In the biofloc technology culture system (BFT System) the water reuse, with a well-established microbial community is important to obtain a better stability of nitrogen compounds throughout a new production cycle. The aim of this study is evaluate the zootechnical performance and water quality parameters in treatments with different levels of inoculum (mg L-1) in order to determine the minimum inoculum necessary for a rapid stabilization of nitrogen compounds in a new culture cycle of L. vannamei.
The experiment was carried out in 18 tanks with a volume of 400 L, in six treatments, with different initial concentrations of total suspended solids (inoculum): 0 mg L-1, 2.5 mg L-1, 5 mg L-1, 10 mg L-1, 20 mg L-1 and 40 mg L-1. During the experiment no water changes were made. L. vannamei juveniles were stocked (0.8g±0.1), at a density of 400 shrimp m-³. The animals were fed twice a day with specific commercial feed (38% CP). The water with mature bioflocs, it means, was collected from a raceway with L. vannamei culture in progress. Sugarcane molasses, with about 36% of carbon in its composition, was used as a source of organic carbon in the experimental units when the ammonia exceeded 1 mg L-1. The study lasted 45 days, until the nitrification cycle was completed in the control treatment with the detection of nitrate in the culture water.
There was no significant difference between the physicochemical parameters of water, except for nitrogen. Ammonia and nitrite were higher in the control and lower in the treatments with the addition of inoculum, and nitrate showed an inverse pattern, it was lower in the control treatment and higher in the inoculum treatments. More sugarcane molasses and water changes were used in the control treatment. The use of molasses was inversely proportional to the inoculum concentration, the higher the inoculum concentration, the lower the use of molasses. Regarding the zootechnical performance of shrimp, survival was lower in the control treatment, followed by the 2.5 mg L-1 treatment. In treatments 5, 10, 20 and 40 mg L-1, survival ranged from 93 to 95%. Final weight was higher in treatments with lower survival and the opposite was observed for FCR (Feed conversion rate). Biomass and productivity were higher where inoculum was used, with better results in the 5 mg L-1 treatment.
The use of a mature inoculum at the beginning of a new culture was essential to avoid the oscillation of nitrogen compounds. This stabilization of nitrogen compounds directly reflected the better zootechnical performance of shrimp and the lower use of water and molasses to maintain water quality. The results demonstrate that an inoculum concentration of 5 mg L-1 used in the starting of the culture of L. vannamei in a biofloc system, maintains water parameters more stable, uses fewer inputs of water and molasses and obtains a better zootechnical performance of shrimps.