Mucosal and systemic physiological changes underscore the welfare risks of environmental hydrogen sulphide in post-smolt Atlantic salmon (Salmo salar).

Atlantic salmon (Salmo salar) might encounter toxic hydrogen sulphide (HS) gas during aquaculture production. Exposure to this gas can be acute or chronic, with heightened levels often linked to significant mortality rates. Despite its recognised toxicity, our understanding of the physiological implications of HS on salmon remains limited. This report details the mucosal and systemic physiological consequences in post-smolt salmon reared in brackish water at 12 ppt after prolonged exposure to elevated HS levels over 4 weeks. The fish were subjected to two concentrations of HS: 1 µg/L (low group) and 5 µg/L (high group). An unexposed group at 0 µg/L served as the control. Both groups exposed to HS exhibited incremental mortality, with cumulative mortality rates of 4.7 % and 16 % for the low and high groups, respectively. Production performance, including weight and condition factors, were reduced in the HS-exposed groups, particularly in the high group. Mucosal response of the olfactory organ revealed higher tissue damage scores in the HS-exposed groups, albeit only at week 4. The high group displayed pronounced features such as increased mucus cell density and oedema-like vacuoles. Transcriptome analysis of the olfactory organ unveiled that the effects of HS were more prominent at week 4, with the high group experiencing a greater magnitude of change than the low group. Genes associated with the extracellular matrix were predominantly downregulated, while the upregulated genes primarily pertained to immune response. HS-induced alterations in the metabolome were more substantial in plasma than skin mucus. Furthermore, the number of differentially affected circulating metabolites was higher in the low group compared to the high group. Five core pathways were significantly impacted by HS regardless of concentration, including the phenylalanine, tyrosine, and tryptophan biosynthesis. The plasma levels of phenylalanine and tyrosine were reduced following exposure to HS. While there was a discernible distinction in the skin mucus metabolomes among the three treatment groups, only one metabolite - 4-hydroxyproline - was significantly impacted by HS. Furthermore, this metabolite was significantly reduced in the plasma and skin mucus of HS-exposed fish. This study underscores that prolonged exposure to HS, even at concentrations previously deemed sub-lethal, has discernible physiological implications that manifest across various organisational levels. Given these findings, prolonged exposure to HS poses a welfare risk, and thus, its presence must be maintained at low levels (<1 µg/L) in salmon land-based rearing systems.