Interactions of plant-based feeding and handling stress on the expression of selected immune markers in rainbow trout (Oncorhynchus mykiss)

Substitutions for fishmeal in fish feed are important in the context of sustainability. However, nutrition has a decisive influence on well-being. An understanding of the impacts of botanicals on health or microbiomes is crucial when assessing the effect of potential immune modulators in the body of carnivorous fish. The present study examined the effect of a plant-protein based diet on the expression of selected immune markers in rainbow trout (Oncorhynchus mykiss) under handling stress. The amount of fishmeal was reduced to 7% in the predominantly plant-protein based diet (P), in contrast to a diet based on 35% fishmeal (F). Triplicate groups of rainbow trout were fed for 59 days with experimental diets, and one experimental group of each feed type was stressed by shooing twice a day. The expression of the genes coding for tumour necrosis factor (TNF) alpha, interleukin (IL) 1 beta, immunoglobulin (Ig) T and IgD was profiled in whole blood samples using real-time RT-qPCR. Significant expression differences between the F and P diets were observed for IL1 beta and IgD in nonstressed fish and TNF alpha, IgT and IgD in stressed fish. No differences were detected between nonstressed and stressed fish fed the same diet. After repetitive stress, gene expression patterns differed between trout fed the F or P diets, as IgT, IgD and TNF alpha were significantly downregulated in the stressed fish fed the P diet. In conclusion, growth and performance did not differ significantly with diet, although a pathohistological assessment of two intestinal segments revealed a histological response to the P diet-feed ingredients. The molecular results indicate that fish performance should also be monitored under challenging conditions to fully assess the impact of dietary fishmeal substitutes.