Integrated Analysis of Physiological, Histological, and Transcriptomic Responses to Transport Density in Schizothorax...

Transport density is a pivotal regulatory factor during live fish transport, as it determines the intensity of stress responses and subsequent survival. With increasing demands for artificial propagation, stock enhancement, and scientific research, the frequency of transporting Schizothorax nukiangensis has risen substantially. This study evaluated the effects of different transport densities (100, 150, and 200 kg/m³) after 4 h on water quality, plasma biochemistry, tissue morphology, and transcriptional responses. High-density transport elevated ammonia nitrogen and temperature, reduced pH, and caused mortality in the highest-density group. Gill and liver tissues exhibited progressive structural damage, accompanied by increased plasma ROS, SOD, cortisol, and glucose levels, while IgG, CAT and T-AOC decreased under higher densities. Transcriptomic analyses showed that upregulated DEGs in the gill were primarily enriched in Antigen processing and presentation, whereas innate immune pathways-including RIG-I-like receptor, IL-17, and cytosolic DNA-sensing-were suppressed. This pattern suggests that increasing transport density suppresses innate immune responses, thereby shifting immune regulation toward enhanced adaptive immune activation via antigen processing and presentation. Both gill and liver exhibited pronounced enrichment of lipid metabolism-related pathways-including steroid biosynthesis, fatty acid metabolism, glycerolipid metabolism, and fat digestion and absorption-accompanied by extensive upregulation of these genes. These changes are tightly associated with activation of the PPAR signaling pathway, which facilitates fatty acid transport, degradation, and β-oxidation, thus sustaining energy supply under transport-induced stress. These findings clarify the physiological and molecular mechanisms of transport stress in S. nukiangensis and provide guidance for optimizing short-distance transport strategies.