Physiological Responses and Serum Metabolite Alterations in Grass Carp (Ctenopharyngodon idellus) Under Chronic Salinity Exposure

Salinity is a pivotal environmental factor that significantly influences the survival, growth, development, and reproduction of aquatic organisms. However, the characteristics of serum metabolites and their mechanistic roles in mediating the response of grass carp (<i>Ctenopharyngodon idellus</i>) to long-term salinity stress remain incompletely understood. Therefore, the present study exposed grass carp to different salinity levels (0, 4, and 8 g/L) for 60 days to evaluate the associated physiological alterations and metabolic responses. The results revealed that high salinity (8 g/L) significantly suppressed growth performance (<i>p</i> < 0.05), whereas low salinity (4 g/L) caused no significant reduction in growth or survival. Physiological analyses indicated that fish in the 8 g/L group exhibited markedly reduced levels of lactic acid and total protein, along with elevated concentrations of total cholesterol, triglycerides, glucose, and glutamic-oxaloacetic transaminase (<i>p</i> < 0.05). Serum ion homeostasis was also disrupted under high salinity, characterized by increased Ca²⁺, Na⁺, and Cl^- levels and decreased Mg²⁺ (<i>p</i> < 0.05). Furthermore, oxidative stress was evident in the high-salinity group through heightened activities of antioxidant enzymes (SOD, CAT, GPx), accumulation of oxidative damage markers (protein carbonyl, 8-OHdG) (<i>p</i> < 0.05). Metabolomic profiling identified 367 and 403 significantly altered metabolites in the 4 g/L and 8 g/L groups, respectively, primarily belonging to lipids and lipid-like molecules along with organic acids and derivatives. KEGG enrichment analysis revealed that these differential metabolites were chiefly involved in amino acid biosynthesis, glycerophospholipid metabolism, biosynthesis of unsaturated fatty acids, and glycine, serine, and threonine metabolism. Trend analysis further uncovered eight distinct expression patterns of metabolites across salinity gradients. These results provide novel insights into the metabolic adaptations of grass carp to salinity stress, demonstrating that high salinity induces oxidative stress, disrupts ion regulation, and drives extensive metabolic reprogramming. The study offers valuable theoretical support for improving salinity tolerance management in aquaculture and informs the selective breeding of salt-tolerant fish strains.