Effect of salinity on natural community and production of Litopenaeus vannamei (Boone), within experimental zero-water exchange culture systems

Recent efforts have been made to culture marine shrimp in systems operating under low or zero-water exchange and with decreased water salinity. The aim of this study was to investigate the impact of various salinity levels on qualitative and quantitative characteristics of the natural community and, more particularly, ciliated protozoa, and compare this information with shrimp growth and survival. Tanks with 9parts per thousand salinity were characterized by a higher pH, but also by a significantly higher concentration of chlorophyll a (Chl a ) per weight of suspended matter (1.93+/-0.72 mug Chl a/mg TSS) than tanks with 18parts per thousand (1.29+/-0.68 mug Chl a/mg TSS) or 36parts per thousand (1.37+/-0.61 mug Chl a/mg TSS) salinity. Concentrations of ciliates (max 6000 cells mL(-1) ) showed considerable fluctuations over the sampling period, reflecting the impact of water salinity, dynamic interactions between ciliates and their diverse roles within the shrimp production system. There was no significant difference between survival rates of shrimp reared at 9parts per thousand, 18parts per thousand or 36parts per thousand, but decreasing salinity from 36parts per thousand to 9parts per thousand led to a significant decrease in final shrimp body weight (from 13.40+/-0.26 g to 10.23+/-2.72 g). Future work should address the potential of ciliates as an indicator of aquaculture water quality, as is currently being done in the wastewater industry, and the contribution of ciliates as food sources.