Contribution of the diatom <i>Navicula</i> sp. to the growth of <i>Penaeus vannamei</i> post-larvae in biofloc system: a quantitative stable isotope assessment

The addition of <i>Navicula</i> sp. to shrimp nurseries can improve the growth of <i>Penaeus vannamei</i> reared in biofloc systems. However, the contribution of microalgae to the biofloc formation and the effective contribution to shrimp nutrition remain unknown. In this study, <i>Navicula</i> sp. was added to biofloc nursery systems of <i>P. vannamei</i> at distinct time frequencies for evaluating its nutritional contribution to shrimp growth. Nursery rearing was carried out in bioflocs for 35 days at a stocking density of 3000 post-larvae m^-3. Shrimp were fed using a commercial feed plus fresh culture of <i>Navicula</i> sp. at different frequencies: no addition of <i>Navicula</i> sp. (WN - control), the addition of 10 × 10⁴ cells mL^-1 of the diatom every 5, 10 and 15 days (N5, N10 and N15, respectively). Food sources relative contribution to <i>P. vannamei</i> development was estimated using a Bayesian mixture model. The isotopic discrimination factor (Δ¹⁵N and Δ¹³C) for each food source was determined experimentally. After 35 days of culture, survival (∼93 %) was similar across all treatments but there was a significant difference in weight gain and feed conversion ratio. The N10 treatment (0.50 ± 0.05 g, 0.99 ± 0.01) exhibited better growth parameters when compared to the WN treatment (0.33 ± 0.07 g, 11.46 ± 0.30). Biofloc was the food source most assimilated by shrimp followed by <i>Navicula</i> sp. and commercial feed. Contribution of <i>Navicula</i> sp. was higher in the N5 treatment. In the treatments with diatom addition, an inverse correlation was observed between the relative contributions of biofloc and <i>Navicula</i> sp., indicating that <i>Navicula</i> sp. is not in the biofloc composition, but it is directly consumed by <i>P. vannamei</i> post-larvae. Biofloc and <i>Navicula</i> sp. exhibited larger contributions to the growth of shrimp, reinforcing the importance of natural food sources to the aquaculture of <i>P. vannamei</i> post-larvae.