EFFECT OF WATER EXCHANGE AND STOCKING DENSITY IN A BIOFLOC-BASED SYSTEM ON SALINITY STRESS TOLERANCE AND PERFORMANCE OF Litopenaeus vannamei POSTLARVAE
The effect of water exchange and stocking density was evaluated on the salinity stress tolerance (by low and high salinity) and performance of Litopenaeus vannamei postlarvae reared in a biofloc-based system at nursery level. A bioassay was conducted with four treatments in triplicate: T1 = 8250 orgs/m3 without water exchange, T2 = 16,500 orgs/m3 without water exchange, T3 = 8250 orgs/m3 with 50% water exchange per week, and T4 = 16,500 orgs/m3 with 50% water exchange per week. At 0, 15, 30, and 45 days of culture, a group of shrimp from each treatment was exposed to acute stress by low (27 to 2 ‰) and high salinity (27 to 60 ‰). After 49 days, the water exchange rate and water exchange rate × stocking density interaction affected shrimp survival, but shrimp growth was affected only by stocking density. The mean weight of shrimp in the treatments with the lowest stocking density was higher than that with the highest stocking density, independently of the water exchange rate. In the salinity stress tests, shrimp survival was only affected by the interaction between the salinity stress tests and treatments (T1, T2, T3, and T4). Generally, shrimp showed a greater tolerance when were subjected to low salinity (27 to 2 ‰). In the culture with biofloc at nursery level, apparently, shrimp from the treatments with water exchange improved their tolerance to stressful events caused by changes in salinity, but this response was not related to growth and survival.