World Aquaculture Magazine - June 2020

WWW.WA S.ORG • WORLD AQUACULTURE • JUNE 2020 63 mg/L. Corn starch (10 mg/L) was added to each bag. One set of bags received chicken manure (5 mg/L) and another set did not receive chicken manure. Each treatment was replicated in three bags (Fig. 1). Addition of chicken manure increased the dry weight biofloc produced and the biofloc protein content, irrespective of N:P ratio. Organic manure, which includes detritus, is a rich source of nutrients (Wade and Stirling 1999b). The dry weight biofloc production and biofloc protein content were similar (around 26 percent) at N:P ratios of 4:1 and 8:1 (Fig. 2). Thus, an N:P ratio of 4:1 with chicken manure at 5 mg/L as an organic fertilizer is sufficient to maximize the protein content of biofloc. Effect of C:NRatio Five carbon to nitrogen (C:N) ratios were evaluated, 1:1, 2:1, 4:1, 8:1 and 16:1, with three replicates for each level. The N:P ratio was fixed at 4:1 and obtained with additions of 8 mg/L of urea and 2.5 mg/L of super phosphate. As in the evaluation of N:P ratio, 5 mg/L of chicken manure was added to each bag. Corn starch was added at 4 mg/L for a 1:1 C:N ratio up to 120 mg/L for a 16:1 C:N ratio. The dry weight biomass produced was similar (3-4 g) for C:N ratios of 1:1 to 8:1 (Fig. 3). However, a C:N ratio of 16:1 resulted in a significantly greater dry weight production (9 g) than any other C:N ratio treatment evaluated. In contrast, the greatest protein content of biofloc occurred with a C:N ratio of 2:1 and decreased as C:N ratio increased. The protein content of biofloc at a C:N ratio of 2:1 was around 35 percent, whereas the protein content of biofloc at a C:N ratio of 16:1 was around 25 percent. Thus, a high C:N ratio produced more biofloc but it was of lower quality as a potential nutritional supplement for shrimp or fish. Effect ofOrganicCarbonSource The effect of three organic carbon sources —corn meal, rice bran and rice meal —was evaluated, with three replicates for each treatment. The N:P ratio was 4:1 and the C:N ratio was 16:1, with 120 mg/L of each organic carbon source added to bags. The quantity of floc produced (8.9-9.5 g) was similar to the 9 g of floc produced at a C:N ratio of 16:1 in the trial to evaluate the effect of C:N ratio. The dry weight of biofloc using the different organic carbon sources was ranked rice meal (9.5 g) > corn meal (9.2 g) > rice bran (8.9 g). The crude protein percentage of the biofloc produced using the different sources was similar and was ranked corn meal (45 percent) > rice bran (44 percent) > rice meal (42 percent) (Fig. 4). The amino acid profile for the biofloc produced using the different sources of organic carbon was quite variable. Corn meal had a greater content of leucine, valine, threonine, isoleucine and glutamine that rice bran or rice meal (Table 1). Although the productivity of floc produced with corn meal was greatest, satisfactory results were also obtained from rice meal. This material has the advantages that it is a local product and that it is lower cost for farmers because they can use broken rice. Effect of Salinity The effect of salinity on dry weight biofloc productivity and biofloc crude protein content was evaluated in six 100-L plastic tanks (Fig. 5), with three replicates for each water source. The seawater source (30 g/L) was the same as described previously. Fresh water was collected fromAl-Raswa Farm, Damietta, Egypt. For each tank, additions were made of urea (8 mg/L), super-phosphate (2.5 mg/L), chicken manure (5 mg/L) and corn starch (120 mg/L). FIGURE 2. Dry weight and crude protein percentage of the biofloc obtained at different levels of N/P ratios. FIGURE 3. Dry weight and crude protein percentage of the biofloc obtained at different levels of C/N ratios. FIGURE 4. Dry weight and crude protein percentage of the biofloc obtained from different carbon sources. FIGURE 5. Three 100-L tanks used to evaluate the effect of salinity on biofloc production and protein content. ( C O N T I N U E D O N P A G E 6 4 )