52 JUNE 2015 • WORLD AQUACULTURE • WWW.WAS.ORG TABLE 1. Parameters of each batch. Batch Number of oysters Density of algal cells (n/ml) Temperature range (C) Water Transfer 1 60 50000 27-31 200 L/day 2 60 50000 20-22 200 L/day 3 60 50000 20-22 500 L/15 days 4 60 50000 20-22 200 L/day TABLE 2. Results of the experimental onshore culture. Batch Mortality out Pearls Luster of Uniformity of of 60 oysters formed the Pearls pearl surface 1 27 9 Mostly poor in luster Most had stains and tails 2 0 37 Deep metallic 21 uniform, remaining 16 had tails 3 0 49 Deep metallic 42 clean and uniform, 7 had stains 4 16 22 Deep metallic 7 uniform, remaining 14 stained using new experimental culture techniques. We evaluated various sea-based cages (Figs. 8 and 9) that provided protection from parrotfish. We also reared four batches of oysters onshore with various parameters to determine the most suitable environmental conditions. Onshore Culture Study The four batches of oysters were maintained in fiberglass conical tanks of 2000 L each (Fig. 10). The conical bottom was covered with pieces of dead coral (Fig. 11). Wastes and discharges of the oysters were concentrated at the central drain of the conical bottom. Aerators and wavemaker devices were installed in each tank. Wavemaker devices were installed in the tanks to create waves to simulate the natural sea environment for oysters. All oysters were attached to stones, as they would be under natural conditions. Waves also helped to mix microalgae quickly and uniformly. A biological filter of 1200 L/h filtration capacity was installed in each tank. Biofilters functioned 18 hours per day, helping to reduce the algal density and remove decaying organic matter. This reduced the need for water exchange. Water quality was maintained with a salinity of over 31 ppt and pH of 8.2-8.5. Each tank was covered with black polyethylene sheeting to avoid unwanted photosynthesis. Sixty oysters were scattered over the bottom of each of the four tanks. A total of 240 oysters were grafted and implanted with 6-mm nuclei and reared in the four tanks (Table 1; Fig. 12). A microalgal culture unit was set up to culture Nanochloropsis salina, Chaetoceros calcitrans and Isochrysis galbana for feeding oysters periodically at 50,000 cells/mL (Figs. 13 and 14). Algal culture was done using two different protocols, one with Walne media and the other without media, containing only filtered and autoclaved seawater. The major expenses for algal culture are the chemicals of media preparation and this affects the onshore culture cost. Oysters in batches 1, 3 and 4 were fed with algae cultured with Walne media and in batch 2, algae were cultured without media. However, no detectable impact was identified when the algal culture method was applied. Ten percent (200 L) of the water was exchanged each day in batches 1, 2 and 4. The water was biologically filtered before adding to the tank in batches 1, 2 and 3. In batch 4, however, 200 L of apparently clean natural seawater was added/ transferred directly. Water was exchanged daily in batches 1, 2 and 4; however, 500 L of water was changed every 15 days in batch 3. FIGURE 9. Sea-based cage installation for protection of pearl oysters from predators. FIGURE 10. Onshore tanks for pearl culture.
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