Effects of Chronic Thermal Stress on the Physiology, Metabolism, Histology, and Gut Microbiota of Juvenile Schizothorax grahami

Temperature is a critical factor influencing fish health and aquaculture success. This study investigates the physiological, histological, and microbiota responses of juvenile <i>Schizothorax grahami</i> to chronic thermal stress across six temperature treatments. We found that exposure to temperatures exceeding 27 °C resulted in complete mortality in juvenile <i>S. grahami</i>, with the upper thermal tolerance range between 24 °C and 27 °C. Chronic thermal stress caused a significant decline in serum glucose (Glu), triglycerides (TG), and total cholesterol (TC) at 24 °C, indicating rapid energy depletion, while reduced serum enzyme activity of catalase (CAT) and total superoxide dismutase (T-SOD) at 24 °C group further reflected a decrease in antioxidant capacity. Histological examination revealed adaptive intestinal villus hypertrophy with increased length and muscularis thickness at temperature under 24 °C. Furthermore, the relative abundance of <i>Cetobacterium</i> and Fusobacteriota suggested either adaptive responses or stress-related dysbiosis that may contribute to weakened host immune function. Overall, our findings highlight the vulnerability of juvenile <i>S. grahami</i> to rising temperatures, with moderate thermal stress inducing adaptive responses, while higher temperatures impair metabolism, gut integrity, microbial health, and may even lead to lethality. These results underscore the need for effective conservation strategies to protect this critically endangered species in the face of climate change.