Nitrite
Rainbow trout can absorb nitrite (NO₂⁻) from the water through the gills, where it enters the bloodstream. This results in nitrite reacting in the blood, reducing its ability to carry oxygen, which can cause respiratory and cardiovascular stress and eventually anemia. This can happen even if there is otherwise sufficient dissolved oxygen in the water. While nitrite can be a problem in freshwater systems, in sea cages, nitrite is not usually a problem due to the flowing, changing water, and because chloride ions compete with nitrite for transport.40
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
|
≤ 0.5 mg/L
|
> 0.5 to ≤ 2.5 mg/L
|
> 2.5 to ≤ 5 mg/L
|
> 5 mg/L
|
Protocol application
Nitrite can be measured using colorimetric test kits (e.g., diazotization method) or photometric analysis.
It is generally unnecessary to monitor nitrite levels in seawater conditions. Nitrite only needs to be observed in high-risk scenarios, for example, where poor water exchange is suspected, or a harmful algal bloom is present.
It is recommended to take readings at different depths (e.g., surface, mid-column, and the bottom). Take the worst measurement from the assessment period.
Scoring notes
- Nitrite is generally unlikely to impact rainbow trout welfare in sea cages.
- In lower-salinity systems such as brackish water systems (e.g., 10–20 ppt), chloride protection is reduced, so more conservative thresholds may need to be applied.
- Validation: These thresholds have not been validated for rainbow trout and should be applied with extreme caution in lower-salinity systems.
- The thresholds have been developed by first considering freshwater rainbow trout data (Kroupova et al., 2008 and Wedemeyer and Yasutake, 1978). From that data, thresholds were determined to be < 0.01, 0.01–0.05, 0.05–0.1, and > 0.1 mg/L, corresponding to level 0, level 1, level 2, and level 3.
- Nitrite was determined to be 50–100 times less toxic in seawater than in freshwater, based on a study conducted on Chinook salmon (Crawford and Allen, 1977), with a seawater salinity of 32.5 ppt.
- The freshwater rainbow trout thresholds were multiplied by 50 (taking the most conservative approach).
- However, this does not account for any species-specific differences between rainbow trout and Chinook salmon, although chloride-competitive inhibition mechanisms are well established across teleost species.
Key evidence
| Source | Summary of source | Key result |
|---|---|---|
| Kroupova et al., 2008 | Experimental study investigating the effects of subchronic nitrite exposure on 240 juvenile rainbow trout (19 g) in freshwater laboratory conditions over 28 days. 5 nitrite concentrations were tested: 0.01, 0.1, 0.6, 1.0, and 3.0 mg/L. A control group was kept in nitrite-free tap water. |
There was no mortality up to 1.0 mg/L nitrite; however, 65% mortality occurred at 3.0 mg/L nitrite. However, even at the lowest tested concentration (0.01 mg/L), trout showed significant welfare impacts, including respiratory damage (abnormal cell growth in gill tissue) and elevated blood glucose levels. As concentrations increased, gill damage progressively worsened, hemoglobin and hematocrit decreased, and white blood cell counts increased. |
| Wedemeyer and Yasutake, 1978 | Experimental study investigating the effects of acute and chronic nitrite exposure on juvenile rainbow trout in freshwater systems. For the acute exposure tests, static 96-hour bioassays were conducted, using duplicate groups of 20 fish per nitrite level. In the chronic exposure tests, trout were exposed to 0.015, 0.03, and 0.06 mg/L of nitrite over 6 months. |
The 96-hour LC50 ranged from 0.5 mg/L to 12.1 mg/L depending on water hardness and pH. Acute exposure caused methemoglobinemia (brown blood disease). For the chronic exposure experiments, the study concluded there were no significant reductions in growth or the ability of smolts to adapt to seawater following the chronic exposure. While exposure to 0.06 mg/L caused minimal gill epithelial changes, these issues began to resolve after 7 weeks as the fish adapted to the new water conditions. |
| Crawford and Allen, 1977 | Experimental study investigating the effects of acute nitrite exposure on 140 Chinook salmon (10.2–13.4 g) in both seawater and freshwater laboratory environments over 48 hours. 31 fish were treated in seawater environments (32.5 ppt), and 109 fish were treated in freshwater environments. In freshwater, 4, 19, and 27 mg/L NO₂⁻ were tested. In seawater, 100, 815, and 1070 mg/L NO₂⁻ were tested. |
In freshwater, 19 mg/L NO₂⁻ caused 50% mortality, and 27 mg/L NO₂⁻ caused 70% mortality over 48 hours. In seawater, 815 mg/L and 1070 mg/L caused 10% mortality in 48 hours. Nitrite was determined to be 50–100 times less toxic in seawater than in freshwater. |
Supporting evidence
- Brown, D. A., and D. J. McLeay. “Effect of Nitrite on Methemoglobin and Total Hemoglobin of Juvenile Rainbow Trout.” The Progressive Fish-Culturist 37, no. 1 (1975): 36–38. https://doi.org/10.1577/1548-8659(1975)37[36:EONOMA]2.0.CO;2.
- Crawford, Richard E., and George H. Allen. “Seawater Inhibition of Nitrite Toxicity to Chinook Salmon.” Transactions of the American Fisheries Society 106, no. 1 (January 1977): 105–9. https://doi.org/10.1577/1548-8659(1977)106%3C105:siontt%3E2.0.co;2.
- Jafari, Leila, Gerard Ayuso-Virgili, Kari Johanne, Sveinung Fivelstad, Ida Pettersen, Tommy Refvik Engevik, and Camilla Diesen Hosfeld. “Water Quality in a Commercial Ras for Atlantic Salmon Post-Smolt: Drum Filter and Biofilter Efficiency,” January 1, 2024. https://doi.org/10.2139/ssrn.5017195.
- Kroupova, H., J. Machova, V. Piackova, J. Blahova, R. Dobsikova, L. Novotny, and Z. Svobodova. “Effects of Subchronic Nitrite Exposure on Rainbow Trout (Oncorhynchus mykiss).” Ecotoxicology and Environmental Safety 71, no. 3 (November 2008): 813–20. https://doi.org/10.1016/j.ecoenv.2008.01.015.
- 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. Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
- Russo, Rosemarie C., Robert V. Thurston, and Kenneth Emerson. “Acute Toxicity of Nitrite to Rainbow Trout (Salmo Gairdneri): Effects of pH, Nitrite Species, and Anion Species.” Canadian Journal of Fisheries and Aquatic Sciences 38, no. 4 (April 1, 1981): 387–93. https://doi.org/10.1139/f81-054.
- Sahhafi, H. Hosseinzadeh, and M. Nafari Yazdi. “Growth Performance of Rainbow Trout (Oncorhynchus mykiss) with Respect to Nutritional Factors in North Iran (Haraz River).” Iranian Journal of Fisheries Science 13, no. 3 (2014): 509–21.
- Wedemeyer, Gary A., and W. T. Yasutake. “Prevention and Treatment of Nitrite Toxicity in Juvenile Steelhead Trout (Salmo Gairdneri).” Journal of the Fisheries Research Board of Canada 35, no. 6 (June 1, 1978): 822–27. https://doi.org/10.1139/f78-132.
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Lewis, William M., and Donald P. Morris. “Toxicity of Nitrite to Fish: A Review.” Transactions of the American Fisheries Society 115, no. 2 (March 1986): 183–95. https://doi.org/10.1577/1548-8659(1986)115%3C183:TONTF%3E2.0.CO;2.