Effects of different carbon sources and carbon/nitrogen ratios on water quality and growth performance of nile tilapia (Oreochromis niloticus) in biofloc system

In this thesis, it is aimed to use of different carbon sources [barley flour (AU), chickpea bran (NK), corn bran (MK) and molasses (M)] to produce Biofloc (BF) and is to evaluate in b iofloc technology (BFT) system where tilapia (Oreochromis niloticus) were cultured. The effects of carbon sources on growth performance of the fish, water quality and biofloc composition were determined. For this purpose, the study was carried out with three trials. In the first trial of thesis, BF production was carried out by using the carbon sources mentioned above (AU, MK, NK and M) and the powder feed with 35% crude protein as nitrogen source. In all trial groups, protein contents of BFs produced with different carbon sources were measured in the range of 36.18%-39.85% and found to be sufficient in terms of tilapia protein requirement. The high essential amino acid indexes (EAAI) ranged of 0.73 and 0.86 were calculated for the BFs produced in the trial groups of AU, MK and M, while a lower EAAI (0.39) was estimated for BFs produced in the trial group of NK. This clearly shows that BFs produced with AU, MK and M can be used as additional feed in aquaculture, while BFs produced with NK are insufficient and so cannot be used. The crude oil content of the BFs in all trial groups varied between 0.76-3.78% and the crude ash content changed between 12.30-16.33%. Although BFs in trial groups have a low oil contents, it has been determined that BFs obtained from the trial groups, except NK (0.76%), are rich in essential fatty acid content. In addition, it was determined that iso15:0 and anteiso15:0 fatty acids, which are an indicator of heterotrophic bacterial density, were found in BFs produced in all trial groups. In the second trial of the thesis, different carbon sources developed by combining 50%AU and 50% MK grain mixture (TK) with molasses in different proportions were tested in tilapia culture. The study was carried out with 6 trial groups (0TK, 20TK, 40TK, 50TK, 60TK, 80TK) with molasses ratio between 100% and 20%, TK ratio between 0% and 80%, and a control group (C) in which no carbon source was added. A total of 525 tilapia with an average weight of 2.34 g were used in the trial. 25 fish were randomly stocked to 21 plastic culture tanks (100 L) with three replicated. The fish were fed with a feed containing 35% crude protein, with a rate of 6% of body weight twice a day for 56 days. During the trial, TAN, NH3-N NO2-N and NO3-N values were measured between 2.96-4.48 mg/L; 0.27-0.36 mg/L; 0.96-2.16 mg/L and 35.24-47.21 mg/L, respectively. Carbon source mixture were added to each culture tank with a C/N ratio of 20:1, 1 hour after each feeding every day. The nutrient contents of the BFs were significantly different among the trial groups (P<0.05). The highest crude protein and crude lipid were determined in the trial group of the 60TK, as 40.50% and 1.59%, respectively. The lowest crude protein rate was measured as 31.56% in the 20TK trial group, while the lowest crude lipid of 0.69% was determined in the 0TK trial group. Given the EAAI values calculated; BFs of 60TK, 40TK and 50TK groups were determined as high quality, while BFs of 80TK, 0TK and 20TK groups were determined as useful protein sources. Considering the growth and feed efficiency parameters of the fish, there were no significant difference among the trial groups (P>0.05) but the difference was significant between control and trial groups (P<0.05). The best SGR (2.03%), PER (1.71) and FCR (1.68) were determined in the 60TK trial group that significantly different from the control (P<0.05). In the third trial of the thesis, the best carbon source mix [60%(AU+MK) + 40% M] determined in the previous set of grain mixture trial was tested in tilapia production with different C/N rates [12:1 (12C), 14:1 (14C), 16:1(16C), 18:1(18C), 20:1(20C), 22:1(22C), 24:1(24C)]. A total of 525 fish with an average weight of 3.31 g randomly stocked 21 culture tanks in three replicates and fed with the same feeding procedure used in the second experiment in a biofloc environment for 56 days. Carbon source mixture were added to each culture tank with related C/N rates, mentioned above, 1 hour after feeding every day. As the C/N rate increased with added carbon source to the test tanks increased, an increase BV and amount of TSS, and a decrease in the concentrations of nitrogen compounds was observed. The lowest TAN and NO3-N values were measured as 3.46 mg/L and 32.66 mg/L in the 20C group, respectively, while the lowest NH3-N concentration value was at 18C as 0.17 mg/L, and the lowest NO2-N value was in the 16C group as 0.12 mg/L. The highest protein content in BF samples were determined in 16C, 18C, 20C, 22C groups. It was determined that the obtained BFs reached sufficient values in terms of EAA, total amino acid and EAAI, the 20C group was the best in supplying the amino acid requirement of tilapia. The BFs obtained from the 16C, 18C, 20C and 24C trial groups were found to be a quality protein source considering the EAAIs calculated. When the fatty acid profiles of the BF samples were examined, the highest values were determined in the 16C, 18C, 20C and 22C groups. The 24C trial group showed the best performance in terms of tilapia growth (3.01%), protein efficiency rate (2.57) and feed conversion rate (1.15). Although the results obtained from the third trial show that C/N can be applied from 16C onwards, considering the results of the economic analysis, it has been concluded that 24:1 C/N ratio can be recommended in tilapia culture. Keywords: Biofloc, carbon source, Oreochromis niloticus, tilapia, growth, water quality, C/N ratio