Hydrogen sulfide
Hydrogen sulfide (H₂S) is a dissolved gas that is particularly toxic in its unionized form. Higher levels of hydrogen sulfide are more likely in seawater systems compared to freshwater systems due to higher sulfate concentrations. Hydrogen sulfide tends to be more of an issue in closed systems than in open systems, though accumulation has been noted in lower levels of fjords.41 Hydrogen sulfide impedes respiration, and mass mortalities related to hydrogen sulfide have been reported in RAS farms.42
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
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≤ 0 µg/L
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> 0 to ≤ 0.3 µg/L
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> 0.3 to ≤ 1 µg/L
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> 1 µg/L
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Protocol application
Measure H₂S directly using a portable colorimetric kit, methylene blue method, or electrochemical sensor. If measuring total sulfide, calculate the H₂S fraction using pH, temperature, and salinity (see scoring notes). Sample and analyze promptly, as H₂S is volatile and concentrations can change rapidly after collection.
Hydrogen sulfide is generally not required to be measured in sea cages or other open systems. If measuring hydrogen sulfide in sea cages, it is recommended to measure at different cage depths (e.g., 3 m, 5 m, 15 m, and especially at maximum depth, as hydrogen sulfide tends to accumulate near the bottom). Scoring classification should be based on the maximum detected concentration at any depth where fish are active.
Scoring notes
- If total sulfide is being measured, the hydrogen sulfide levels must be calculated from pH, temperature, and salinity, or derived from reference tables such as those in the Australian & New Zealand Guidelines for Fresh & Marine Water Quality.
- Water quality parameters such as dissolved oxygen, temperature, and pH can affect hydrogen sulfide levels. For example, low pH is a risk factor for increased hydrogen sulfide.
- While short-term exposure (e.g., for a few hours) to elevated hydrogen sulfide may be tolerated, exposure over days or months poses a greater risk to rainbow trout.
- Validation: Extremely limited rainbow trout-specific evidence exists. One study on juvenile rainbow trout in freshwater was published in 1976, but more conservative thresholds have been suggested based on a more recent salmon study in brackish water, and the possibility that hydrogen sulfide tolerance levels may have been overestimated in older literature.
Key evidence
| Source | Summary of source | Key result |
|---|---|---|
| Lazado et al., 2024 | Experimental study investigating the effects of hydrogen sulfide exposure on 450 post-smolt Atlantic salmon (162 g) in brackish water laboratory conditions over 4 weeks. The fish were split into 3 treatment groups, with exposure to 0 (background level < 0.3 µg/L recorded), 1, and 5 µg/L of hydrogen sulfide, respectively. |
The low-exposure group had a mortality rate of 4.7% over the trial period, and the high-exposure group had a mortality rate of 16%. Mortalities were only observed in the third week, with the authors suggesting that the fish were actively deploying countermeasures until their defenses were depleted. The control group had less than 1% mortality. Both exposure groups had significantly lower weights than the control group, with the low-exposure group weighing 26% less than controls and the high-exposure group 22% less. Exposed fish also showed signs of lethargy and exhaustion, with significant tissue damage present in both exposure groups. The authors explicitly propose that hydrogen sulfide levels should not exceed 1 µg/L in brackish water RAS systems for salmon. |
| Smith et al., 1976 | Experimental study investigating the effects of hydrogen sulfide exposure on juvenile rainbow trout in freshwater conditions over 4–17 days. Acute toxicity tests were conducted on two groups of fish with average lengths of 45 and 54mm, respectively, with hydrogen sulfide concentrations ranging from 1.4–29.2 µg/L over 4–17 days. Chronic exposure tests on 50-day-old juveniles were also conducted at exposure levels of 0, 1.1, 3.7, 5.9, and 10.2 µg/L. 30 fish per treatment group were tested. |
In the acute exposure tests, it was determined that, for 50% of fish to suffer mortality within 96 hours, the hydrogen sulfide level was 12.5–13 µg/L. Over a more extended period, a lower hydrogen sulfide level was required to cause 50% mortality. For 17 days, only an 8.6 µg/L exposure was required. There were also some impacts on growth. Fish exposed to 6.2 µg/L or below showed equal or slightly higher growth compared to the control, while those exposed to 9.5 µg/L had about half the average weight compared to the control. For the chronic exposure tests, the group exposed to 10.2 µg/L had a mortality rate of 57% over 50 days. In contrast, the other groups exposed to lower levels of hydrogen sulfide had mortality rates at or below 10%. The authors determined that to ensure no demonstrable adverse effect, the hydrogen sulfide levels should remain below 3–5 µg/L. |
| Ciani et al., 2024 | Experimental study investigating the impact of short hydrogen sulfide exposure on 340 post-smolt Atlantic salmon (114 g) over 1 hour in brackish water RAS conditions. Fish were stocked at densities of 10 kg/m³ in the control and one exposure tank, and 30 kg/m³ in a second exposure tank. Fish were exposed to increasing hydrogen sulfide concentrations once daily for 10 consecutive days, with concentrations doubling each day, starting at 1 µg/L and reaching 68 µg/L. Swimming behavior was recorded using overhead and underwater cameras. |
Fish rapidly responded to increases in hydrogen sulfide, exhibiting faster swimming speeds, erratic zigzagging patterns, and a loss of schooling. These behavioral changes increased linearly from no exposure to concentrations of 30–40 µg/L. Between 40–50 µg/L, behavior significantly shifted, with speed increasing by 65%, dispersion increasing by 33%, and pattern quality decreasing by 18% compared to the previous group. Further increases in hydrogen sulfide beyond this level did not result in additional changes in swimming parameters. |
Supporting evidence
- Bergstedt, Julie Hansen, Carlos Octavio Letelier-Gordo, and Peter Vilhelm Skov. “Exposure to Sublethal Levels of H2S Does Not Influence the Short-Term Growth or Feed Utilization in Post-Smolt Atlantic Salmon (Salmo Salar) during Restrictive Feeding.” Aquaculture 590 (September 2024): 741093. https://doi.org/10.1016/j.aquaculture.2024.741093.
- Bergstedt, Julie Hansen, and Peter Vilhelm Skov. “Acute Hydrogen Sulfide Exposure in Post-Smolt Atlantic Salmon (Salmo Salar): Critical Levels and Recovery.” Aquaculture 570 (May 2023): 739405. https://doi.org/10.1016/j.aquaculture.2023.739405.
- Ciani, Elia, Bjarne Kvæstad, Marit Stormoen, Ian Mayer, Shruti Gupta, Deni Ribičić, and Roman Netzer. “Early Warning through Video Monitoring: Dissolved Hydrogen Sulphide (H2S) Affects Atlantic Salmon Swimming Behavior in Recirculating Aquaculture Systems.” Aquaculture 581 (February 25, 2024): 740201. https://doi.org/10.1016/j.aquaculture.2023.740201.
- Lazado, Carlo C., Kevin T. Stiller, Gerrit Timmerhaus, Britt Kristin, Ilona Lorraine Nicolaysen, Danilo Carletto, Hanna Ross, Julie Hansen Bergstedt, and Øivind Andersen. “Mucosal and Systemic Physiological Changes Underscore the Welfare Risks of Environmental Hydrogen Sulphide in Post-Smolt Atlantic Salmon (Salmo Salar).” Ecotoxicology and Environmental Safety 270 (January 1, 2024): 115897–97. https://doi.org/10.1016/j.ecoenv.2023.115897.
- Nicolaysen, Ilona Lorraine. “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.” MSc Thesis, 2023. https://nva.sikt.no/registration/0198f13c3755-4e86d005-11f2-4203-a77b-cefa95255530.
- Reynolds, Fayette A. “The Effects of Long-Term Exposure to Low Concentrations of Hydrogen Sulfide on Brown Trout, Salmo Trutta.” MSc Thesis, 1976. https://soar.suny.edu/server/api/core/bitstreams/42a43361-bce2-45fc-a750-39cbb91ff7bb/content.
- Ross, Hanna, Julie Hansen Bergstedt, and Carlo C. Lazado. “Differential Sensitivity of Mucosal Organs to Transient Exposure to Hydrogen Sulphide in Post-Smolt Atlantic Salmon (Salmo Salar).” Aquaculture 573 (August 1, 2023): 739595–95. https://doi.org/10.1016/j.aquaculture.2023.739595.
- Smith, Lloyd LeRoy, Steven J. Broderius, Donavon M. Oseid, and Ira R. Adelman. Effect of Hydrogen Sulfide on Fish and Invertebrates. United States Environmental Protection Agency, 1976.
- Sommerset, Ingunn, Cecilie S. Walde, Britt Bang Jensen, Geir Bornø, Asle Haukaas, and Edgar Brun. “The Health Situation in Norwegian Aquaculture 2019.” Norwegian Veterinary Institute, 2020. https://www.vetinst.no/rapporter-og-publikasjoner/rapporter/2020/fish-health-report-2019/_/attachment/inline/7507c5b2-df54-4028-aee8-da5ae3010d0d:728b5711c9fb60a0ba16803064780727f5bc3b6b/Fish%20health%20report%202019.pdf.
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Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. “Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare.” Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
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Sommerset, Ingunn, Cecilie S. Walde, Britt Bang Jensen, Geir Bornø, Asle Haukaas, and Edgar Brun. “The Health Situation in Norwegian Aquaculture 2019.” Norwegian Veterinary Institute, 2020. https://www.vetinst.no/rapporter-og-publikasjoner/rapporter/2020/fish-health-report-2019/_/attachment/inline/7507c5b2-df54-4028-aee8-da5ae3010d0d:728b5711c9fb60a0ba16803064780727f5bc3b6b/Fish%20health%20report%202019.pdf.