Stress Responses and Gut Health of Post-Smolt Atlantic salmon (Salmo salar) Exposed to Chronic Sub-Lethal Level of Hydrogen Sulphide in a Recirculating Aquaculture System

In recent years, episodes of acute Atlantic salmon mass mortalities in recirculating aquaculture systems (RAS) have been associated with the formation of hydrogen sulphide (H2S), a toxic gas known for its distinct "rotten eggs" odor. Our understanding of how H2S affects salmon mucosal defenses is limited, thereby, risk assessment can be challenging and does not reflect the extent of the threat. This study investigated the health and welfare consequences of exposure to chronic sub-lethal concentrations of H2S, focusing on the stress responses and gut health of Atlantic salmon post-smolt. Mixed-sex post-smolt Atlantic salmon in brackish water RAS were exposed continuously to 1 µg/L and 5 µg/L H2S for four weeks. The group that was not exposed to H2S served as a control. Thereafter, fish were allowed to recover for 2 weeks before they were subjected to a handling-confinement stress test. Plasma and gut samples for histology, gene expression and biochemical analyses were collected at weeks 2 and 4 during H2S exposure and after 2 weeks of recovery. Additionally, plasma samples were collected post-stress test. The results indicated that chronic exposure to H2S influenced key defense mechanisms in the distal intestine of Atlantic salmon. Specifically, the high dose of 5 µg/L H2S upregulated the expression of sulfide: quinone oxidoreductase (sqor2), bclassociated x protein (bax), caspase-3a (casp3a) and cluster of differentiation 4 (cd4). The handling-confinement stress test revealed that chronic exposure to 5 µg/L significantly reduced the ability of salmon to mount an appropriate cortisol response, even after two weeks of recovery. Furthermore, a strong inverse relationship between H2S concentration and oxidation-reduction potential (ORP) was identified, suggesting that ORP can serve as a potential proxy indicator of H2S levels in RAS. This research advances our understanding of the physiological impacts of chronic H2S exposure on salmon, which could assist in reducing H2S-related fatalities in RAS by identifying potential biomarkers and informing risk assessments and monitoring strategies.