Water quality and plankton densities in mixed shrimp-mangrove forestry farming systems in Vietnam

Water quality and plankton densities were monitored in shrimp ponds at 12 mixed shrimp-mangrove forestry farms in Ca Mau province, southern Vietnam, to detail basic water chemistry and assess whether conditions are suitable for shrimp culture. In general, water quality was not optimal for shrimp culture. In particular, ponds were shallow (mean ± 1SE, 50.5 ± 2.8 cm), acidic (pH < 6.5), had high suspended solids (0.3 ± 0.03 g l-1), low chlorophyll a/phytoplankton concentrations (0.2 ± 0.05 μg l-1 and 8600 ± 800 cells l-1 respectively) and low dissolved oxygen (DO) levels (3.7 ± 0.15 mg l-1). Eight out of the 12 farms sampled had potentially acid sulphate soils (pH < 4.2). Salinity, DO and pH were highly variable over short time-periods (hours); DO in particular was reduced to potentially lethal levels (1-2 mg l-1). Seasonal variations in water chemistry and plankton communities (i.e. salinity, DO, phosphate, temperature, phytoplankton and zooplankton densities) appear to be driven by differences in rainfall patterns. The presence or absence of mangroves on internal pond levees ('mixed' versus 'separate' farms) and the source of pond water (rivers versus canals) were of lesser importance in determining water quality patterns and plankton biomass. Zooplankton and macrobenthos densities were sufficient to support the current (low) stocking densities of shrimp. However, natural food sources are not adequate to support increases in production by stocking hatchery reared post larvae. Increasing productivity by fertilization and/or supplemental feeding has the potential for adverse water quality and would require improvements to water management practices. Some practical strategies for improving water quality and plankton densities are outlined.