Effects of different flow velocity on growth, intestinal digestion and antioxidant function of juvenile largemouth bass (Micropterus salmoides)

The cultivation of largemouth bass (Micropterus salmoides) fry is a crucial phase for successful aquaculture. Traditional rearing methods often face challenges such as slow growth rates, weak disease resistance, and low survival rates. With the development of facility-based fisheries, controlled aquaculture environments have significantly enhanced production efficiency. However, research remains insufficient regarding the impacts of environmental factors on the growth and health of M. salmoides juveniles in industrialized seedling rearing systems. To improve survival rates and growth performance during fry cultivation, this study investigates the effects of different flow velocities and photoperiods on growth performance, digestive function, antioxidant capacity in M. salmoides fry. In the present study, health M. salmoides (0.53±0.08) g was randomly selected and treated with 4 different flow (0, 2.5, 4.5 and 6.5 bl/s) for 6 weeks. Their growth performance, intestinal oxidative stress and digestive function were tested to elucidate the effects of flow rate on M. salmoides juveniles. The present results showed that the 2.5 bl/s increased the growth rate and decreased the mortality rate of juvenile M. salmoides (P<0.05). Meanwhile, 2.5-6.5 bl/s led to the significant increases of intestinal MDA concentration, and decreases of SOD activity. However, the flow rate caused the different degree of increase of CAT and GPX activity from 24 h to 48 h, and then the significant decrease were observed after 96-hour treatment (P<0.05). This results indicated that the intestinal oxidative stress of M. salmoides juveniles was induced by 96-hour flow treatments, and CAT and GPX activities play an important role in response to oxidative damage induced by acute flow treatment in intestine of M. salmoides. Additionally, 2.5-6.5 bl/s flow led to a significant increases of trypsin activities, and 6.5 bl/s flow significantly down-regulated the activities of amylase and lipase after two-week flow treatment. Thus, different flow increased the intestinal digestion and absorption of amino acids, but the higher flow rate inhibited the growth through decreasing the activities of amylase and lipase. Analysis of intestinal transcriptome sequencing suggested that 48-hour treatment of 2.5 bl/s flow resulted in the most differential genes (DEGs) enriching in the pathways mainly related to neuroactive ligand-receptor interactions and amino acid metabolism, and the DEGs were significantly enriched in the signaling pathways related to amino acid and fatty acid metabolism in the 4.5 and 6.5 bl/s groups, indicating that the short-term flow treatment enhanced the intestinal metabolism of fatty acids and amino acids. Under 6-week flow treatment, DEGs were enriched in the signaling pathways related to nucleotide, amino acid and lipid metabolism in the 4.5 and 6.5 bl/s groups, indicating that M. salmoides juveniles still need to increase lipid metabolism to adapt to the energy consumption induced by higher flow treatment, in which PPAR and FoxO signaling pathways played important roles. In summary, the flow rate of 2.5 bl/s was the optimal flow rate for the growth and development of larval M. salmoides juvenile during the early development. In addition, there was a time-dependent effect of flow that short-term flow treatment inhibited and long-term flow treatment promoted the growth performance. Therefore, the stress damage induced by the short-term flow change should be avoided during the seedling cultivation stage of M. salmoides, and long-term appropriately flow treatment should be selected for improved the growth performance and health of M. salmoides.