22 MARCH 2013 • WORLD AQUACULTURE • WWW.WAS.ORG conducted at the Vietnamese Institute for Aquaculture No. 3 (RIA 3), which lies at the heart of an extensive network of coastal prawn ponds in Khanh Hoa province (Duy 2010, Duy 2012). Regrettably prawn farmers in this region have fallen victim to the devastating effects of disease outbreaks that have swept through Asia, robbing farmers of their livelihoods and leaving them with abandoned ponds. Naturally researchers looked to other species that could be farmed in ponds and sandfish represented a promising candidate. Initial trials in Vietnam demonstrated that sandfish grew well in prawn ponds at low densities, with growth rates of 2.2-3.2 g/d, exceeding growth in the natural environment (Pitt and Duy 2004, Agudo 2006). Ponds are generally between 0.5-1 ha with a water depth of 0.8-1.5 m. To be suitable for sandfish culture, ponds should meet the biophysical requirements of being situated in the intertidal zone with good water exchange (full salinity) and sandy-muddy substrate. Prior to stocking, ponds should be prepared by removing predators, drying, tilling and liming the sediment to raise the pH and ensure that sandfish can burrow (Duy 2012). Juvenile sandfish (2 g) reared in nursery-phase hapa nets are stocked into ponds at 1/m2. Survival rates are high (8087 percent) and growth rates range from 1.0-1.8 g/d, with sandfish reaching a harvest size of 300-350 g within 6-14 mo (Duy 2012). Pond carrying capacities are linked to quality of the substrate, natural productivity, and rate of water exchange with reported yields generally between 2.6-2.8 t/ha. Sandfish monoculture is proving to be viable economic alternative for former prawn farmers in coastal provinces in Vietnam. Capital outlay for famers is minimal (mainly related to purchasing juveniles; US$ 0.14/individual) because ponds are already leased or owned outright. Economic returns are high, ranging from US$ 1700 to 2200/ha per crop, with profit margins of 33-45 percent (Duy 2012). Farmers sell market-size sandfish to processors for US$ 2.00-2.20/kg (Fig. 9). Now that earlier problems associated with managing ponds and gaining acceptable prices for harvested stock have been overcome, the remaining constraints to sandfish pond farming relate to density limitation and reducing the duration of the culture period (Duy 2012). Integrated Multi-Trophic Aquaculture Sandfish are detritivores, capable of processing large quantities of sediment and extracting energy from organic matter, including detritus, waste feed and feces. Given their mode of nutrition, sandfish are excellent candidates for integrated multi-trophic aquaculture (IMTA) — the practice of culturing several species together, with one species consuming waste of another species. IMTA systems have the potential to significantly reduce environmental impacts of aquaculture by reducing waste output and increase production through farming of several complimentary and valuable species simultaneously within a production unit. Given the excellent growth rates obtained in ponds and their potential for sediment bioremediation, early research focused on the potential of sandfish co-cultured with prawns. While results from tank-based experiments showed promise, largescale trials with tiger shrimp Penaeus monodon and Pacific blue shrimp Litopenaeus stylirostris were largely unsuccessful due to mortality of sandfish from predation by shrimp (Mills et al. 2012). Rotational culture, where production cycles of shrimp and sandfish culture are alternated, is practised by farmers in Vietnam (Mills et al. 2012). Co-culture of sandfish with some finfish species is also showing promise in Vietnam and the Philippines (Mills et al. 2012). Recently IMTA of the commercially valuable red seaweed Kappaphycus striatum, farmed extensively in the tropics for carageenan production, and sandfish has been demonstrated in Zanzibar, Tanzania (Fig.10). Growth rates of seaweed and sandfish in co-culture were comparable with monoculture systems and no negative effects were observed on the performance of either co-cultured species (Gutierrez 2012). Intensive Aquaculture in South Africa In South Africa, research is currently underway to develop technologies for intensive, land-based sandfish culture in recirculating systems. Density limitation, thought to be linked to food availability, is a common problem in sandfish aquaculture (Robinson and Pascal 2012). Estimates for current grow-out techniques range from 250 g/m2 for sea ranching (Purcell and Simetoga 2008) to 300-400 g/m2 for pond culture FIGURE 7. Sea pen farming of sandfish in southwest Madagascar. Photo: G. Robinson. FIGURE 8. Market-size sandfish. Photo: G. Robinson.
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