuri) under the West Barabheola union in Ilishia village (Figure 1). There were three types of coastal shrimp ponds selected
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