Peaks and accumulation of nitrogen compounds in biofloc technology (BFT) systems frequently occur during system establishment, causing stress to cultured organisms and compromising their growth and survival. This study investigated the effects of chronic exposure to ammonia, nitrite, and nitrate on Litopenaeus vannamei, focusing on both immediate stress responses and compensatory growth during recovery phases.
Three experiments (ammonia, nitrite, and nitrate) were conducted in a 2 x 3 design with two "concentrations" and three "stress" exposure times, divided into two phases: stress and recovery. In the first experiment, shrimp were exposed to high ammonia concentrations (two- and three-fold the recommended safe level, i.e., 7.10 mg·L⁻¹ and 10.65 mg·L⁻¹) for periods of 5, 10, and 15 days, followed by 38, 33, and 28 days of "recovery time," respectively. For nitrite (experiment 2), animals were exposed to two concentrations (30.4 and 45.6 mg·L⁻¹), representing two- and three-fold the recommended safe level, for 7, 14, and 21 days, followed by 42, 35, and 28 days of "recovery time," respectively. In the third experiment, chronic exposure was conducted with two nitrate concentrations (0.5- and one-fold the recommended safe level, i.e., 139.45 mg·L⁻¹ and 278.91 mg·L⁻¹) over three "stress" exposure times of 10, 20, and 30 days, with “recovery times” of 48, 38, and 28 days, respectively. In all experiments, a control was maintained with ammonia (experiment 1), nitrite (experiment 2), and nitrate (experiment 3) levels below 25% of the safe threshold.
In experiment 1, results indicated that, after the stress phase, survival was impacted by a 10-day exposure to three-fold the safe ammonia level and by 15-day exposures in both high-ammonia treatments. Growth was reduced in all stressed treatments, except for those exposed to two-fold the safe level for 5 days. Results from experiment 2 (nitrite) showed that survival rates were significantly reduced after the stress phase in all groups exposed to three-fold the safe level, as well as in the group exposed to two-fold the safe level for 21 days. Growth was reduced in treatments exposed to high nitrite concentrations for 14 and 21 days. In the third experiment, results indicated that L. vannamei displayed higher resistance to nitrate; however, prolonged exposure (more than 20 days) to concentrations above 139.45 mg·L⁻¹ negatively affected growth rates. After the recovery phase, shrimp from experiments one (ammonia) and three (nitrate) exhibited total compensatory growth, nullifying significant differences from the control group. In experiment 2, although the animals achieved satisfactory growth rates when nitrogen compound levels were re-established, they did not match the control group, demonstrating partial compensatory growth.
Therefore, if it is necessary to maintain shrimp exposed to high nitrogen compounds, such as during system establishment or when water replacement is limited, the following exposure limits are recommended: (1) for ammonia, exposure should not exceed two times the safe level for a maximum of 10 days; (2) for nitrite, exposure should be limited to two times the safe level for up to 14 days; and (3) for nitrate, exposure should remain within the safe level for a maximum of 20 days.