World Aquaculture Magazine - June 2020

WWW.WA S.ORG • WORLD AQUACULTURE • JUNE 2020 21 environmental friendliness and food safety. With the advance of super-intensive and high-tech shrimp culture systems, this is an important national strategy, with a target of 150,000 ha and 700,000 t produced from these type of systems by 2025 (Decision No. 79-TTg, 2018). Mud Crab Culture There are two species of mud crabs in Vietnam, Scylla paramamosain and S. olivacea , of which the first is more abundant (Keenan 1999, Macintosh et al. 2002). Mud crab culture in the Mekong Delta started in the early 1980s with different systems. Currently, grow-out of mud crab in integrated mangrove- shrimp farming systems and improved-extensive shrimp farming systems are the most important (Hai 2017). In the early stages of development, mud crab seed for farming mainly came from the wild catch or natural recruitment to farms through water supply gates. With the possibility of replacing rotifers with umbrella Artemia as a starter food, great progress was made in seed production of mud crabs, with over 600 hatcheries and 1.5 billion crablets now produced yearly in the region (Hai et al. 2018). Mud crab farming currently relies mainly on hatchery-produced seed of S. paramamosain . In grow-out, mud crabs are stocked extensively with 500- 1000 crabs/ha per month and the crabs rely mainly on natural food. Crabs are partial-harvested year-round with production of 50-150 kg/ha per year. With over 350,000 ha of culture area in the Mekong Delta, total production of mud crab in the region is about 38,000 t (Hai 2017), out of about 53,000 t produced in the whole country (Hungria et al. 2017). Mud crab is a euryhaline species, with a high tolerance to poor water quality and disease pathogens, and a high value for domestic and export markets. It is thus a very important candidate for culture in the coastal areas of the Mekong Delta in different systems to adapt to climate change and saltwater intrusion (Hai 2017). Giant Freshwater Prawn Culture in BrackishWater Giant freshwater prawn Macrobrachium rosenbergii is an important species in traditional freshwater aquaculture in the Mekong Delta in garden ponds or integrated with rice farming (Phuong et al. 2003). In recent years, prawn culture has strongly expanded in brackishwater areas of the region (Fig. 4), with a total culture area of more than 15,000 ha and production of more than 5,000 t. Prawn culture in brackish water accounts for 90 percent of the total area and 65 percent of the total production (Hai et al. 2017). In freshwater areas, the most common system is the alternative rice-prawn farming system (Fig. 5) in which prawns are cultured at semi-intensive levels in rice fields during the height of the rainy season, with production of about 0.5-1.5 t/ha per crop. The two main prawn farming systems in coastal areas are prawn and rice farming alternatively with tiger shrimp culture in rice fields and prawn culture in ponds alternatively with tiger shrimp culture. Prawn stocking density and production are about 1.1 ± 0.6/ m 2 and 110 ± 52 kg/ha per crop for the first system; and 9.0 ± 3.6/ m 2 and 886 ± 642 kg/ha per crop for the second system (Hai et al. 2017). Mixed-sex and all-male post-larvae are mainly supplied from local hatcheries in the region with different production systems. Water salinity during prawn culture ranges from 0-10 g/L. Giant freshwater prawn can tolerate a wide range of salinity (0-25 g/L) and grow well at 0-15 g/L (Huong et al. 2010). Prawn cultured in brackish water grow well, are high quality, have high resistance to diseases and provide a very good income. The coastal zone also has a large potential area of brackish water less than 15 g/L. These areas are very important to further promote the industry in the region, to adapt to climate change and saltwater intrusion, to diversify culture species and income, and especially to rotate culture species that reduce the risk of diseases and environmental problems due to over-development of shrimp culture in the area. The target for giant freshwater prawn culture industry is 50,000 ha and 55,000 t by 2025 (Decision No 79 – TTg, 2018). Mollusk Culture The total culture area of mollusk culture in Vietnam is 41,200 ha and production of mollusks reached 300,000 t recently (MARD 2019b). The Mekong Delta has about 60 percent of the culture area and about 31 percent of total production (MARD 2018). In the Mekong Delta, mollusk culture is popular with different ( C O N T I N U E D O N P A G E 2 2 ) FIGURE 4. Giant freshwater prawn culture in brackishwater ponds. FIGURE 5. Rotational rice-freshwater prawn farming system in the rainy season.