Reducing water use is essential for shrimp farming in super-intensive systems, especially when located away from the coast or in subtropical and temperate regions, as it reduces water heating costs. Reducing total suspended solids production is also important for reducing oxygen consumption and waste/effluent production. There are two main groups of bacteria involved in nitrogen removal in biofloc technology culture systems (BFT) . The absorption of inorganic nitrogen through heterotrophic bacterial growth promotes a faster reduction in ammonia concentrations than nitrifying process. The growth rate and bacterial biomass production of heterotrophic bacteria is 10 times higher than that of nitrifying bacteria. Autotrophic bacteria perform nitrification through the oxidation of ammonia to nitrite and then to nitrate. However, due to the slow growth of autotrophic bacteria, nitrite tends to increase significant in the environment culture. In this way, bacteria degrade excess organic matter and allow successive cycles of shrimp production without the need for water renewal culture. In general, organic fertilizations used to be done to increase the C:N ratio (15:1) for increase of heterotrophic bacterial population to convert inorganic nitrogen in bacterial protein. However, this growth of bacterial population used to increase the amount of total suspended solids (sludge) in shrimp culture, and sometimes can become a problem for BFT systems.
The aim of this study was to evaluate the effect of Carbon:Nitrogen ratio on reduction of water use and production of total suspended solids in shrimp culture in a biofloc system and heterotrophic- and nitrifying bacteria composition in BFT system.
A 70 day-trial was conducted at the Marine Station of Aquaculture, University of Rio Grande, Rio Grande do Sul State, Brazil. After nursery, Pacific white shrimp Penaeus vannamei (1.46g ±0.52) juveniles were stocked at 400/m3 in a twelve-1.000 L tanks . Four treatments (three replicates) were tested using four different C:N rates : 9:1, 11:1, 13:1 and 15:1. Water temperature, salinity, dissolved oxygen, pH, TA-N, NO2-N, NO3-N, Alkalinity and Settleable solids were daily monitored. BFT water samples were collected to detect the growth of the population of nitrifying and heterotrophic bacteria by Fluorescent IN SITU Hybridization determination - FISH. The results were analyzed by one-way analysis of variance (ANOVA) and Tukey’s test (α = 0.05).
The water quality parameters were influenced by the treatments (P<0.05), but were not limiting for shrimp survival and growth in all treatments. Shrimp survivals were not influenced (P>0.05) by the treatments. The growth parameters were also different among treatments , (P<0.05), except FCR (P>0.05). The different C:N fertilization rate influenced significantly ((P<0.05) bacterial population composition . Results showed the importance of decreasing the amount of Carbon to decrease total suspended solids and water use in BFT system.