WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2015 35 are stocked at 2-4/m2, two to eight times per year through two seasons (December-April and June-November). Mud crabs, blood cockles (Anadara granosa), and some brackishwater fish are also stocked at low density to make use of natural food and for diversification of products and income. Almost no supplemental feed is used in these systems. Two to three months after stocking, shrimp are partially harvested on every new moon and full moon by collecting in a bag net during outgoing tides. Annual shrimp production is about 300-400 kg/ha, of which black tiger shrimp account for 50-70 percent, and the rest are other wild shrimps such as banana shrimp (P. merguiensis), Indian shrimp (P. indicus) and sand shrimp (Metapenaeus ensis). Large black tiger shrimp of over 50 g are commonly harvested. Next to shrimp, mud crab is a very important and regularly harvested product. Recently over 6,000 ha of integrated mangrove-shrimp farming systems have been certified as organic by Naturland (Minh et al. 2011). With simple techniques, low capital investment, environmental friendliness, regular harvests, and with low disease and economic risks, integrated farming systems (50,000 ha) and the improved extensive shrimp farming system (330,000 ha) are very important for sustainable development. However, further improvements in farming technology are needed for these systems to achieve better shrimp survival rates, production and income. Alternative rice-shrimp farming systems. Alternative riceshrimp farming systems (Fig. 5c) are very typical in the Mekong Delta, currently covering more than 160,000 ha. In these systems, black tiger shrimp are cultured in the dry season with brackish water and rice is cultivated in the rainy season with fresh water. The major characteristics of traditional systems and improved systems are summarized in Table 4. Traditional systems are surrounded by a deep ditch along the dike but have shallow water level (30 cm) on the platform. Improved systems are considered semi-floating ponds and land is prepared by machines, and thus a deeper water level (70-80 cm) can be maintained. Stocking densities, feeding and water management of the two systems differ. In the traditional system, shrimp are stocked at a low density (2-5 shrimp/m2), feeding is casual and management is simple. Shrimp yields of 200-300 kg/ha/ crop are normally obtained. In improved systems, shrimp are stocked at a higher density (6-8 shrimp/m2), fed well and water is managed carefully. Shrimp yields of 800-1,500 kg/ha/crop can be harvested. The 3-4 month shrimp crop is cultured in the dry season and a traditional variety of rice is planted during the rainy season. Due to enhanced soil fertility after the shrimp crop, very limited or no fertilizer is used for rice cultivation. Rice production can be reach 3-5 t/ha/crop. Giant freshwater prawn Macrobrachium rosenbergii is also normally stocked during rice cultivation at low stocking density (1-2 prawns/m2) with casual feeding; prawn production is about 50-100 kg/ha/crop. This system – with diversified culture species for different seasons, simple techniques and low investment – is important for incomes of local people, strategically important for food security and environmentally friendly. Intensive farming systems. Intensive farming of black tiger shrimp and Pacific white shrimp developed in the central provinces before the Mekong Delta. However, the Mekong Delta currently has a larger culture area due to its greater overall potential. Recently the majority of intensive shrimp farms shifted from farming of black tiger shrimp to Pacific (CONTINUED ON PAGE 36) TABLE 3. Technical characteristics of extensive shrimp farming systems and mangrove-shrimp farming systems in the Mekong Delta (Hai et al. 2007). Characteristic Unit Mangrove-shrimp Improved extensive Total farm area ha 4.4 ± 2.6 3.6 ± 1.6 Mangrove area percent 38 ± 24 0 Pond (ditch) area percent 25 ± 14 29 ± 24 Tree types Mainly Rhizophora Without or with some aquatic plants and Rhizophora Stocking shrimps times/yr 4.8 ± 2.6 5.8 ± 2.8 inds/m2 13 ± 11 15 ± 14 Stocking crabs times/yr 1.3 ± 1.1 1.7 ± 1.2 inds/m2 0.06 ± 0.08 0.06 ± 0.04 Water exchanges d/mo 7.1 ± 2.8 6.7 ± 2.7 percent/d 41 ± 15 40 ± 15 Total production kg/ha per yr 492 ± 462 432 ± 370 Cultured shrimp kg/ha per yr 192 ± 231 213 ± 314 Wild shrimp kg/ha per yr 153 ± 247 96 ± 80 Crab kg/ha per yr 61 ± 78 80 ± 112 Fish kg/ha per yr 70 ± 116 36 ± 60 Blood cockle kg/ha per yr 15 ± 115 8 ± 34
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