Schizochytrium sp. as a sustainable docosahexaenoic acid source improves growth performance, fatty acid profile, and hepato-intestinal health in juvenile darkbarbel catfish (Pelteobagrus vachelli)

Schizochytrium sp. has become a potential feed additive due to its rich contents of functional components such as docosahexaenoic acid (DHA), but its dose effects and physiological effects on darkbarbel catfish have not yet been clarified. This study evaluated the effects of six graded levels of dietary Schizochytrium sp. powder (0, 5, 10, 15, 20, and 25 g/kg; designated D0, D1, D2, D3, D4, and D5, respectively) on 30-d post-hatching juvenile darkbarbel catfish ( Pelteobagrus vachelli ) with an initial body weight of 0.38 ± 0.011 g, using a total of 1800 fish (three replicates per group, 100 fish per replicate) over a 10-week feeding trial. The optimal inclusion level of Schizochytrium s p. powder was also determined. Results showed that fish in the D4 group exhibited the highest weight gain rate (WGR) and specific growth rate (SGR) ( P < 0.05), accompanied by significantly decreased feed conversion ratio (FCR) and feed intake (FI) compared with the D0 group ( P < 0.05). Nutrient composition analysis revealed no significant changes in crude fat or moisture ( P > 0.05), but compared with the D0 group, crude protein increased significantly in D2, D3, and D5 groups ( P < 0.001). For fatty acids, D4 group exhibited the highest polyunsaturated fatty acid (PUFA) and ∑n-6 PUFA contents, the lowest saturated fatty acids (SAFA) content and health indices (atherogenic index and thrombosis index) ( P < 0.05), while monounsaturated fatty acid (MUFA) content significantly decreased with increasing addition ( P < 0.001). Principal component analysis confirmed PUFA, C18:1n-9c and MUFA as key factors driving intergroup differences. Histologically, the D4 group had significantly increased intestinal villus length and muscularis thickness compared with the D0 group ( P < 0.05), whereas the D5 group showed obvious liver damage. Physiologically, D4 group had the highest activities of superoxide dismutase (SOD), catalase (CAT) and total antioxidant capacity (T-AOC), the lowest content of malonic dialdehyde (MDA), and the activities of digestive enzymes (lipase and protease) were also significantly enhanced in liver and intestine ( P < 0.05). In conclusion, 20 g/kg Schizochytrium sp. powder promotes growth, optimizes fatty acid profiles, and enhances intestinal health and physiological function in juvenile darkbarbel catfish, while excess supplementation (25 g/kg) causes liver damage. This study revealed the application value and dose-effect relationship of Schizochytrium sp. powder as an aquatic feed additive, and provides a comprehensive theoretical basis for its precise application in freshwater fish farming.