Stress and gut responses of post-smolt Atlantic salmon (Salmo salar) to elevated levels of hydrogen sulphide

Summary

Hydrogen sulphide (H2S), a toxic gas recognised for its distinctive rotten egg odour, has been implicated in acute mass mortalities in recirculating aquaculture systems (RAS) for Norwegian Atlantic salmon in recent years. This study delved into the health and welfare ramifications of prolonged exposure to H2S, focusing on stress and gut responses in post-smolt Atlantic salmon. Post-smolt of mixed sexes in brackish water RAS were subjected to prolonged exposure of 1 μg/L and 5 μg/L H2S for 4 weeks. The non-exposed group served as the control. Subsequently, a 2-week recovery period was granted before subjecting the fish to a handling-confinement stress test. Plasma and distal gut samples were collected at weeks 2 and 4 during H2S exposure, with additional samples taken 2 weeks post-recovery for histology, gene expression, and biochemical analyses. Stress parameters in plasma were also analysed post-stress test. The findings indicated that prolonged exposure to elevated H2S levels influenced the expression of critical defence genes in the distal gut of salmon. Notably, the high 5 μg/L dose of H2S upregulated the expression of sulfide:quinone oxidoreductase 2 (sqor2), bcl-associated x protein (bax), caspase-3a (casp3a), and cluster of differentiation 4 (cd4). However, there were no evident substantial changes in gut structures following prolonged H2S exposure. The handling-confinement stress test conducted after the cessation of H2S dosing revealed that prolonged exposure to 5 μg/L significantly diminished the ability of salmon to mount an appropriate cortisol response, even after a 2-week recovery period. Additionally, a robust inverse correlation between H2S concentration and oxidation-reduction potential (ORP) was observed. This research enhances our understanding of the physiological effects of prolonged H2S exposure on salmon, which could aid in mitigating H2S-related fatalities in RAS by identifying potential biomarkers and informing risk assessments and monitoring strategies.