Modeling the water movement and water quality of a mangrove-shrimp system in Vietnam

Abstract

Shrimp farming is an important source of income in Vietnam. In the Mekong Delta several shrimp farms are established, due to its stable weather and environmental conditions making it possible to farm throughout the whole year. The farm where this research was done is an extensive culture system. In these systems, the water is exchanged using the tidal range. During low water, water is let out and with the incoming tide water is let in. No feeding or other improvements to increase the shrimp harvest are done. These systems are often combined with another agricultural production system like for instance rice or mangroves. The extensive culture system used for this research is a so-called mangrove-shrimp system. Channels are used to farm shrimp and the land between the channels is used to grow mangroves. A good harvest is important for the farmers to earn money, manage the pond, and stay independent. It is said that during the last decade the harvest of shrimp has decreased. A large number of factors can cause this decrease of the harvest. A computer model is made describing the water movement in combination with the water quality to understand the system and to find alternatives to improve the dissolved oxygen concentration. Measurements were done in Vietnam concerning the water movement and water quality. Their analysis showed that the sediment consumed a lot of dissolved oxygen. The model showed that the dissolved oxygen concentration in the forest was lower compared to the channels. During inlet water from the forest, containing very low dissolved oxygen concentrations flowed into channels and decreased the dissolved oxygen concentrations in it. During inlet the dissolved oxygen concentrations increased rapidly. The sections situated further from the sluices were not good influenced, by the time the water reached these parts the dissolved oxygen concentration already decreased due to the high consumption of dissolved oxygen by the sediment. Several different options were looked at to improve the dissolved oxygen concentrations. These options concerned increasing the influence oxygen providers, and decreasing the influence of the oxygen consumers. The most important conclusions found after this research were that - If the water level inside the pond follow the tide, the water level changed constantly causing a permanent flow in the system. Together with exchanging more water, more leaf litter will be transported out of the pond. Less leaf litter present decreases dissolved oxygen consumption resulting in higher dissolved oxygen concentrations. - If the pond is managed using sluices, the combination of mangroves and shrimp decreases the amount of water daily exchanged and so decreases the self-cleaning of the system. The combination of the mangroves and shrimp in a system managed with sluices is not a good one. - More research must be done concerning the functioning of the pond (nursery or shrimp trap), population dynamics and other factors for the decreasing harvest that like predators and diseases.