Feasibility of Rainbow Trout (Oncorhynchus mykiss) Fry Rearing in Biofloc System: Effect of Total Suspended Solid Levels on Zootechnical Performance, Intestinal Condition and Antioxidant Enzyme Activity

This study evaluated the effects of different total suspended solid (SST) levels (0, 250 and 350 mg L^-1) on the productive performance, intestinal condition and antioxidant enzyme activity of rainbow trout (<i>Oncorhynchus mykiss</i>) fry reared in a biofloc technology (BFT) system. The experimental design was completely randomized with three treatments and five replicates. A total of 225 rainbow trout fry (0.81 ± 0.06 g) were distributed into 15 tanks (70 L working volume) at a stocking density of 15 fish per tank. Tanks designated for the control treatment (clear water) were connected to a recirculating aquaculture system, whereas BFT treatments were maintained in independent units. Performance analyses were conducted and fish were sampled for intestinal histomorphometry, intestinal microbial counting and antioxidant enzymatic activity. Rearing rainbow trout fry in BFT systems with TSS levels up to 250 mg L^-1 proved feasible and preserved zootechnical performance, intestinal condition and antioxidant enzyme activity during the early stages of development. The higher TSS concentration negatively affected fish growth performance. However, increased colonization by lactic acid bacteria was observed, indicating that biofloc may serve as a source of probiotic bacteria. The BFT system is a viable alternative for rearing rainbow trout fry, enhancing biosecurity and reducing water consumption.