Effects of Varying Salinity Levels on Growth, Antioxidant Defense, and Digestive Enzymes Activity of Whiteleg Shrimp ( <i>Litopenaeus vannamei</i> ) Postlarvae Reared in the Biofloc System

This study evaluated the effects of different salinities on the growth and physiology of whiteleg shrimp ( Litopenaeus vannamei ) postlarvae reared in the biofloc system. A total of 4200 shrimps (PL20) were randomly distributed in 12 850 L biofloc (BFT) tanks (350 PL20 in each tank). Shrimps divided into four experimental groups in the biofloc system with salinities of 3 (BFT3), 8 (BFT8), 10 (BFT10), and 12 ppt (BFT12). Salinities were made by adding sea salt to tap water. During the rearing period, for the optimal growth and development of biofloc in the tanks, a carbon source (brown sugar and wheat flour in equal proportions) was calculated and added to the tanks to adjust the ratio of carbon to nitrogen (12:1). From the third to the sixth week of rearing, the water nitrate concentration was significantly higher in BFT12 and BFT10 ( p &lt; 0.05). The total ammonia nitrogen and nitrite concentration at different sampling times did not show significant differences among experimental groups ( p &gt; 0.05). After 6 weeks, salinity had no effect on the growth indices including final weight, weight gain (WG), WG rate, specific growth rate (SGR), and survival ( p &gt; 0.05). The highest levels of intestinal amylase (28.97 ± 3.46 U/mg protein) and protease (13.28 ± 2.19 U/mg protein) activities were also observed in the BFT3 group ( p &lt; 0.05), while lipase activity did not changed. Muscle glutathione (GSH) and malondialdehyde levels and catalase activity did not show significant differences among the experimental treatments ( p &gt; 0.05). On the other hand, the highest superoxide dismutase activity and total antioxidant capacity (TAC) was observed in BFT10 ( p &lt; 0.05). Body proximate composition of L. vannamei was not significantly different in experimental groups ( p &gt; 0.05). Our findings showed that biofloc system could overcome the reported adverse effects of salinity on L. vannamei growth and possibility of rearing L. vannamei in low salinity in the biofloc system.