In Vietnam, the giant freshwater prawn Macrobrachium rosenbergii is becoming an increasingly important targeted species, as its culture, especially in rice fields, is considered to have the potential to raise income among impoverished farmers. The production of M. rosenbergii based on aquaculture reached over 10 000 tons per year in 2002, having increased from about 2500 tons since the 1990s. Until recently, lack of a stable supply of seed had been an important obstacle to the further expansion and development of M. rosenbergii culture, but cumulative research on larval rearing, especially in the 1990s, has led to the development of new seed production technology based on the 'modified stagnant green water system'. Following its dissemination by the efforts of provincial authorities, hatchery operators, and farmers, the freshwater prawn seed production industry developed rapidly in the Mekong Delta with over 90 hatcheries producing 76.5 million postlarvae in 2003. This is considered to have affected the expansion of rice-prawn farming in the Mekong Delta, leading to increased aquacultural production in the region. This paper reviews the current status of freshwater prawn culture in Vietnam and background history, and presents a socioeconomic evaluation of seed production technology implementation.KEY WORDS: freshwater prawns, green water, Mekong Delta, rice-prawn farming, seed production.

Production management of intensive prawn farming strongly differs between dry and wet season in terms of availability and quality of prawn seeds and water quality. Secondary data implied that prawn seed batches from the market had a total pathogen infection rate of 53%, predominantly caused by 37% white spot syndrome virus (WSSV), 19% monodon baculovirus (MBV) and 4% yellow head virus (YHV). Pathogen-free seed batches averaged to 52% during the dry season compared to the wet season (42%). Survey revealed average stocking density of 17 PL m -2 with a survival rate of 55%, a crop yield of 2,470 kg ha -1 crop -1 and a net income of 6,768 USD ha -1 crop -1 . Average production cost amounted to 3.4 USD kg -1 , and feed cost accounted for 58% of the production cost. Technical efficiency with respect to prawn yield and survival rate was higher during the dry season than the wet season. The probability of yield loss was 15.6 times higher in the wet than in the dry season. A positive net income can be generated for farms operating in the dry season using new ponds and performing pathogen laboratory tests on prawn seeds before stocking.

A technical and socio-economic survey conducted in the Dong Thap province of Vietnam to assess the current status of river pen culture of giant freshwater prawn Macrobrachium rosenbergii (De Man) showed that pen culture, which has been developed and operated by farmers' indigenous knowledge, requires more study in order to optimize stocking density and to assess its environmental impacts. In this study, the prawn pens were of rectangular shape, with an average size of 209 m 2 . Prawns were stocked in June at an average density of 62 pieces m À 2 and fed on farm-made feed and were harvested 2^4 times starting from 4 months after stocking until late December or the following January. The average prawn yield was 0.52 kg m À 2 year À1 , ranging from 0.14 to 1.6 kg m À 2 year À1 . The average net return was US$0.71m À 2 year À1 , ranging from US$1.24 to 4.37 m À 2 year À1 . About 73% of the farmers achieved positive net returns. The top ¢ve constraints for prawn pen culture were lack of knowledge of culture technologies, poor water quality, disease problems, poor quality of wild seed and lack of capital. Almost all farmers have no environmental awareness, and environmental regulations for prawn pen culture have not been established. Field measurements showed that all measured water quality parameters were within the range for good growth of giant freshwater prawns, and indicated that there was no signi¢cant accumulation of nutrients and organic matter at the bottoms of rivers or canals.