pH
Stable pH within an appropriate range supports normal gill function, osmoregulation, and blood pH regulation in fish. pH is a measure of the acidity or alkalinity of water, reflecting the concentration of hydrogen ions. pH in seawater typically ranges from 7.5 to 8.5. Rapid changes to pH should be avoided. Extreme pH values can cause direct tissue damage, particularly to gill structures, compromising oxygen uptake and ion regulation. Maintaining stable, appropriate pH levels is essential for minimizing stress and ensuring proper physiological function in sea cage ongrowing systems.
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
|
> 7.8 to ≤ 8.2 pH
|
> 7.5 to ≤ 7.8 pH
OR > 8.2 to ≤ 8.5 pH |
> 7 to ≤ 7.5 pH
OR > 8.5 to ≤ 9 pH |
≤ 7 pH
OR > 9 pH |
Protocol application
Calibrate the pH meter or probe (glass electrode recommended for accuracy in seawater) daily, using at least two buffer solutions spanning the expected range (e.g., pH 4.0, 7.0, and 10.0).
It is recommended to measure pH once a day. Measuring pH 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 the bottom) and at different horizontal locations.
When taking multiple readings in a pen, use the worst score within the assessment period for overall scoring.
If there is a change in pH, note whether it is due to a brief event (e.g., a storm or algal bloom) or to a chronic condition.
Scoring notes
- Acute (e.g., < 24 hours) and infrequent (e.g., once per month) exposure to suboptimal pH levels will have less severe welfare impacts than chronic (e.g., > 7 days) or frequent (e.g., > 30 days per year) exposure.
- If level 3 pH occurs frequently at the site, the site is not suitable as welfare cannot be maintained at the location.
- pH levels should not be considered in isolation, but also in reference to total alkalinity, dissolved carbon dioxide, and other substances such as ammonia.
- Validation: The evidence on the impacts of different pH levels for rainbow trout in seawater is extremely limited, with the majority of data investigating rainbow trout in freshwater. These findings were adjusted upwards to account for naturally higher pH levels in seawater. Level 0 was based on typical seawater pH ranges.
Key evidence
| Source | Summary of source | Key result |
|---|---|---|
| Laurent et al., 2000 | Experimental study investigating the effect of highly alkaline freshwater on the morphology of the gills in ongrowing rainbow trout (175–250 g) in laboratory conditions over 72 hours. Fish were either exposed to pH 9.5 or to pH 8 (control). |
Fish in the 8 pH group showed normal physiological functions. The 9.5 pH group had negative welfare impacts, even in the first 10 hours. This included a reduction in essential ion uptake, decreased ammonia excretion, and loss of functional gill cells. After 24 hours, blood ammonia rose to nearly seven times normal. |
| Winter et al., 2005 | Experimental study investigating the effects of aluminium exposure on juvenile rainbow trout (~0.6 g) in laboratory conditions over ~2 days. Water pH ranged between 4 and 10. The trout were exposed to 3 µmol Al/L in ion-poor water. There was a control group with no aluminium, with a pH of 7.2. |
Aluminium accumulation was pH-dependent. The highest was at pH 6–8, then pH 5 and 9, with no accumulation at pH 4 and 10. The pH 7–8 groups had no mortality over the course of the experiment, despite aluminium accumulation. There was a 20% mortality rate in the groups with pH 9, 6, 5, and the control. There was a 100% mortality rate in the pH 10 and 4 groups, caused by gill damage, decreased plasma ion concentrations, impaired ammonia elimination, and disrupted ion homeostasis. It is difficult to separate the effects of pH and aluminium. |
| Mota et al., 2016 | Experimental study investigating the effects of water exchange rate and pH on hormonal levels in 250 juvenile rainbow trout (90 g) in laboratory conditions over 70 days. Three treatments were tested: High water exchange and neutral pH (~pH 7.3), low water exchange and neutral pH (~pH 7.3), and low water exchange and low pH (~pH 5.8). |
Fish in low-pH systems had significantly higher plasma cortisol levels at the end of the experiment, indicating stress. Water cortisol concentrations and testosterone levels were also significantly higher in low pH systems halfway through the experiment. These concentrations were also higher at the end of the experiment, but these results were not significant. No significant differences were found between survival, growth, or feed conversion. |
| Calabrese et al., 2023 | Experimental study investigating the effects of water flow rate on 2500 post-smolt Atlantic salmon in intensive seawater laboratory flow-through systems. Four treatments were tested: 0.2, 0.3, 0.4, and 0.6 L/kg of fish/min. In these treatments, pH was 6.94, 7.2, 7.37, and 7.46 respectively. With increased water flow, there was lower carbon dioxide and ammonia. |
Blood pH was significantly elevated in the lowest flow treatment (and therefore lowest pH), indicating compensation. After 2 weeks, blood pH had been restored in the 0.3, 0.4, and 0.6 L/kg of fish/min (corresponding to pH of 7.2, 7.37, and 7.46), but not in the 0.2 L/kg of fish/min treatment (corresponding to a pH of 6.94). |
Supporting evidence
- Al-Bairuty, Genan A., David Boyle, Theodore B. Henry, and Richard D. Handy. “Sublethal Effects of Copper Sulphate Compared to Copper Nanoparticles in Rainbow Trout (Oncorhynchus mykiss) at Low pH: Physiology and Metal Accumulation.” Aquatic Toxicology 174 (May 2016): 188–98. https://doi.org/10.1016/j.aquatox.2016.02.006.
- Bertin, Matthew J., Delia C. Voronca, Robert W. Chapman, and Peter D.R. Moeller. “The Effect of pH on the Toxicity of Fatty Acids and Fatty Acid Amides to Rainbow Trout Gill Cells.” Aquatic Toxicology 146 (January 2014): 1–11. https://doi.org/10.1016/j.aquatox.2013.09.026.
- Calabrese, Sara, Albert K. D. Imsland, Tom Ole Nilsen, Jelena Kolarevic, Lars O. E. Ebbesson, Camilla Diesen Hosfeld, Sveinung Fivelstad, et al. “Water Flow Requirements of Post-Smolt Atlantic Salmon (Salmo Salar L.) Reared in Intensive Seawater Flow-through Systems: A Physiological Perspective.” Fishes 8, no. 6 (May 26, 2023): 285–85. https://doi.org/10.3390/fishes8060285.
- Dussault, Ève B., Richard C. Playle, D. George Dixon, and R. Scott McKinley. “Effects of Sublethal, Acidic Aluminum Exposure on Blood Ions and Metabolites, Cardiac Output, Heart Rate, and Stroke Volume of Rainbow Trout, Oncorhynchus mykiss.” Fish Physiology and Biochemistry 25, no. 4 (December 2001): 347–57. https://doi.org/10.1023/a:1023295413119.
- Laurent, Pierre, Michael P. Wilkie, Claudine Chevalier, and Chris M. Wood. “The Effect of Highly Alkaline Water (pH 9.5) on the Morphology and Morphometry of Chloride Cells and Pavement Cells in the Gills of the Freshwater Rainbow Trout: Relationship to Ionic Transport and Ammonia Excretion.” Canadian Journal of Zoology 78, no. 2 (March 2000): 307–19. https://doi.org/10.1139/z99-207.
- Morgan, Ian J., Leela Marie D’Cruz, J. J. Dockray, Tyler K. Linton, and Chris M. Wood. “The Effects of Elevated Summer Temperature and Sublethal Pollutants (Ammonia, Low pH) on Protein Turnover in the Gill and Liver of Rainbow Trout (Oncorhynchus mykiss) on a Limited Food Ration.” Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 123, no. 1 (May 1999): 43–53. https://doi.org/10.1016/s1095-6433(99)00036-7.
- Mota, Vasco C., Catarina I. M. Martins, Ep H. Eding, Adelino V. M. Canário, and Johan A. J. Verreth. “Cortisol and Testosterone Accumulation in a Low pH Recirculating Aquaculture System for Rainbow Trout (Oncorhynchus mykiss).” Aquaculture Research 48, no. 7 (August 29, 2016): 3579–88. https://doi.org/10.1111/are.13184.
- 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.
- Winter, Anna R., Joel W. Nichols, and Richard C. Playle. “Influence of Acidic to Basic Water pH and Natural Organic Matter on Aluminum Accumulation by Gills of Rainbow Trout (Oncorhynchus mykiss).” Canadian Journal of Fisheries and Aquatic Sciences 62, no. 10 (October 2005): 2303–11. https://doi.org/10.1139/f05-063.