Changes in mucosal epithelia of marine Atlantic salmon (Salmo salar) under chronic heat stress

Farming of Atlantic salmon in Tasmania, Australia, is becoming more challenging as water temperatures can exceed the species' upper thermal limit during summer, with the frequency and duration of exposure to heat stress exacerbated by ocean warming driven by climate change. Although heat-related disorders are characterized by skin, gills and gut pathologies, the effects of heat stress on the mucosal epithelia of Atlantic salmon remain poorly understood. In the present study, Atlantic salmon smolts were exposed to a gradual temperature increase from 15 to 20°C over six weeks, while a control group remained at 15°C, simulating the natural conditions experienced by fish in Tasmanian summers. After four weeks at 20°C, samples were collected from skin, gills, mid- and distal-intestine, and their epithelia were analysed using quantitative histology with Fiji and RT-qPCR for mucin ( muc2 and muc5 ) relative gene expression. A cumulative mortality was observed in the heat-stressed fish compared to the control fish. Heat stress also resulted in the skin epidermal thickening with hyperplasia of mucous cells and the atrophy in surface area of simple folds in distal intestine with hypoplasia of mucous cells. A close-to-significant down-regulation in the relative expression of muc2 ( p = 0.0519) was also observed in the distal intestine. In conclusion, exposure to 20°C heat stress can affect the epithelial barriers of Atlantic salmon, likely impairing functions such as osmoregulation or nutrient absorption. Further research is needed to determine whether these changes indicate impaired mucosal function or compensatory adaptation under heat stress. This will better inform strategies for maintaining mucosal health in order to ameliorate pathologies of the mucosal epithelia that are frequent sequelae to heat stress. • Atlantic salmon smolts were subject to a thermal stress by increasing seawater temperature from 15 °C to 20 °C over two weeks and holding at 20 °C for 4 weeks. • Temperature-stressed fish experienced significantly higher mortality than control fish held at 15 °C. • Heat stress resulted in epidermal thickening in the skin. • Atrophy in the surface area of simple folds in the distal intestine of heat-stressed fish was correlated with reduced goblet cell numbers and down-regulation in expression of muc2.