Strategies for ammonium and nitrite control in Litopenaeus vannamei nursery systems with bioflocs

In biofloc technology (BFT) rearing systems, nitrogen compounds, specially ammonia and nitrite, have to be controlled by microbial pathways, mainly through the activity of heterotrophic and chemoautotrophic bacteria. The objective of this work was to assess different water preparation strategies (heterotrophic, chemoautotrophic and mature) in BFT system for nursery of Pacific white shrimp (Litopenaeus vannamei). A 35-day study was conducted with post-larvae shrimp (0.08 g) stocked in twelve 300 L tanks at a stocking density of 2000 shrimp m^?3. The water preparation strategies for shrimp rearing that were evaluated in this study included: i) Heterotrophic treatment, where the water received sugar as a carbon source; ii) Chemoautotroph treatment, where ammonium and nitrite salts were added to the water; and iii) Mature treatment, which was created by the addition of a significant amount of water containing mature biofloc from another established BFT system. In both mature and chemoautotrophic treatments, the nitrification process was able to keep toxic nitrogen compounds (ammonia and nitrite) at low levels without the addition of carbohydrates. In contrast, heterotrophic system showed peaks of ammonia and nitrite during the rearing cycle, and the level of these compounds were found to be higher in this treatment (relative to the mature and chemoautotrophic treatments). The chemoautotrophic system exhibited a lower abundance of bacteria from the family Vibrionaceae in the beginning of the experiment compared to the heterotrophic and mature treatments. The combination of low Vibrionaceae abundance and good water quality resulted in improved growth performance in this treatment. These findings demonstrate the importance of manipulating the environment of BFT systems to induce an enrichment of nitrifying bacteria before stocking shrimp. We have also found that the addition of a carbon source to BFT systems is necessary only in emergency situations, when ammonia spikes need to be controlled.