Determination of acute toxicity and evaluation of the chronic effect of nitrate on compensatory growth of Litopenaeus vannamei reared in a biofloc technology system

Understanding the effect of acute and chronic nitrate stress on Litopenaeus vannamei grown in intensive biofloc technology systems is essential for production optimization. The present study evaluated L. vannamei exposed to nitrate extremes at a salinity of 25 g.L-1 measuring survival and possible recovery through compensatory growth. An initial clear-water lethal test, revealed an LC50-96 h for nitrate of 2789.11 mg.L-1. The nitrate safe level was determined as 278.91 mg.L(-1)by the Sprague Factor (0.1 of the LC50-96 h). Consequently, a chronic exposure experiment was carried out in a Biofloc Technology System. A 3 x 2 experimental design was applied, with two nitrate concentrations (278.91 mg.L-1 and 139.45 mg.L-1) and three "stress" exposure times (10, 20 and 30 days) followed by 48, 38 and 28 days "recovery" time, respectively. A control, in which the nitrate was maintained below 25% of the calculated safe level was maintained for 58 days. The experiment was divided into two phases: (1) stress and (2) recovery. Survival did not differ among treatments during the experimental period. Shrimp exposed to concentrations of 278.91 mg.L-1 and 139.45 mg.L-1 of nitrate over 20 and 30 days exhibited reduced growth, while growth of shrimp exposed 10 days did not differ from the control. After the end of the recovery phase (day 58), no significant differences were found in shrimp weights among treatments. Results indicate L. vannamei presents higher resistance to nitrate, but prolonged exposures (>20 days) to concentrations above 139.45 mg.L(-1)may negatively affect growth rates. However, when lower nitrate values are re-established, the animals exhibit compensatory growth nullifying significant differences to cohort members maintained at optimal nitrate throughout.