Effects of dietary synbiotic supplementation on growth performance, digestive enzyme activities, and physiological resistance against high stocking density in rainbow trout (Oncorhynchus mykiss)

The current research aimed to assay the role of dietary supplementation of Lactobacillus delbrueckii subsp. bulgaricus (LDB) and Asparagus officinalis L. (AR) root in a single form or combination (as synbiotics) on growth performance, digestive enzyme activities, gut lactic acid bacteria (LAB), and multi-biomarkers against crowding stress in rainbow trout. During a 60-day feeding trial, rainbow trout (initial weight, 35.48 ± 0.34 g) were subjected to seven experimental diets including CG (control group without LDB and/or AR inclusion), LDB7 (1 × 107 CFU/g CG), LDB9 (1 × 109 CFU/g CG), AR5 (5 g/kg CG), AR10 (10 g/kg CG), LDB7 + AR5 (1 × 107 CFU/g CG + 5 g/kg CG), and LDB9 + AR10 (1 × 109 CFU/g CG + 10 g/kg CG). After 60 days, all supplemented diets with LDB and/or AR significantly improved growth performance, feed utilization, and digestive protease enzyme. Moreover, these feed additives could mitigate the stress-related effects on serum immune responses, hematocrit percentage (%), hemoglobin value, white blood cell and neutrophil counts, glutathione peroxidase and superoxide dismutase activities, malondialdehyde content, and cortisol and glucose value, as well as alkaline phosphatase and aspartate aminotransferase levels (P < 0.05). The highest red blood cell count was obtained in the fish of the LDB7 + AR5 group at both experimental times (before and after stress). Dietary supplementation of synbiotics markedly improved amylase and lipase activities and total bacterial count (TBC) (P < 0.05). Serum catalase activity was significantly improved in response to diets supplemented with LDB7, AR5, and synbiotics at both times (P < 0.05). After stress, monocyte count in fish fed with dietary supplementation of LDB and/or AR except for the LDB7 group was significantly higher than that in the control group (P < 0.05). Although all dietary supplements of LDB and/or AR exhibited beneficial effects on most tested parameters, synbiotic diets, especially LDB7 + AR5, provided higher efficiency on metabolic processes and microbial function. Furthermore, they strongly alleviated the negative effects of crowding stress on immune, hematological, antioxidant, and serum biochemical markers. Therefore, dietary AR5 + LDB7 is recommended for high-stocking density rainbow trout rearing systems and aquaculture operations such as fish transportation.