Evaluation of the breeding potential of juveniles of mullets Mugil cf. hospes and Mugil liza in a biofloc system
In order to evaluate the possibility of maintaining juvenile mullets in a biofloc system, three experiments were carried out at the Marine Aquaculture Station of the Federal University of Rio Grande, Rio Grande, RS, using juveniles from Mugil cf. hospes and Mugil liza. The experiments were carried out in a completely randomized design, with three replications in each treatment and the results were subjected to one-way ANOVA and Kruskal-Wallis analyzes of variance (= 0.05), considering the premises of normality (Kolmogorov-Smirnov ) and homoscedasticity (Cochran C) of the data. When a significant difference was detected between treatments after ANOVA, the Tukey test was applied to compare the means. The first study was carried out to verify the possibility of maintaining bioflocs in the rearing of Mugil juveniles cf.hospes (initial average weight of 4.55 ± 0.15 g). Three treatments were evaluated: mullets with inoculum of bioflocs (T), mullets with bioflocs from the creation of shrimp Litopenaeus vannamei (TFC) and without animals (SEM). All treatments received commercial feed (44% CP). After 21 days the final concentration of total suspended solids did not differ (P> 0.05) between the TFC treatments (785.33 ± 269.05 mg L-1), T (310.67 ± 126.0 mg L-1 ) and SEM (298.67 ± 30.28 mg L-1), while the final volume of TFC bioflocs (76.66 ± 5.77 mL L-1) was higher (P <0.05) than SEM (3.1 ± 2.48 mL L-1) and T (18.66 ± 5.50 mL L-1) were similar to all treatments. These results demonstrate that it was possible to maintain bioflocs in rearing juveniles of Mugil mullets. hospes, making it possible to create this type of system.In order to evaluate the importance of natural feeding in the growth performance of juveniles of Mugil liza mullets (initial average weight of 0.66 ± 0.21 g), three different rearing systems were used for 30 days, one system being carried out in clear water, a system that consisted of water containing bioflocs and artificial substrate with a periphyton, and a third system performed in clear water containing artificial substrate with a periphyton. All treatments received commercial feed (42% CP), and the treatments with clear water had water changes daily. The final survival of juveniles was over 90% in all treatments, with no difference between them. The treatment that used periphyton and bioflocs together resulted in greater growth in weight of juvenile mullets and lower apparent feed conversion rate,compared to other treatments. Additionally, this treatment registered a lower average concentration of total ammonia in the water of the rearing tanks. A third study aimed to evaluate the effect of adding different sources of organic carbon on water quality and zootechnical performance of juveniles of Mugil liza mullets kept in BFT system. During 45 days, four treatments were tested: dextrose, liquid sugar cane molasses, rice bran and a treatment in clear water was used as a control (three replicates each). 15 juveniles of M. Liza (7.99 ± 2.57g) in each experimental unit (160 L, salinity 14.0 ± 4.2 and temperature of 27.0 ± 3.3 oC). The mullets were fed commercial feed (55% CP) four times a day. At the end of the experiment the weights of juvenile mullets,weight gain and specific growth rate showed no significant difference (P> 0.05) between the different treatments. Survival was greater than 90% in all treatments, with no significant difference between them. The highest average volume of bioflocs was registered in the rice bran treatment (161.4 ± 66.29 ml L-1), followed by molasses (96.46 ± 96.24 ml L-1), and the lowest volume was observed in the treatment of dextrose (52.54 ± 57.29 ml L-1), all significantly different from each other. The average concentration of SST (mg L-1) in water was higher (P <0.05) in treatments with bioflocs than in clear water (492.05 ± 456.83 mg L-1), but it did not differ between dextrose, molasses and rice bran treatments (926.16 ± 299.03; 932.18 ± 309.72 and 809.31 ± 217.77 mg L-1, respectively). The dextrose treatment showed the highest (P <0,05) plenty of total bacteria adhered to than molasses, rice bran and clear water treatments. The addition of a supplementary source of organic carbon, molasses or rice bran, proved to be efficient in the development of bioflocs and control of ammonia and nitrite levels in the water, in comparison to the use of dextrose and the clear water system.