World Aquaculture Magazine - June 2020

22 JUNE 2020 • WORLD AQUACULTURE • WWW.WA S.ORG species and systems, especially white clam culture on muddy sand beaches, blood cockle culture in mangrove-shrimp farms or on mud flats of estuaries and beaches, and oyster culture on rafts in estuaries, rivers and canals (Fig. 6). For white clam Meretrix lyrata culture, the activities have been underway for many years along the coastline of the Mekong Delta. This is considered fisheries-based aquaculture and mostly under cooperatives. The culture area of each farm is about 6.4 ± 1.1 ha, which is simply fenced with nets on the beach. Although seed production of white clams is practiced around the country, clam seed for culture in the Mekong Delta are mainly fromwild sources, or farmers procure seed from other places for supplemental stocking on farms. Clam culture relies entirely on natural food and tides on the beach. After 17 months of culture, white clams reach 15-20 g and are ready for harvest, with production of about 13.0 ± 1.3 t/ha per crop (Phong et al. 2018). White clam is an important culture species for export and the culture in some provinces have achieved Marine Stewardship Council (MSC) certification. Blood cockle Anadara granosa culture is widely practiced on mudflats of estuaries or beaches, and in mangrove-shrimp farms. For culture on mudflats, blood cockle seed of 900-1000/ kg collected from the wild are stocked at a density of 1-2 t/ha. After 7-12 months, blood cockle reaches 40-60/kg and are ready for harvest, with production of about 20 t/ha per crop. For blood cockle culture in mangrove-shrimp farms, seed of 700/kg are stocked at 1.5 t/ha. After six months of culture, cockles reach harvest size of 70-100/kg (Thao and Phu 2012). Oyster Crassostrea belcheri culture is increasingly important and widely practiced in estuaries of most coastal provinces in the Mekong Delta where water salinity is 9-27 g/L. Culture is normally practiced with rafts of 40-1000 m 2 . Different kinds of substrates are used for natural seed collection and grow-out. After 17 months of culture, oysters reach commercial size of 300-500 g. Average production is 3,560 ± 1,440 kg per crop from a 100-m 2 raft (Thao et al. 2018). Generally, mollusks are an important commodity for coastal aquaculture for local markets or for export. With simple culture techniques, low capital investment, various species available, high culture potential in different conditions, this industry is increasingly important for the region. Further improvement and investment in culture technology, seed production, environmental monitoring and management are needed. Brackishwater andMarine Fish Culture Vietnam has a long coastline of 3260 km and thousands of islands and the Mekong Delta has over 700 km of coastline and hundreds of islands. With large areas of brackish water and seawater, Vietnam, and the Mekong Delta in particular, have great potential for brackishwater and marine fish culture. Since the early 1990s, brackishwater and marine fish culture have developed different species and culture systems: 1) extensive, integrated pond culture of mullet, milkfish and tilapia with shrimp and mud crab; 2) semi-intensive or intensive pond culture of seabass, grouper, marine goby and eel; and 3) marine cage culture of grouper, cobia, snubnose pompano, sea bream, snapper and red drum (Fig. 7). Production of brackishwater and marine fish was about 29,770 t in 2017, with a production target of 600,000 t by 2030 (MARD 2018). In the Mekong Delta, integrated mangrove-shrimp farming systems and improved extensive shrimp farming systems are very good for integration with brackishwater fish culture. Natural fish seed may enter farms through gates during water exchange, or wild- caught fish are stocked additionally to farms. No feed is used for fish. Fish production from these systems are rather low (100-150 kg/ha per yr) but it contributes important and regular income for households. With more than 350,000 ha of these extensive systems in the region, total production of fish is considered large for local use but is not statistically recorded (Hai et al. 2017) For semi-intensive and intensive fish pond culture, euryhaline and high-value fish such as goby ( Pseudapocryptes elongates ), eels ( Anguilla mamorata ), seabass ( Lates calcarifer ), Nile tilapia ( Oreochromis niloticus ) and recently spotted scat ( Scatophagus argus ) are very important culture species. Goby is cultured intensively at 80-150 fish/m 2 for 4-5 months, using pelleted feed, resulting in production of 5-16 t/ha per crop. The culture is very dynamic due to good acceptance of pelleted feed, short culture period, high yield, high profit; and fish species that are euryhaline with high tolerance of poor water quality. However, the fish seed supply remains dependent on wild-caught fry or fingerlings (Hai et FIGURE 6. Raft culture of oysters in an estuary. FIGURE 7. Cage culture of marine fish.