| Summary: | The skin and associated mucus layer of Atlantic salmon constitutes its first line of defence against the aqueous environment. Through intensive farming, a range of stressors including both mechanical and environmental factors are known to have an impact on the skin condition of fish. Damaged skin can serve as a portal of entry for primary pathogens and secondary infections. Two of the current main problems in the salmon farming industry are skin related: ectoparasitism with sea lice and skin wounds of diverse origin, often related to extreme temperatures. Four experiments with salmon were conducted to investigate the response of skin and the epidermal mucus layer to challenges representative of normal farming conditions, and to assess the possibility to modulate the skin response through diet. The factors studied included water temperature change, sea lice infection and mechanical wound infliction. New methodologies were developed or existing methods optimised to assess the impact of these factors on the composition, structure and functionality of skin. Quantitative histology using digital image analysing, proteomics of mucus and transcriptomics of skin were all demonstrated to be valuable tools in defining differences between groups exposed to distinct treatments. The chemical composition of the skin was studied at 3 temperatures; 4°C, 10°C and 16°C. Increasing temperature resulted in higher level of protein and collagen related amino acids, while moisture level was reduced. Altering the dietary levels of minerals and vitamins also resulted in changes in their concentration in the skin, demonstrating dietary modulation of skin composition. The structure of the epidermis was also affected by temperature. Quantitative histology assessments showed that the epidermal thickness decreased from low to high temperature, whereas the epidermal area comprising mucous cells increased. Temperature also impacted on the skin transcriptome. A subtle increase in skin-mediated immunity was observed at low temperature, suggesting ...
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