The relationship between shrimp (Litopenaeus vannamei) size and biofloc consumption determined by the stable isotope technique

Biofloc technology (BFT) is considered one of the most promising methods for the sustainable development of shrimp culture. Bioflocs keep the water quality at good standards and represent a complementary food source, allowing the production of high stocking densities of the white shrimp Litopenaeus vannamei. Microorganisms present in flocs are an important source of lipids and proteins; however, laboratory observations indicate that floc consumption is not the same for all shrimp stages. We have, therefore, established the hypothesis that bioflocs are more important food sources for larger shrimps that can better manipulate the flocs and pick up and consume specific microorganisms. The stable isotope technique has been recently applied in aquaculture to quantify the importance of different food sources to the growth of target species. In this study, this technique was employed in two experiments in order to test the established hypothesis, measuring biofloc consumption by 0.01 g L. vannamei postlarvae in nursery tanks and by 0.80 g shrimp in the grow-out phase. Both experiments were conducted in greenhouse tanks (12-800 L) with a stocking density of 2000 shrimp/m 3 in the nursery phase and 400 shrimp m 3 in the grow-out phase. The experiment was designed with two treatments with four replicates each: (I) F, with feed only; and (II) BF, with bioflocs and feed. Shrimp were fed a commercial diet twice a day in the nursery and growout stage with 40 and 38% crude protein, respectively. Differences were found between water quality parameters for pH, ammonia, nitrite, nitrate, phosphate, SST and turbidity (p < 0.05).Zootechnical performance also showed differences between shrimp of different treatments for final weight, final biomass, survival, yield m(3) and m (2) (p < 0.05). The results of the stable isotope technique indicate that bioflocs contributed 22-43% C and 0-43% N to the shrimp tissue composition during the nursery phase, whereas in the grow-out phase, the contribution of bioflocs varied between 63 and 100% C and 35-86% N. Differences in the abundances of Oocystis sp., pennate diatoms and flagellates (p(<)0.05) indicate that these microorganisms were preferentially consumed by larger shrimp. Thus, the results of this study corroborate the hypothesis that the contribution of bioflocs as a supplementary source of natural food depends on the size of the shrimp.