World Aquacluture Magazine - September 2020

WWW.WA S.ORG • WORLD AQUACULTURE • SEP TEMBER 2020 21 mostly microalgae in a size range from a fewmicrometers to 50 µm in the case of adults. Green water rich in microalgae can be prepared by application of manure and fertilizer directly or indirectly into the Artemia pond. Green water blooms of naturally occurring microalgae are dependent on species composition, nutrient availability or deficiency, salinity levels and light intensity and do not always result in an optimal feeding regime for Artemia. As food is the key factor for successful Artemia production, several approaches have been explored to improve feeding conditions: 1) supplementary feeding with finely-ground agricultural-by products such as rice bran or wheat bran (Fig. 7) or with formulated feeds (Fig. 8) (e.g., using the specific formula for Artemia of Cantho University or commercial shrimp feeds); 2) application of biofloc technology by creating a C/N ratio >10 through the addition of locally available cheap carbon sources (e.g., molasses, tapioca). This technique is gaining more interest as it permits better control of feeding conditions and maximization of production potential. Flexible Farming In the first period of Artemia production in the early dry season (January and February), when maximumwater temperatures are in the lower 30s C, the main mode of reproduction is by cysts, although some nauplii are also produced and these can compensate for natural mortalities and contribute to further expansion of the Artemia population. By the end of March and into early April, as maximum temperatures reach close to 40 C, the dominant mode of reproduction switches to ovoviviparity and the remaining population no longer produces cysts. If the remaining period of the dry season is sufficiently long (minimum 2 months), it may be economical to consider a complete new culture with newly hatched nauplii. However, in the Mekong Delta, the production season is becoming too short to start a new cycle (as has been practiced over a number of seasons in the past) and farmers start harvesting and marketing Artemia biomass (Fig. 9) in the last part of the dry season. Fortunately this period coincides with the natural spawning season of many local aquaculture species such as marine shrimp, marine fish and mud crab. A small amount of live Artemia biomass (nauplii to juvenile) is sold at about US$ 20/kg wet weight to local nurseries of marine shrimp and all-male Macrobrachium. The preferred form of preservation is frozen as it allows long-distance transportation to aquaculture facilities throughout Vietnam (Fig. 10). Artemia production yields in the Mekong delta now reach about 150-200 kg wet weight cysts per hectare per 4-mo season. With an increased focus on Artemia biomass, harvests can reach 3-4 t wet weight of biomass per hectare per season. Currently, income generated from cyst production (US$ 40- 50/kg raw cysts per ha) or from biomass production (US$ 2-4/kg wet-weight biomass per ha) are very similar. According to farmers, the system for biomass production is easier to manage than for cyst production, as is also reported by Artemia farmers in Thailand. Today in the Mekong Delta more than 700 salt farming house- holds are engaged in Artemia production in 800 ha of area, pro- ducing 60-80 t of raw cysts (Fig. 11) and over 200 t of wet-weight biomass per year for a net-profit of up to US$ 8,000/ha, in compari- son with an income of about US$ 2,000/ha in the case of only salt production. ( C O N T I N U E D O N P A G E 2 2 ) FIGURE 7. Feeding rice bran as supplementary feed to Artemia pond. FIGURE 8. Formulated feed as a supplemental feed source in Artemia farming. FIGURE 9. Artemia biomass production in Vinh Chau solar saltworks.