World Aquacluture Magazine - September 2020

20 SEP TEMBER 2020 • WORLD AQUACULTURE • WWW.WA S.ORG FIGURE 5. Screening greenwater inlet to prevent predator entry. FIGURE 6. Artemia population in peak season for cyst collecting. Year by year more salt producers adopted Artemia farming and, as a result, cyst production increased in terms of scale and total production. However, yields per hectare fluctuated from year to year as several factors would interfere with obtaining consistent yields, e.g. experience of farmers, quality of cyst inoculum, quality of manure, and extra feeding (Figs. 5 and 6). Furthermore, climate change is increasingly interfering as longer rainy seasons with cooler temperature versus shorter dry seasons with high temperature have significantly reduced the suitable culture period for Artemia in the Mekong Delta. In recent years, a lot of effort by Cantho University, local authorities, Artemia-co-operatives and Artemia farmers has been made to overcome some of these problems: Saline Water Preparation Usually seawater is evaporated to a salinity of 80 g/L for transfer to Artemia ponds. However, in the Soc Trang and Bac Lieu saltworks, the salinity of intake water can be as low as 15 g/L at the start of the dry season as the area is close to the brackishwater estuary and freshwater sources. To reach a salinity of 80 g/L, at least 45 days of evaporation time are required. As mentioned, climate change has shortened the dry season, so different approaches have been tried to accelerate start-up of the Artemia culture. These include 1) using high salinity water from the previous dry season stored in a deep pond, 2) dissolving crude salt to increase salinity, 3) accelerating evaporation by mechanical disturbance such as raking the bottom of the evaporation pond with an initial thin layer of seawater to accelerate the evaporation rate. These techniques help to gather a sufficient volume of saline water of 80 g/L within two weeks. As a result, salt farmers can gain an extra month for Artemia production. Inoculation Strategy From the time of inoculation of ponds with newly hatched nauplii, 2-3 weeks are required before Artemia reach adult stage and start to reproduce. To shorten the production cycle, farmers have developed several new alternative strategies: 1) Inoculation at a higher density than usual. By this strategy, ponds are stocked with as many as 500 nauplii/L (rather than 100/L) iin a small nursery pond with 80 g/L seawater. Then, after a few days when more saline water is available, juvenile/pre-adult Artemia are transferred to a larger pond. 2) Stocking at lowwater level of 80 g/L salinity and gradually adding more saline water until reaching a minimumwater depth of 30 cm. 3) Stocking on-grown Artemia from another pond where the current population is sufficiently dense to justify a partial harvest. 4) Stocking at lower salinity but assuring that no predators (fish, insects) and competitors (copepods, ciliates) are present. The time period from nauplii to adults is considered to be lag time. Therefore, all possible strategies are considered to reach the reproductive stage sooner, when either the population expands (nauplii production) or first cysts can be harvested. Multiform Feeding Artemia is a non-obligate particle filter feeder, consuming FIGURE 3. Cyst collecting in Artemia pond, Ben Tre. FIGURE 4. Extension team working closely with an Artemia farmer.