Soybean meal-induced enteritis in Atlantic salmon ( Salmo salar) and Chinook salmon ( Oncorhynchus tshawytscha ) but not in pink salmon ( O . gorbuscha )

To improve sustainability in the aquaculture industry plant meals are increasingly used to replace fish meal in fish feed. Solvent-extracted soybean meal (SBM) is an attractive protein source for fish feed because of its high protein content, favorable amino acid profile, and low cost. In Atlantic salmon (Salmo salar), SBM at low levels causes soybean meal-induced enteritis (SBMIE). Few studies have been done with SBM in Pacific salmon, and none of those have included intestinal inflammation analysis. To gain more insight into salmonid responses to SBM, we assessed and compared the effects of SBM on intestinal morphology, inflammation and microbiome composition of Chinook salmon (Oncorhynchus tshawytscha), pink salmon (O. gorbuscha) and Atlantic salmon (Salmo salar). Atlantic, Chinook and pink salmon were fed for three weeks on a diet with 20% inclusion of SBM, or a control diet with fish meal. After one week on the SBM diet, Atlantic and Chinook salmon showed increased submucosa thickness in the distal intestine compared to the fish fed on the fishmeal diet. Intestinal inflammation in these species increased over time, with a further increase in submucosa thickness coincident with an infiltration of eosinophilic granular and mononuclear leucocytes. After 3 weeks on the SBM diet, intestinal inflammation was most severe in Chinook salmon. In contrast, pink salmon only showed a slight increase in submucosa thickness after three weeks on the SBM diet, and no significant increase in inflammatory cell infiltrate. Sequence-based analysis of the intestinal microbiome showed a significant difference in overall microbiome composition between species, but did not show an effect of the SBM diet on microbiome diversity or composition in any of the three salmon species. In conclusion, SBM-fed Chinook salmon were more susceptible to SBMIE than Atlantic salmon whereas pink salmon were not susceptible to SBMIE at the levels of SBM tested.