Salinity acclimatization enhances Atlantic salmon resistance to Tenacibaculum finnmarkense: Skin infection and host response

Tenacibaculosis is a severe bacterial ulcerative disease affecting Atlantic salmon aquaculture. Fish are most susceptible to infection during the first weeks at sea, with higher risks associated with low water temperatures. This study investigated whether water salinity prior to seawater transfer could mitigate disease progression. Two production strategies were compared: gradual acclimatization in low seawater salinity (LSS, 26 ppt) and continuous freshwater (F, 0 ppt) production until seawater transfer. Three challenge trials were conducted with Tenacibaculum finnmarkense, the first with freshwater smolt transferred directly to seawater, then two trials with postsmolts that had been held for a further 25 or 50 days in either F or LSS conditions prior to seawater transfer. Mortality rates were recorded as follows: 15 % in the first trial with smolts, 53 % (F) and 22 % (LSS) in the second trial, and 23 % (F) and 13 % (LSS) in the third trial. Histopathological analysis of skin ulcers from infected fish revealed a clear preference of T. finnmarkense for collagen-rich tissues. The bacterium may exploit the collagen matrix to facilitate translocation into underlying tissues, where it is associated with necrotic muscle fibers and thrombosis. Transcriptome analysis showed largely similar gene expression changes in the dermis and epidermis of infected fish, characterized by an acute inflammatory response. Comparison between the F and LSS groups provided insights into the mechanisms underlying increased resistance. In uninfected LSS fish, immune gene expression was generally lower, while epidermal cell proliferation was more active, alongside the upregulation of diverse collagens, collagen chaperones, and modifying enzymes. Following infection, gene expression differences between groups diminished in the epidermis and nearly disappeared in the dermis. However, the initial disparities contributed to a proportionally greater response in the LSS group. Acclimatization in LSS water may enhance resistance to T. finnmarkense infection potentially improving survival in aquaculture settings.