Biological characteristics of the improved extensive shrimp system in the Mekong delta of Vietnam

Low and unstable shrimp yields of the improved extensive shrimp system has been a tremendous obstacle for economic development in the coastal areas of Southern Vietnam. To investigate the biological characteristics of this system, ponds in the coastal Cai Nuoc district, Mekong delta of Vietnam, were monitored. Results showed that the system was not optimal for shrimp. While chlorophyll a (chl a) (1.5137.2 mu g L-1), phytoplankton density (6333974 444 cells L-1) and zooplankton density (7.1517.2 ind L-1) were abundant and comparable to shrimp farms elsewhere, zoobenthic community was very poor (71971 ind m-2). Toxin-producing cyanobacteria (Oscillatoria limosa, Oscillatoria formosa, Anabaena sp. and Phormidium tenue) were found. Total bacteria and Vibrios were present in large numbers (respectively 1.04 x 105 and 6.64 x 102 CFU mL-1 in pond water, 6.33 x 105 and 9.47 x 103 CFU g-1 in sediment). Presence of toxin-producing organisms, poor zoobenthic community and abundance of Vibrios all can enhance shrimp susceptibility to diseases. The following measures are recommended to improve the situation: (1) complete testing of seeds for pathogens, (2) not to incorporate fish into shrimp ponds and (3) applying no-culture breaks and pathogen-killing chemicals.