Natural defences
Researchers have charted how low levels of hydrogen sulphide in RAS systems trigger a defensive reaction in salmon – and their findings could be used to help manage the risks associated with this toxic gas.
A study has revealed how Atlantic salmon in recirculating aquaculture systems (RAS) deploy natural defences against low levels of hydrogen sulphide.
The findings could help fish farmers to detect underlying problems with hydrogen sulphide (H2S) in RAS facilities.
Hydrogen sulphide is a toxic gas that can form in recirculating aquaculture systems (RAS), particularly at higher salinity levels. Even low levels of H2S have been linked to health problems and mortality in salmon.
The study, published in the journal Ecotoxicology and Environmental Safety, looked at how salmon are affected by being in water with low amounts of hydrogen sulphide over an extended period, which can occur during normal production in RAS.
Researchers from the Norwegian food research establishment Nofima, working with international collaborators, have investigated how salmon mucosal surfaces (skin, gills and nose), which are the first line of defence against environmental threats, react to prolonged exposure to low, non-lethal levels of H2S.
The Nofima hydrogen sulphide experiment
The study was conducted at DTU Aqua, Technical University of Denmark, Hirtshals, using three RAS units to test H2S concentrations at low and high levels in addition to a control unit.
One of the key findings is that when salmon are exposed to hydrogen sulphide, the gills and olfactory organ in particular react by activating genes that help the fish cope with stress and activate the immune responses. Another finding is that salmon mucosae have natural ability for detoxifying hydrogen sulphide. The researchers also identified several protein markers in the mucus from the skin and gills that can be used for non-invasive testing for H2S response.
The study concludes: “Prolonged exposure to sub-lethal H2S concentrations induced immune and metabolic responses in Atlantic salmon mucosa, supported by targeted gene expression and mucus proteomics [referring to the structure and functions of proteins within cells] and metabolomics [referring to the small molecules, commonly known as metabolites, within cells, biofluids, tissues or organisms].
“Both proteomics and metabolomics revealed that gill and skin mucus had little similarity in their response to H2S. Histological and morphological analysis showed that H2S exposure resulted in minimal changes in mucosal structures. Key molecules related to sulphide detoxification were identified and localised in Atlantic salmon mucosa.
“The results offer new insights into hydrogen sulphide biology in fish, especially on key biological processes involved in detoxification and associated physiological countermeasures. The identification H2S-responsive molecules can be further explored as markers for H2S response in salmon.”
Carlo C Lazado
“Our findings suggest that salmon can largely adapt to the low H2S levels we tested, without serious negative health effects,” said Project Leader Carlo C Lazado from Nofima, adding: “This is good news for the aquaculture industry, but also underscores the importance of careful monitoring and control of H2S levels in RAS facilities.”
The results can contribute to better farming of Atlantic salmon in several ways, Nofima said. One of the most important points that the researchers highlight in the report is improved risk assessment and setting of limit values for H2S in RAS facilities. There is potential here for developing new standards to strengthen salmon’s resistance to H2S exposure.
Nofima said the study represents an important step towards more sustainable and efficient land-based salmon farming, with a focus on fish welfare and production quality. The findings are particularly relevant for land-based fish farms where H2S levels may be higher than in open sea facilities. By monitoring and controlling H2S levels, as well as implementing measures to strengthen the fish’s natural defences, farmers can ensure healthier and more productive fish.
Mucosal organs exhibit distinct response signatures to hydrogen sulphide in Atlantic salmon (Salmo salar) (Ara-Díaz, Juan Bosco; Bergstedt, Julie Hansen; Albaladejo-Riad, Nora; Malik, Muhammad Salman; Andersen, Øivind; Lazado, Carlo C) published in Ecotoxicology and Environmental Safety, vol. 281 , p. 1–13 , 2024.
