Introduction
In the biofloc technology culture system (BFT System) the water reuse, with a well-established microbial community where nitrification it is already taking place and in a presence of nitrate, is important to obtain a better stability of nitrogen compounds throughout a new production cycle. Krummenauer et al., (2014) reported that a minimum inoculum of 25% accelerated the formation of microbial aggregates in the BFT system. H owever, information on the minimum inoculum from the reuse of water rich in mature bioflocs is limited. Th e proposed 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.
Material and Methods
The study lasted 45 days, until the nitrification cycle was completed in the control treatment with the detection of nitrate in the culture water. The experiment was carried out in 18 tanks with a volume of 400 liters, 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, unless the nitrite level exceeded 20 mg/L. In this case 30% of total volume was changed with previously chlorinated and de- chlorinated seawater. L . vannamei juveniles were stocked with an initial weight of 0.8g±0.1, at a density of 400 shrimp m- ³. The animals were fed twice a day at 08:00h and 17:00h with specific commercial ration containing 38% crude protein. The water with mature bioflocs, it means, that already had undergone the complete nitrification process until the appearance of nitrate , 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.
Results
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
n itrate showed an inverse pattern, it was lower in the control treatment and higher in the inoculum treatments (Figure 1).
no addition of inoculum; 2.5: addition of 2.5 mg L-1; addition of 5 mg L-1; 10: addition of 10 mg L-1; 20: addition of 20 mg L-1; 40: addition of 40 mg L-1 of biofloc, inoculum in the culture starting .
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.
Discussion
The use of a mature inoculum with a well-established nitrification process at the beginning of the culture was essential to avoid the oscillation of nitrogen compounds, as occurred in the control treatment without inoculum. 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. Shrimp survival was low in the control treatment and 2.5, due to prolonged exposure to high concentrations of ammonia and nitrite that exceeded the safety level proposed by Lin & Chen (2001, 2003), of 3.95 mg L-1 and 25.7 mg L-1
respectively.
Conclusion
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.
References
Krummenauer, D., Samocha, T., Poersch, L., Lara, G., & Wasielesky, W. (2014). The reuse of water on the culture of pacific white shrimp, litopenaeus vannamei, in BFT system. Journal of the World Aquaculture Society, 45(1), 3–14.
Lin, Y. C., & Chen, J. C. (2001). Acute toxicity of ammonia on Litopenaeus vannamei Boone juveniles at different salinity levels. Journal of Experimental Marine Biology and Ecology, 259, 109–119.
Lin, Y. C., & Chen, J. C. (2003). Acute toxicity of nitrite on Litopenaeus vannamei (Boone) juveniles at different salinity levels. Aquaculture, 224(1–4), 193–201.
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