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HAVBRUK2-Stort program for havbruksforskning

Prevalence and consequences of hydrogen sulphide in land-based Atlantic salmon production

Alternative title: Omfang og konsekvenser av hydrogensulfid i landbasert lakseoppdrett

Awarded: NOK 12.0 mill.

Project Number:

300825

Application Type:

Project Period:

2020 - 2024

Partner countries:

Mortality related to hydrogen sulphide (H2S) has recently become a serious concern in Atlantic salmon (Salmo salar) farming, particularly in saline recirculating aquaculture systems (RASs), where the risk of H2S formation is high. H2Salar is an ambitious project that aims to create knowledge and advance our understanding of the risks and impacts of H2S on the physiology of Atlantic salmon in RAS. The H2S monitoring programme conducted at different RAS-based farms identified the levels of H2S under normal operational conditions. The survey data revealed that a reduced number of severe mortality events we first identified in 2020 continued in 2021. Mass-balance evaluations using farm data showed that the presence of NO3 did not prevent the formation of H2S, normal degassing systems did not remove low levels of H2S, and furthermore, the site with fixed bed biofilters produced more CO2 and H2S, whereas the site with moving bed biofilter (MBB) removed more CO2 and H2S. The project identified a potential mitigation strategy for H2S in RAS. Adding hydrogen peroxide (H2O2) is an efficient water treatment technology for H2S removal. Dosage adjustments according to the concentrations of H2S and specific system's water parameters could reduce the level of H2S by about half in less than 30 min. The project identified salmon's critical levels and recovery after exposure to H2S. The results showed that salmon have a lower tolerance to H2S than previously estimated, with a mean H2Scrit of 1.78±0.39 µM H2S, independent of fish size. Further, the results revealed that H2S exposure had a more significant effect on the recovery phase of the smaller individuals. Behavioural studies demonstrated an avoidance response at an H2S concentration of 1.8 µM. This concentration has been shown to cause respiratory problems in salmon. To provide an in-depth understanding of the physiological consequences of H2S exposure, we conducted transient and chronic sublethal H2S exposure experiments. In the transient exposure trial, the fish were exposed to one of three levels of H2S for 1 h: 0 µM (unexposed), 0.6 µM (low), and 1.2 µM (high). We found that the skin and gills were the most sensitive mucosal organs to H2S as revealed by molecular and histological changes following exposure. We further determined that transient H2S exposure had a substantial mucosal impact rather than a systemic impact, as shown by changes in skin mucus metabolome. For the chronic exposure trial, we continually exposed salmon to one of three levels of H2S for 4 weeks (unexposed/control, low [1µg/L] and high [5µg/L]). Thereafter, the fish were allowed to recover for 2 weeks before subjecting them to a secondary stressor. The two H2S levels resulted in progressive mortality, with the high group having around 16% mortality after 4 weeks. Transcriptomic analysis revealed that the olfactory organ was the most sensitive mucosal organ, an oppositive trend from the transient exposure. Comparing the transcriptomes of the gills and olfactory organ, the commonly upregulated genes were related to immunity, while the commonly downregulated genes were related to the extracellular matrix. Histology showed some signs of recovery 2 weeks after terminating the H2S exposure. Preliminary data suggest that chronic exposure to high H2S affected the kinetics of stress responses to a secondary stressor. In vitro models were employed for mechanistic insights into the interactions of H2S and salmon. Sulphide donors upregulated the expression of sulphide detoxification genes, while a significant downregulation was observed with mucins in gills and olfactory organ explant models. Pharmacological interference demonstrated that mucins played a crucial role in mucosal protection against H2S toxicity. The sensitivity of olfactory organ to H2S was further studied using the nasal leukocyte model. We have provided the first evidence that H2S is a potent modulator of nasal leukocytes of salmon. We found that the VEGF receptor-ligand interaction was the pathway significantly affected by H2S. By inhibiting the function of VEGF kinase, we proved that the VEGF pathway is crucial for the ability of nasal leukocytes to respond to H2S. We employed a hepatocyte model to explore further the physiological implications of H2S as a xenobiotic. Fast and slow releasing sulphide donors engender distinct transcriptomic alterations in hepatocytes. Sulphide donors induced oxidative stress, affected cellular proliferation and viability and perturbed the pathways involved in mitochondrial metabolism and immunity in hepatocytes. There are several ongoing in vitro and in vivo trials to further elucidate the biological functions of H2S in salmon.

Problems related to hydrogen sulphide (H2S) have become increasingly prevalent in Norwegian Atlantic salmon recirculating aquaculture system (RAS) facilities, with several cases of mass mortality reported in the last years. There is, however, a significant lacuna in the current understanding concerning the biology and physiology of H2S-fish interactions, specifically in salmon. To solve this challenge, a unique and strategic project consortium has been assembled, including two of Norway’s leading R&D institutes on the environmental and biological aspects of recirculation technology (Nofima, NIVA) and a top-rank academic institution with strong expertise in fish physiology (DTU). In addition, an Industry Reference Group will be set up composed of key industry actors in land-based salmon production. In Work Package 1, a programme for detailed monitoring of H2S and tank water quality will be implemented in several salmon farms operating in RAS in Norway and Denmark. This will be supplemented with routine monitoring of health and welfare status of fish reared in those systems and a series of batch reactor experiments to identify how H2S is formed under different production scenarios. Work Package 2 aims to provide molecular and mechanistic insights into exogenous H2S-host interactions in salmon. We will elucidate how salmon cells respond to H2S and identify molecules that may be exploited as biomarkers for H2S response. We will combine the characterised genetic markers and behavioural reaction to further elucidate the sensing response of salmon to environmental H2S. Work Packages 3 and 4 will employ a series of small-scale and large-scale studies to identify the impacts of acute and chronic exposure to H2S on metabolism, welfare and production performance (WP3), as well as on mucosal health and stress resilience (WP4) in salmon. The expected results will offer new frontier in H2S research in fish, particularly in developing knowledge-based mitigating measures.

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Funding scheme:

HAVBRUK2-Stort program for havbruksforskning