Dissolved oxygen

Adequate dissolved oxygen is fundamental for all metabolic processes in fish, and therefore dissolved oxygen is one of the most critical environmental parameters to measure. Dissolved oxygen levels can fall due to high stocking density, elevated water temperature, poor water exchange, or other environmental factors such as algal blooms. Both hypoxia (low oxygen) and hyperoxia (excessive oxygen) are major physiological stressors, compromising health, immunity, and overall welfare, and can lead to morbidity and mortality.36

Level 0 Level 1 Level 2 Level 3
> 90 to ≤ 110 %
> 80 to ≤ 90 %
OR
> 110 %
> 60 to ≤ 80 %
≤ 60 %
Protocol application

Regular (e.g., weekly) calibration of monitoring devices against known standards is required to ensure accurate measurements.

Measuring dissolved oxygen at various points is crucial for a comprehensive understanding of the system’s environment. This involves taking readings at different depths (e.g., surface, mid-column, and bottom) as well as at different horizontal locations (e.g., center and cage edge). Readings should also be taken from areas with the poorest expected circulation, such as the depth with the highest fish density. This is important because oxygen levels can stratify in specific areas due to factors such as temperature and stocking density.

It is recommended to monitor oxygen levels continuously, with manual verification at least twice daily, at dawn and dusk due to the photosynthesis and respiration cycles of algae/plankton impacting oxygen levels. The duration of suboptimal conditions should also be noted, with increased monitoring of fish behavior during these periods. When taking multiple readings in a pen, use the worst-scoring reading within the assessment period for overall scoring.

Scoring notes
  • If the oxygen monitoring device measures the absolute oxygen level and does not provide a saturation reading, temperature compensation must be applied. Specific calculators can be used to determine the saturation.
  • Due to diurnal variation, expect daily fluctuations, with morning readings typically being the lowest.
  • Depth stratification can occur, with oxygen depletion possible in deeper layers.
  • Feeding may affect the availability of oxygen, with temporary oxygen depletion occurring post-feeding in intensive systems.
  • At level 2 and level 3, monitor fish behavioral changes, such as an increase in surface swimming.
  • Acute oxygen depletion or oversaturation (e.g., < 4 hours) is less concerning than sustained exposure.
  • Oxygen interacts with temperature. Higher temperatures reduce the water’s oxygen capacity while increasing fish oxygen demand, meaning monitoring oxygen levels is especially important during high-temperature periods.
  • Validation: The above oxygen levels have been validated only for temperatures of 13–21°C, based on available rainbow trout studies. The effects of oxygen levels outside this range are unclear, though as temperature declines, lower saturation levels should be tolerable. If using these thresholds for temperatures outside of this range, proceed with caution. Further, very high saturations (e.g., > 110%) have only been studied to a limited extent. Very high saturation systems should be treated with caution. The health and welfare status of the fish is also important, as gill issues or high stress levels may impact the levels of oxygen tolerated, with the above thresholds needing to be treated conservatively in these cases.
Key evidence
Source Summary of source Key result
Waldrop et al., 2020 Experimental study investigating the impacts of dissolved oxygen concentration and swimming exercise on rainbow trout (from 18 g to 1 kg) in freshwater laboratory conditions over 341 days.

Dissolved oxygen was measured at high (100% saturation) and low (70% saturation) levels. Swimming exercise was also tested at high levels (1.5–2.0 body lengths/second) and low levels (~0.5 body lengths/second).		 Fish in the high dissolved oxygen groups showed significantly better growth, reaching 1019 g vs. 878 g.

The higher oxygen group also had reduced caudal fin damage.

Mortality in the 100% dissolved oxygen group was 1.95%, and in the 70% dissolved oxygen group was 1.30%, but this was not statistically significant.
Park et al., 2023 Experimental study investigating the impact of dissolved oxygen concentration on 1080 rainbow trout (123 g) in freshwater RAS over 8 weeks.

4 different dissolved oxygen levels were tested: 5–6 mg/L, 9–10 mg/L, 14–15 mg/L, and 17–18 mg/L. The temperature was 17.6°C.

Converting these to oxygen saturation levels (assuming 0 salinity and sea-level atmospheric pressure of 101.32 kPa), the four oxygen saturations tested were: ~52%–62%, ~93%–104%, ~145%–155%, and ~176%–186%.		 Fish in the lowest oxygen group had significantly reduced growth rates, weight gain, length growth, and daily feed intake. They also had elevated growth hormone and hemoglobin/hematocrit levels, indicating physiological stress.

The higher oxygen groups did not show significant differences in growth performance.

The ~93%–104% group had the best growth performance and the most balanced hormone levels. The hormone Insulin-like Growth Factor-1 was also highest in this group, indicating better conditions.
Jiang et al., 2021 Experimental study investigating the impact of temperature and oxygen on 360 ongrowing rainbow trout (110 g) in freshwater RAS over 28 days.

Three temperatures (13°C, 17°C, and 21°C) and two dissolved oxygen levels (4.2 mg/L (hypoxic) and 9.6 mg/L (hyperoxic)) were tested. There were 6 treatments for each combination of temperature and oxygen, with 3 replicates each, resulting in 18 tanks with 20 fish per tank.

Converting to oxygen saturation levels (assuming 0 salinity and a sea-level atmospheric pressure of 101.32 kPa), the low-oxygen treatments (4.2 mg/L) resulted in 40% saturation at 13°C, 43% saturation at 17°C, and 47% saturation at 21°C. The high-oxygen treatments (9.6 mg/L) resulted in 91% saturation at 13°C, 99% saturation at 17°C, and 108% saturation at 21°C.		 High dissolved oxygen levels improved survival at all temperatures and enhanced growth performance and feed consumption.

Under low oxygen conditions, the 13°C treatment (40% saturation) achieved 86.7% survival with 0.89% growth rate, the 17°C treatment (43% saturation) achieved 86.7% survival with 1.15% growth rate, and the 21°C treatment (47% saturation) showed severely compromised welfare with only 70% survival and 0.36% growth rate.

Under high oxygen conditions, the 13°C treatment (91% saturation) achieved 93.3% survival with 1.20% growth rate, the 17°C treatment (99% saturation) achieved the best performance with 96.7% survival and 1.46% growth rate, and the 21°C treatment (108% saturation) maintained 90.0% survival with 0.95% growth rate.
Supporting evidence
  • Aksakal, Ercüment, and Deniz Ekinci. “Effects of Hypoxia and Hyperoxia on Growth Parameters and Transcription Levels of Growth, Immune System and Stress Related Genes in Rainbow Trout.” Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 262 (December 2021). https://doi.org/10.1016/j.cbpa.2021.111060.
  • Duran, Utku, and Sena Çenesiz. “The Effect of Different Oxygen Concentrations on Oxidative Stress and Some Biochemical Parameters in the Transfer of Adult Rainbow Trout (Oncorhynchus mykiss).” BMC Veterinary Research 21, no. 1 (April 30, 2025). https://doi.org/10.1186/s12917-025-04770-4.
  • Han, Buying, Yuqiong Meng, Haining Tian, Changzhong Li, Yaopeng Li, Caidan Gongbao, Wenyan Fan, and Rui Ma. “Effects of Acute Hypoxic Stress on Physiological and Hepatic Metabolic Responses of Triploid Rainbow Trout (Oncorhynchus mykiss).” Frontiers in Physiology 13 (June 24, 2022). https://doi.org/10.3389/fphys.2022.921709.
  • Jiang, Xuyang, Shuanglin Dong, Rongxin Liu, Ming Huang, Kang Dong, Jian Ge, Qinfeng Gao, and Yangen Zhou. “Effects of Temperature, Dissolved Oxygen, and Their Interaction on the Growth Performance and Condition of Rainbow Trout (Oncorhynchus mykiss).” Journal of Thermal Biology 98 (May 2021). https://doi.org/10.1016/j.jtherbio.2021.102928.
  • MacIntyre, Craig. “Water Quality and Welfare Assessment on United Kingdom Trout Farms.” PhD Thesis, 2008. https://dspace.stir.ac.uk/bitstream/1893/434/1/FINAL%20THESIS.pdf.
  • 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.
  • Park, Kunhong, Jinseo Choi, Younghun Lee, and Jeonghwan Park. “Evaluation of the Optimal Dissolved Oxygen Level for Rainbow Trout (Oncorhynchus mykiss) in the Recirculating Aquaculture System.” Journal of the Korean Society of Fisheries and Ocean Technology 59, no. 4 (November 30, 2023): 387–98. https://doi.org/10.3796/ksfot.2023.59.4.387.
  • Ritola, O., K. Tossavainen, T. Kiuru, P. Lindström-Seppä, and H. Mölsä. “Effects of Continuous and Episodic Hyperoxia on Stress and Hepatic Glutathione Levels in One-Summer-Old Rainbow Trout (Oncorhynchus mykiss).” Journal of Applied Ichthyology 18, no. 3 (June 26, 2002): 159–64. https://doi.org/10.1046/j.1439-0426.2002.00324.x.
  • Waldrop, Thomas, Steven Summerfelt, Patricia Mazik, P. Brett Kenney, and Christopher Good. “The Effects of Swimming Exercise and Dissolved Oxygen on Growth Performance, Fin Condition and Survival of Rainbow Trout Oncorhynchus mykiss.” Aquaculture Research 51, no. 6 (March 15, 2020): 2582–89. https://doi.org/10.1111/are.14600.

  1. 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.