World Aquaculture Magazine - June 2020

WWW.WA S.ORG • WORLD AQUACULTURE • JUNE 2020 23 result of a collaboration among KWT (The Netherlands), Ghent University (Belgium) and Cantho University (Brands 1992, Hoa, 2002, Hoa 2014). After a few years to adapt to the new habitat (Hoa 2002), the first harvest of more than 1 t wet weight cysts was collected (Hoa and Van 2019), with almost 90 kg/ ha per 4-mo season collected in the same decade. From then, cyst production in Vinh Chau has expanded to saltfields in Soc Trang and Bac Lieu provinces and trials have also been made in other provinces in the Mekong Delta (e.g. Tra Vinh, Kien Giang, Ben Tre). Great efforts have been made to evaluate and improve Artemia farming in solar-saltworks like 1) culture system (monoculture vs. integrated system, static vs. flow-through system, one-cycle vs. multi-cycle system), 2) fertilization procedures, 3) feeding protocols and type of feed, all of which have helped increase productivity. In a suitable environments and with appropriate pond management, a pond can produce 200 kg wet-weight cysts/ha per season and, if biomass is the main harvested product, production can reach 1 t wet- weight biomass/ha per month and harvest up to 4-5 t/ha per season from a site (Brands et al. 1995, Anh 2009). Currently, almost 800 ha of saltfields have been switched to Artemia farming (Fig. 8). There are more than 700 households involved and they can earn from US$3,000-8,500/ha per season, which corresponds to 3-5 times greater income compared to traditional salt production. The area now produces 40-60 t of cysts and 300-400 t of wet-weight biomass annually. The common technique for Artemia farming in the site is stocking Artemia in saltwater of 80 g/L and supplying green water from a fertilization pond as feed, sometimes with extra feeding with rice bran, fish meal, or formulated feed, as needed. Artemia will reach adulthood 2-3 weeks after inoculation and start to reproduce (either ovoviviparous or oviparous). Cysts or biomass are collected and processed prior to marketing as canned cysts or frozen biomass. As with other aquaculture activities, Artemia farming has to cope with climate change, although saltwater intrusion is not the al. 2017). Eel culture has also been practiced in the Mekong Delta for many years, especially in Ca Mau province. Wild- caught eels of about 90-110 g are stocked into ponds at 0.9-1/ m 2 . Salinity during culture is around 2-5 g/L. Fish are fed with trash fish. After 14-19 months, eels are harvested at a market size of 1.3-1.6 kg, with production of 42-95 kg per 100-m 2 pond. Eels are euryhaline, high value, and grow well in ponds or tanks. However, the culture remains dependent on wild-caught seed (Long and Hai 2014). For seabass culture, pond culture of this species is practiced at household and commercial scales. Hatchery-produced fingerings are stocked at 5 ± 2 fish/m 2 in brackishwater or freshwater ponds and fed mostly with pelleted feed. After 6-7 months, fish are harvested with a yield of 20-33 t/ha (Ha et al. 2016). With availability of seed from hatcheries, good acceptance of pelleted feed, good growth in fresh or brackish water, and rather good price, seabass is an excellent candidate for further pond culture expansion in the Mekong Delta under climate change and saltwater intrusion. Marine cage culture of fish in the Mekong Delta is focused mainly in Kien Giang and Ca Mau provinces, with more than 7,000 marine fish cages in the region. Each household commonly operates 4-10 wood-frame cages of 60-70 m 3 each. Cobia or grouper are stocked at 2.7 ± 1.8/m 3 or 9.5 ± 3.2/m 3 , fed mostly with trash fish for about 10 months to reach harvest size of 7.8 ± 1.3 kg or 1.0 ± 0.1 kg, respectively. Fish production averages 17 ± 10 kg/m 3 for cobia and 6.5 ± 2.5 kg/m 3 for grouper (Khanh et al. 2015). Recently, marine fish cage culture in the region has developed at a large commercial scale with round HDPE cages for cobia, grouper and snubnose pompano. Marine fish cage culture is one of the important trends to make use of the great potential of the sea surface as well as to mitigate the impacts of climate change in the region. Artemia Culture With no native Artemia in Vietnam, Artemia franciscana strain from San Francisco Bay, USA was first introduced into the Vinh Chau saltfields (Soc Trang province) in the early 1980s as a ( C O N T I N U E D O N P A G E 2 4 ) FIGURE 8. Artemia culture in a salt production field. FIGURE 9. Striped catfish (tra) culture in ponds in freshwater or in areas with seasonal saltwater intrusion. Artemia was first introduced in the early 1980s. Currently, almost 800 ha of saltfields have been switched to Artemia farming. There are more than 700 households involved and they can earn from US$3,000-8,500/ha per season, which corresponds to 3-5 times greater income compared to traditional salt production.