World Aquaculture Magazine - June 2020

24 JUNE 2020 • WORLD AQUACULTURE • WWW.WA S.ORG main constraint. High water temperature is the major concern because Artemia are typically grown in shallow water to facilitate the high salinity required. Moreover, water temperature is positively related to salinity so high water temperature is the major problem in Artemia culture. Several efforts have been made to address the need to reduce water temperature (e.g. increasing water depth, control of salinity, partial shading of the pond surface). Secondly, unforseen precipitation during the dry season together with cool temperatures are also a disaster for Artemia farming as the season has shortened as low salinity is more prolonged than usual. A number of technical advances have been recommended: 1) keep saltwater from last season for inoculation in the next season, 2) facilitate evaporation by pumping and raking frequently and 3) salinize sea water with crude salt, although this is more expensive than other options. Artemia farming in the Mekong Delta currently is the main subsistence for salt farmers in Soc Trang and Bac Lieu provinces compared to only salt production. However, the activity needs to be sustained, especially in the context of climate change as appropriate solutions need to pinpoint and to clarify per site/location in order to help salt farmers now become Artemia farmers for sustainable development. Freshwater Fish Culture in Brackishwater Areas Coastal aquaculture in the Mekong Delta is now dominated by brackishwater and marine species but is also very dynamic with many freshwater fish due to their high tolerance to saltwater up to 10 g/L, depending on species. Important species includes striped catfish ( Pangasianodon hypophthalmus ), sand goby ( Oxyeleotris marmoratus ), snakehead ( Channa striata ), snakeskin gourami ( Trichopdus pectoralis ) and Nile tilapia ( Oreochromis niloticus ). Striped catfish culture is a big industry in the Mekong Delta with a current area of 6,600 ha and a production of 1.42 million t (MARD 2019). The culture is practiced with intensive pond culture systems (Fig. 9) at stocking density of 40-70/m 3 and production of 300-400 t/ha. Catfish culture is operated by large companies, entrepreneurs or cooperatives and mostly certified by VietGAP, GlobalGAP, BAP and ASC. Studies on the simulated impacts of climate change and saltwater intrusion to catfish culture have been implemented and come out with valuable recommendations, especially to the catfish farming at some coastal provinces that are seasonally affected by low salinity. However, catfish can tolerate salinity up to 18 g/L and grow well at 9-12 g/L. Work on selective breeding of catfish for higher salinity tolerance and innovative culture systems (Fig. 10) are being implemented for future farming in higher- salinity brackishwater areas (Huong and Quyen 2011, Anh 2014, Phuong and Tuan 2016). Pond culture of sand goby in Ca Mau province, at a stocking density of 0.8-2/m 2 at salinities of 1-8 g/L, gave production of 69 kg/100-m 2 pond after one year. This is a very high-value species for export and local markets (Viet et al. 2014, Long 2013). Snakeskin gouramy fish pond culture at 2-3 g/L of salinity produces 3-8 t/ha (Long 2013, Mai 2016). Snakehead culture in ponds at 50-70/m 2 at 0-5 g/L salinity yields 160-190 t/ha per crop (Tuan et al. 2014). Snakehead is an important indigenous fish in the region that are very common but with high value in local markets. Total culture production of snakehead in the Mekong Delta is currently about 40,000 tons in different systems of pond culture, hapa-in-pond culture and indoor or outdoor tank culture (Quyen et al. 2017). This fish is an air-breathing species, readily accepts pelleted feed, grows well in intensive culture, easy to breed seedlings and tolerates high salinity and poor water quality. Those enable the fish to be a very important species for aquaculture under climate change and saltwater intrusion. Outlook for Further Development of Coastal Aquaculture Based on scenario and the status of the climate change and saltwater intrusion in the Mekong Delta as well as information from a large survey of 168 farmers on aquaculture status and perceptions, as well as the information on salinity tolerance of different species, Mai (2016) proposed a zoning map for aquaculture of various species in specific saltwater intrusion areas in the Mekong Delta. Under saltwater intrusion, aquaculture can adapt well. Diversification and selection of suitable species for culture in different saltwater-affected areas are thus necessary, based on farmers’ experience and technical and local manager support. Although well documented, for better adaptation to climate change and saltwater intrusion, more research on salinity and high temperature tolerance as well as methods of acclimation during culture for different locally potential freshwater and brackish and marine species are needed. There have been big achievements in seed production of indigenous brackishwater and marine species (Hai et al. 2013) but more work is needed to improve on current results or explore new species. FIGURE 10. Striped catfish (tra) culture in a raceway. Considering that climate and saltwater intrusion are critical issues, the government has been effectively serving the aquaculture industry in the Mekong Delta particularly and the country generally with legal and institutional management as well as other solutions.