The main objective of this study was to increase the knowledge about the mucosal immunity of Salmo salar, using soybean meal‐induced enteritis as a model of inflammation. A control fish meal (FM) and a diet containing 20% soybean meal (SBM) were fed to salmon for seven weeks in seawater. There was no growth difference between groups. However, histology of distal intestine (DI) showed a mild inflammation in the fish fed SBM. Proteomic results revealed differences between the diets. Among the proteins detected uniquely in DI mucus of SBM group, complement C5, Galectin and Glutathione synthetase are involved in innate and adaptive immunity, inflammation, redox signalling and detoxification of xenobiotics in mammals, and similar roles are hypothesized in salmon. Adenylosuccinate synthetase and putative aminopeptidase were uniquely detected in the skin mucus of SBM group. Trypsin enzymatic activity was significantly decreased in the DI of SBM group. Significantly higher production of immunoglobulin M and Mucin‐like protein in DI mucus in SBM group was observed, while an increase in immunoglobulin D and lysozyme but decrease in chymotrypsin was detected in the skin mucus of the same group. We propose mucosal immunoglobulins as diagnostic biomarkers for assessment of novel feed ingredients and aquafeeds.

Ensuring salmon health and welfare is crucial to maximize production in recirculation aquaculture systems. Healthy and robust mucosal surfaces of the skin and intestine are essential to achieve this goal because they are the first immunological defenses and are constantly exposed to multistressor conditions, such as infectious diseases, suboptimal nutrition, and environmental and handling stress. In this work, Atlantic salmon, split from a single cohort, were subjected to acute hypoxia stress or 15-min crowding stress and observed over a 24-h recovery period. Samples were collected from fish at 0, 1, 3, 6, 12 and 24 h post-stress to analyze plasma-circulating markers of endocrine function (cortisol), oxidative stress (glutathione peroxidase) and immune function (interleukin 10 (IL-10), annexin A1). In addition, mucosal barrier function parameters were measured in the skin mucus (Muc-like protein and lysozyme) and distal intestine (simple folds, goblet cell size and goblet cell area). The results showed that both acute stress models induced increases of circulating cortisol in plasma (1 h post-stress), which then returned to baseline values (initial control) at 24 h post-stress. Moreover, the hypoxia stress was mostly related to increased oxidative stress and IL-10 production, whereas the crowding stress was associated with a higher production of Muc-like protein and lysozyme in the skin mucus. Interestingly, in the distal intestine, smaller goblet cells were detected immediately and one hour after post-hypoxia stress, which could be related to rapid release of the cellular content to protect this organ. Finally, the correlation of different markers in the hypoxic stress model showed that the circulating levels of cortisol and IL-10 were directly proportional, while the availability of Muc-like proteins was inversely proportional to the size of the goblet cells. On the other hand, in the crowding stress model, a proportional relationship was established between plasma cortisol levels and skin mucus lysozyme. Our results suggest key differences in energy partitioning between the two acute stress models and support the need for further investigation into the interplay of multistressor conditions and strategies to modulate immunological aspects of mucosal surfaces.