Carbon dioxide
Carbon dioxide (CO₂) is a waste product of fish respiration and so will always be present to some degree in water where fish are produced. However, elevated levels can pose a threat to fish welfare, particularly when other water quality parameters, such as low dissolved oxygen, are present. High CO₂ levels can impede a fish’s ability to excrete CO₂ from its blood, which is vital for maintaining blood pH and the blood’s capacity to transport oxygen effectively. Rainbow trout in seawater systems are more sensitive to CO₂ than in freshwater systems.
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
|
≤ 5 mg/L
|
> 5 to ≤ 8 mg/L
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> 8 to ≤ 15 mg/L
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> 15 mg/L
|
Protocol application
Regular calibration of the sensor is especially important for detecting low CO₂ levels.
It is unlikely that CO₂ will need to be assessed in sea cages, and so only needs to be monitored on a risk-assessed basis. It is recommended to measure CO₂ in areas of highest fish density and poorest water circulation (near the bottom in stratified systems, such as in sea cages).
Use a reliable method to measure CO₂, rather than calculating it from pH levels, for example. A temperature correction may be needed, as CO₂ solubility and toxicity vary with temperature.
Scoring notes
- Account for carbonate buffering in seawater measurements.
- Reduced appetite is an early indicator of CO₂ stress.
- Monitor for reduced growth rates and changes in blood pH under moderate CO₂ exposure, indicating physiological impacts.
- Diurnal variation results in CO₂ being typically highest in the early morning.
- CO₂ and pH are closely linked, and both should be measured.
- Post-feeding spikes of CO₂ may result in temporary increases, but should resolve within 2–4 hours.
- Acute CO₂ increases (e.g., for less than 4 hours) are less concerning than sustained exposure.
- Monitor for behavioral changes, including reduced feeding and surface gasping during elevated CO₂ periods.
- Validation: Limited rainbow trout-specific studies exist, with evidence from Atlantic salmon also relied upon. Threshold validation is also limited.
Key evidence
| Source | Summary of source | Key result |
|---|---|---|
| Pfalzgraff et al. (2025) | Experimental study investigating growth, appetite, and mineral deposition under different CO₂ regimes in juvenile rainbow trout in RAS over 5 weeks. Three trials were conducted, each involving 360 trout: two in freshwater and one in seawater. 4 CO₂ conditions were tested: control (≤ 3 mg/L CO₂), medium (10 mg/L CO₂), high (25 mg/L CO₂), and fluctuating (daily oscillation between 10–25 mg/L CO₂). |
In freshwater, only 25 mg/L CO₂ significantly reduced growth (by 8–13%). However, the effects were more severe in seawater: 10 mg/L CO₂ reduced growth, and 25 mg/L reduced growth by 44%. Condition factor was also measured in the seawater trial. The high CO₂ group had a significantly lower condition factor than the control. Similarly, in the high CO₂ group, blood pH was significantly higher compared to all other groups. In seawater, fish had a 35% reduction in feed intake. In freshwater, fish ate their full ration. Blood pH increased in high CO₂ groups, and kidney mineral deposits also increased. |
| Khan et al., 2018 | Experimental study investigating the effects of acute and long-term exposure to CO₂ on 32 juvenile Atlantic salmon (20 g) in laboratory conditions over 47 days. For the chronic exposure experiments, fish were raised in 4 different CO₂ concentrations, 2.9, 10, 15, and 25 mg/L. |
There was a 17% decrease in growth performance and a 21% increase in feed conversion between 2.9 mg/L and 25 mg/L CO₂. There was a linear relationship between decreased growth performance and feed conversion across all doses, with both 10 mg/L and 15 mg/L being progressively worse than the 2.9 mg/L control. The condition factor was significantly higher at 2.9 mg/L than in the other groups. |
| Good et al., 2010 | Experimental study investigating the effects of chronic exposure to CO₂ on 4800 rainbow trout in freshwater RAS over 6 months. Fish were raised in replicated RAS systems at either 8 mg/L or 24 mg/L CO₂ exposure, with flow-through tanks serving as controls. |
The control achieved a survival rate of 100%, while the low- and high-CO₂ groups had 96.5% and 98.3% survival rates, respectively. The final weight was 938 g in the control group, approximately 13% lower in the low CO₂ group (817 g), and 10% lower in the high CO₂ group (849 g). Lymphocytic hepatitis was significantly more likely in the high CO₂ group, while epithelial hyperplasia was significantly more likely in the low CO₂ group. The prevalence of lesions associated with reduced water quality was similar across all groups, if not higher in the control. |
Supporting evidence
- Good, Christopher, John Davidson, Carla Welsh, Kevin Snekvik, and Steven Summerfelt. “The Effects of Carbon Dioxide on Performance and Histopathology of Rainbow Trout Oncorhynchus mykiss in Water Recirculation Aquaculture Systems.” Aquacultural Engineering 42, no. 2 (March 2010): 51–56. https://doi.org/10.1016/j.aquaeng.2009.11.001.
- Khan, Javed, Dag Johansen, and Peter Vilhelm Skov. “The Effects of Acute and Long-Term Exposure to CO₂ on the Respiratory Physiology and Production Performance of Atlantic Salmon (Salmo salar) in Freshwater.” Aquaculture 491 (April 1, 2018): 20–27. https://doi.org/10.1016/j.aquaculture.2018.03.010.
- Mota, Vasco C., Tom Ole Nilsen, Jascha Gerwins, Michele Gallo, Elisabeth Ytteborg, Grete Baeverfjord, Jelena Kolarevic, Steven T. Summerfelt, and Bendik Fyhn Terjesen. “The Effects of Carbon Dioxide on Growth Performance, Welfare, and Health of Atlantic Salmon Post-Smolt (Salmo salar) in Recirculating Aquaculture Systems.” Aquaculture 498 (January 1, 2019): 578–86. https://doi.org/10.1016/j.aquaculture.2018.08.075.
- 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.
- Pfalzgraff, Tilo, Helene Volkoff, Tine Moesgaard Iburg, Heidi Amlund, and Peter Vilhelm Skov. “Growth, Appetite, and Mineral Deposition in Rainbow Trout Reared in Fresh- or Seawater under Different CO2 Regimes.” Aquaculture 600 (April 30, 2025). https://doi.org/10.1016/j.aquaculture.2025.742234.