WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2015 53 Frequent water exchange is required in land-based oyster rearing because the carbonate minerals calcite and aragonite are available in the water in trace amounts. Oysters use these for shell formation and growth by secretion. Therefore, new aseptic water was added daily to maintain the availability of these minerals for better pearl formation. Water chillers were installed in batches 2, 3 and 4 to maintain a uniform temperature range of 20-22 C whereas the temperature of batch 1 was not controlled and varied naturally; at night it was about 27 C and in the day was 29-31 C. Ammonia and other contaminants, produced from the decomposition of uneaten food and other forms of organic matter, were monitored frequently because oysters consume plankton, detritus, bacteria and other particulate organic matter, but no contamination was detected in the water column. Water chemistry was the same in batch 3 where water was transferred every 15 days. Results and Observations The aseptic condition of water, optimum algal density and adequate oxygen play important roles in oyster rearing in onshore tanks. However, the uniformity of pearl formation and luster of pearls were determined by water temperature and food quality. Oyster survival in onshore rearing tanks was high (Table 2); there was no mortality in batch 2 and 3. High temperature resulted in poor survival of oysters; 27 oysters died during 7 months in batch 1. Pearl quality was also compromised in this batch; most pearls were dull in luster and formation of pearl sacs was not uniform. Sixteen oysters died in batch 4, indicating that, without filtration, direct use of seawater is not sufficient for onshore pearl oyster culture. Pearls of the best quality were formed in batches 2 and 3. Batch 3 produced 49 pearls with 11 implantation rejections. Of the 11 rejections, 7 rejected both nucleus and graft tissue and 4 contained graft pieces that formed irregular pearls. Rearing oysters in the sea under floating rafts have several drawbacks, such as predation on oysters. Numerous biofouling organisms grew on oyster shells; some bore into shells, causing oysters to repair shells by secretion, which, in turn, affected pearl quality. To overcome this problem, oysters and their cages in the sea must be cleaned twice per month. Scrubbing the outer surface of oyster shells causes shock, affecting normal pearl formation. However, in onshore rearing, no such growth on the shell was detected and it was not necessary to clean oysters. The results of studies indicated the potential for mass pearl production with low operational cost. In onshore pearl culture, major expenses were associated with microalgae culture but, when it is done without media, the cost goes down substantially. This raises the possibility of producing pearls along any part of the coast, although in conventional pearl culture operations, a protected harbor is necessary. However, if large expensive pearls can be produced onshore, it would be a highly lucrative and environmentally friendly project. Notes Ajai Kumar Sonkar, ajai@pearlindia.com FIGURE 11. Penguin’s wing oyster reared in onshore tanks. FIGURE 12. A pearl oyster reared onshore. FIGURE 13. Microalgae culture for feeding pearl oysters reared in onshore tanks. FIGURE 14. Feeding pearl oysters onshore with cultured microalgae.
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