Ammonia
NH₃ is the unionized form of ammonia, which can cross from water into fish. Where there are high levels of ammonia in the environment that the rainbow trout are in, this can mean that they reduce the amount of ammonia they excrete, or even uptake ammonia from the water. The pH and temperature of the water significantly affect the amount of NH₃, which increases with higher pH and temperature. Higher salinities result in lower NH₃ levels. Ammonia can cause physiological welfare issues, such as gill damage, osmoregulatory stress, and even mortality. In sea cages, ammonia levels are typically low due to high water exchange rates, but can accumulate in areas of poor flow, high stocking density, or during harmful algal blooms that elevate pH.
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
|
≤ 0.01 mg/L
|
> 0.01 to ≤ 0.02 mg/L
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> 0.02 to ≤ 0.04 mg/L
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> 0.04 mg/L
|
Protocol application
Most test kits measure TAN (Total Ammonia Nitrogen), not NH₃ directly. If using such a kit, calculate NH₃ from TAN by also recording pH and temperature. The simplified formula for this is: NH₃ (mg/L) = TAN × f, where f is the fraction of NH₃ that depends on salinity, pH, and temperature.
It is recommended to measure ammonia monthly in the winter (or other low-risk periods) and weekly in the summer, with increased monitoring during high-risk periods (such as periods of elevated pH or temperature).
It is recommended to take readings at different depths (e.g., surface, mid-column, and bottom), at multiple horizontal locations, and at any location where ammonia accumulation is suspected. Take the worst measurement from the assessment period.
Note the ammonia source when identifiable, as well as the spatial distribution (e.g., whether it is localized or system-wide).
Scoring notes
- Some examples of the value of f at different salinities, pH, and temperatures are set out on the FAO website.
- Online calculators can also be used by entering the TAN and relevant water quality parameters, such as those published by the Florida Department of Environmental Protection.
- Even at low TAN concentrations, high pH can produce high levels of NH₃. For example, at 15°C and pH 7.5, the percentage of TAN that is NH₃ is 0.665%. At pH 8.5, the percentage is about 10 times higher, at 6.28%.
- During periods of elevated pH (e.g., during algal blooms), especially at pH > 8.5, increased monitoring frequency is essential, as even low TAN readings can result in high NH₃ levels.
- Acute exposure to elevated ammonia levels (e.g., < 24 hours) has less severe welfare impacts than chronic exposure (e.g., > 7 days).
- Persistent level 2 or level 3 scores for ammonia suggest the site’s water exchange is insufficient for the current stocking density or feeding practices. Consider reducing stocking density, improving water flow, or reassessing site suitability.
- Validation: While a limited number of studies were relied on, a range of ammonia concentrations was tested on adult and juvenile rainbow trout.
Key evidence
| Source | Summary of source | Key result |
|---|---|---|
| Vosylienė and Kazlauskienė, 2004 | Experimental study investigating the sublethal effects of ammonia on adult rainbow trout (1 year old) in laboratory conditions over 14 days. Ammonia concentrations between 0.012–0.09 mg/L NH₃ were tested. The control group had a concentration of 0 mg/L NH₃. |
At 0.012 mg/L, there was minimal impact, with most parameters remaining at or near control levels. At 0.024 mg/L, there was a significant increase in old and damaged red blood cell forms, and fewer juvenile red blood cells were being produced. Concentrations of 0.044–0.09 mg/L caused anemia (reduced red blood cells and hemoglobin), immunosuppression, and increased respiratory effort. There was also a tendency for gill ventilation frequency to increase after exposure to levels of 0.024 mg/L or higher. |
| Thurston et al., 1984 | Experimental study investigating the effects of low levels of ammonia on rainbow trout over 5 years. Over these 5 years, the rainbow trout had two generations of offspring. Five ammonia levels ranging from 0.01–0.07 mg/L NH₃ were tested under water conditions with a mean pH of 7.7 and a temperature of 9.3°C. |
Histopathological lesions were common at ≥ 0.04 mg/L NH₃, including significant tissue damage in the gills and kidneys. Blood ammonia levels were positively correlated with water ammonia concentrations. The second generation of offspring had significant welfare impacts at a concentration of 0.02 mg/L NH₃, with histopathological lesions common. However, these fish had a severe protozoan infection, which could have made them more vulnerable. However, the 2 preceding generations did not have significant lesions at this level. |
| Grobler and Wood, 2018 | Experimental study investigating the effects of ammonia on dominance hierarchy formation of 84 juvenile rainbow trout (9.56 g) over 11 days in freshwater laboratory conditions. Fish were size-matched to create 21 groups of 4 fish each. There were 5 treatment groups: the control, 0.0515 mg/L, 0.0884 mg/L, and 0.1105 mg/L NH₃, as well as a sodium bicarbonate control. |
Ammonia effects were concentration-dependent: at 0.0515 mg/L NH₃, stable hierarchies formed despite a 50% reduction in aggression, with lower growth, oxygen consumption, and cortisol (subordinate fish lost weight). At 0.0884 mg/L NH₃, aggression was 75% lower, and hierarchies were severely disrupted. At 0.1105 mg/L NH₃, aggression was abolished and hierarchies could not form. Studies at higher concentrations terminated at 5 days without physiological measurements. The authors attributed decreased swimming ability to an important cause of the lack of aggressive acts and the absence of a social hierarchy in the NH₃-exposed groups. The lower plasma cortisol at 0.0515 mg/L NH₃ was unexplained, with authors suggesting possible reasons being that the effects of reduced aggression in lowering plasma cortisol were higher than the effects of elevated NH₃ in increasing it, NH₃ having a ‘calming’ effect due to overall reduced activity, NH₃ impairing the ability to mobilize cortisol, and acclimation to ammonia. |
Supporting evidence
- Ackerman, Paige A., Beverly J. Wicks, George K. Iwama, and David J. Randall. “Low Levels of Environmental Ammonia Increase Susceptibility to Disease in Chinook Salmon Smolts.” Physiological and Biochemical Zoology 79, no. 4 (July 2006): 695–707. https://doi.org/10.1086/504615.
- Grobler, Josias M.B., and Chris M. Wood. “The Effects of High Environmental Ammonia on the Structure of Rainbow Trout Hierarchies and the Physiology of the Individuals Therein.” Aquatic Toxicology 195 (February 2018): 77–87. https://doi.org/10.1016/j.aquatox.2017.12.006.
- Harsij, Mohammad, Hosna Gholipour Kanani, and Hossein Adineh. “Effects of Antioxidant Supplementation (Nano-Selenium, Vitamin C and E) on Growth Performance, Blood Biochemistry, Immune Status and Body Composition of Rainbow Trout (Oncorhynchus mykiss) under Sub-Lethal Ammonia Exposure.” Aquaculture 521 (May 2020). https://doi.org/10.1016/j.aquaculture.2020.734942.
- Kolarević, J., H. Takle, E. Ytteborg, and B. F. Terjesen. “Effects of Long-Term Sublethal Ammonia Exposure on Expression of Ammonia and Urea Transporting Genes in Gills of Atlantic Salmon (Salmo salar).” In Aquaculture Europe 2011, 568–69, 2011.
- 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.
- Thurston, R. V., R. C. Russo, R. J. Luedtke, C. E. Smith, E. L. Meyn, C. Chakoumakos, K. C. Wang, and C. J. D. Brown. “Chronic Toxicity of Ammonia to Rainbow Trout.” Transactions of the American Fisheries Society 113 (1984): 56–73. https://doi.org/10.1577/1548-8659(1984)113%3C56:CTOATR%3E2.0.CO;2.
- Vosylienė, Milda Zita, and Nijolė Kazlauskienė. “Comparative Studies of Sublethal Effects of Ammonia on Rainbow Trout (Oncorhynchus mykiss) at Different Stages of Its Development.” Acta Zoologica Lituanica 14, no. 1 (2004): 13–18. https://doi.org/10.1080/13921657.2004.10512568.
- Wicks, B. J., and D. J. Randall. “The Effect of Feeding and Fasting on Ammonia Toxicity in Juvenile Rainbow Trout, Oncorhynchus mykiss.” Aquatic Toxicology 59, no. 1-2 (September 10, 2002): 71–82. https://doi.org/10.1016/s0166-445x(01)00237-5.