Effects of different stocking densities and feeding frequencies on growth, physiological and biochemical indexes, and intestinal microflora of largemouth bass (Micropterus salmoides) under land-based round pond

Land-based Round Pond is an emerging and efficient aquaculture technique widely adopted in Southwest China. To study the effects of stocking density and feeding frequency on growth, oxidative stress, physiological char-acteristics, and intestinal flora of largemouth bass (Micropterus salmoides) in Land-Based Round Pond, 3 x 2 two-factor treatment groups were set with three stocking densities (30, 40, and 50 fish/m(3)) and two feeding fre-quencies (2 and 3 times/day). Largemouth bass (initial weight of 155.62 +/- 2.43 g) were stocked for 60 days. The yield of 30 fish/m(3) groups were close to 40 fish/m(3) groups, but the 30 fish/m(3) groups demonstrated significantly higher growth rate, condition factor and lipid content compared to other density groups with a lower feed conversion ratio (p < 0.05). The feeding frequency of 3 times per day resulted in higher growth rate than 2 times per day with a higher feed conversion ratio (p < 0.05). Physiological and biochemical analyses showed that levels of TC, TG, lysozyme, ALP, T-SOD, CAT, AOC, lipase, and trypsin were significantly higher in the 30 fish/m(3) groups than those of other density groups, while MDA content were highest in the 50 fish/m(3) groups than other groups (p < 0.05). The T-SOD and CAT activity were significantly higher in the 3 times per day groups than those in the 2 times per day groups (p < 0.05), and no significant effect of feeding frequency on the immunity was observed. No pathological changes were found in muscle and intestinal histology, but infiltration of hemocytes was found in liver in the 50 fish/m(3) groups. In intestinal microbial analysis, the 40 and 50 fish/m(3 )density groups had a higher relative abundance of Proteobacteria, suggesting a higher probability of disease occurrence and an unstable intestinal microflora. These results demonstrate that the optimal stocking density for largemouth bass in Land-Based Round Ponds is 30 fish/m(3) (155 g/fish) at a size stage ranging from 155.62 g to 353.83 g, with the optimal feeding frequency of 3 times per day. This study provides a theoretical basis for the development of Land-Based Round Ponds.