Tissue-specific effects of stocking density on fatty acid composition and liver metabolism in rainbow trout (Oncorhynchus mykiss).

Rainbow trout (Oncorhynchus mykiss), which is rich in high-quality protein and polyunsaturated fatty acids (PUFAs), is one of the most commercially important aquaculture species worldwide. The fatty acid profiles of trout, however, might be substantially influenced by different stocking densities. The present study aims to evaluate the effects of stocking density on the fatty acid composition and hepatic metabolomics of rainbow trout. Juvenile rainbow trout were reared for 84 days in circular tanks at 3 distinct levels of stocking density, i.e., low (group LD, 9.15 g/L), moderate (group MD, 13.65 g/L), and high (group HD, 27.31 g/L). The results showed that the levels of C22:6 n-3 (Docosahexaenoic acid, DHA) and total n-3 PUFAs in the dorsal muscles of the fish at group MD were significantly higher than those at groups LD and HD (p < 0.05). For ventral muscles, fish at group HD showed significantly decreased content of C16:1n-9 (palmitoleic acid) and n-3 PUFAs compared to those at groups LD and MD (p < 0.05). Hepatic fatty acid profiles showed significant reductions in monounsaturated fatty acids (MUFAs) for the fish at groups MD and HD compared to those at group LD (p < 0.05). Moreover, the concentrations of total n-6 PUFAs in the dorsal muscle were positively correlated with stocking density (R = 0.7357, p = 0.0238), while the levels of total n-3 PUFAs in the ventral muscle (R =  - 0.8478, p = 0.0039) and the levels of total MUFAs in the liver (R =  - 0.7837, p = 0.0101) showed negative correlations to the fish stocking densities. Metabolomics analyses further revealed that stocking density influenced the pathways related to glycerophospholipid metabolism, fatty acid degradation, and arachidonic acid metabolism. The results suggested that high stocking density altered fatty acid profiles and enhanced antioxidant and inflammatory responses in rainbow trout, providing valuable insights for optimizing aquaculture practices to improve the nutritional quality of farmed trout.