Effective immunostimulant for rainbow trout after short term usage: Lepidium sativum and its by-product

In the present study, garden cress ( Lepidium sativum ) cold-pressed seed oil and its pulp ethanolic extract, obtained as a by-product, were evaluated as dietary immunostimulants in rainbow trout ( Oncorhynchus mykiss ). Fish were fed an unsupplemented control diet or diets containing garden cress seed oil (LS) or pulp ethanolic extract (LSE) at 0.1, 0.5, and 1 g kg⁻¹ for 21 days. The seed oil groups were designated LS01, LS05, and LS1, and the pulp extract groups were designated LSE01, LSE05, and LSE1. Sampling was performed at 7-day intervals to assess growth performance, hematological parameters, humoral immune responses, and immune-related gene expression. Digestive enzyme activities and antioxidant status were assessed at the end of the feeding trial, after which fish were subjected to a 10-day experimental challenge with Yersinia ruckeri . Growth performance was significantly enhanced in the LS1 group compared with the control group. Humoral immune responses, including respiratory burst activity and lysozyme levels, increased in both LS and LSE groups compared with the control. Cytokine gene expression was also significantly upregulated, particularly in the LSE-fed groups. Digestive enzyme activities increased notably in the LS1 group and in the higher-dose LSE groups. Antioxidant enzyme activities, including catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx), were significantly higher in the supplemented groups than in the control group. Survival rates after bacterial challenge were higher in all experimental groups than in the control, with the highest survival observed in the LSE1 group. Overall, these results suggest that both garden cress seed oil and its pulp ethanolic extract have immunostimulatory and health-promoting potential in rainbow trout, while the pulp extract showed more pronounced effects in some immune-related responses.