Nitrate
Nitrate (NO₃⁻) is less toxic than nitrite, but elevated levels can still be a cause for concern for welfare by reducing blood oxygen-carrying capacity. The toxicity of nitrate is influenced by environmental factors, with higher temperatures, lower dissolved oxygen, and higher TSS levels potentially exacerbating nitrate stress. In sea cages, nitrate accumulation is generally not a concern due to continuous water exchange and dilution. However, in areas with agricultural runoff, upwelling events, or localized pollution, nitrate levels may impair fish welfare.
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
|
≤ 25 mg/L
|
> 25 to ≤ 50 mg/L
|
> 50 to ≤ 100 mg/L
|
> 100 mg/L
|
Protocol application
Most test kits measure nitrate-nitrogen, not NO₃⁻ directly. If using such a kit, calculate NO₃⁻ from nitrate-nitrogen. The simplified formula for this is: NO₃⁻ (mg/L) = NO₃-N (mg/L) × 4.43, ensuring measurements use the same units.
It is recommended to measure NO₃⁻ monthly in the winter (or other low-risk periods), with increased monitoring during high-risk periods (such as periods of agricultural runoff).
It is recommended to take readings at different depths (e.g., surface, mid-column, and bottom) and at multiple horizontal locations. Take the worst measurement from the assessment period.
Scoring notes
- The standard measurement methods for nitrate are either cadmium reduction (colorimetric) or ion-selective electrode methods.
- High nitrate levels are unlikely in sea cages.
- Where ammonia and nitrate levels are elevated, combined nitrogen stress occurs. In these cases, monitor closely and consider taking action.
- Acute exposure to elevated nitrate levels (e.g., < 24 hours) has less severe welfare impacts than chronic exposure (e.g., > 7 days).
- When chronic exposure lasts more than 30 days per year, the site may be unsuitable or require seasonal production planning.
- Validation: There are few studies on adult rainbow trout, and the thresholds have been derived from studies on juveniles.
Key evidence
| Source | Summary of source | Key result |
|---|---|---|
| Papadopoulos et al., 2024 | Experimental study investigating the influence of water nitrate concentration and elevated temperature on 160 juvenile (8 g) rainbow trout in laboratory conditions over 20 days. Nitrate concentrations of 40 mg/L and 110 mg/L were compared at both 17°C and 21°C. |
The condition factor at 110 mg/L was lower than at 40 mg/L. At 110 mg/L, rainbow trout showed multiple stress responses compared to 40 mg/L: inflammatory activation (elevated TNF-α), cellular stress (increased heat shock protein production - HSP70), oxidative stress (higher catalase and Cu/Zn SOD in early phases), and metabolic disruption (reduced glucokinase expression for the majority of the trial period). |
| Freitag et al., 2015 | Experimental study investigating the effects of elevated nitrate on endocrine function in 54 juvenile Atlantic salmon (102 g) in freshwater laboratory conditions over 27 days. Three concentrations were tested: 23.0, 45.6, and 451.0 mg/L nitrate. |
Plasma nitrate levels increased significantly with increasing concentration. There was minimal impact on welfare in the lowest nitrate group. Testosterone was significantly elevated in fish exposed to 45.6 mg/L nitrate. In the highest nitrate group, testosterone levels returned to normal. Plasma nitrate was 23 times higher than the control. |
Supporting evidence
- Davidson, John, Christopher Good, Carla Welsh, and Steven T. Summerfelt. “Comparing the Effects of High vs. Low Nitrate on the Health, Performance, and Welfare of Juvenile Rainbow Trout Oncorhynchus mykiss within Water Recirculating Aquaculture Systems.” Aquacultural Engineering 59 (March 2014): 30–40. https://doi.org/10.1016/j.aquaeng.2014.01.003.
- Davidson, John, Christopher Good, Christina Williams, and Steven T. Summerfelt. “Evaluating the Chronic Effects of Nitrate on the Health and Performance of Post-Smolt Atlantic Salmon Salmo salar in Freshwater Recirculation Aquaculture Systems.” Aquacultural Engineering 79 (December 2017): 1–8. https://doi.org/10.1016/j.aquaeng.2017.08.003.
- Freitag, Alyssa R., LeeAnne R. Thayer, Christopher Leonetti, Heather M. Stapleton, and Heather J. Hamlin. “Effects of Elevated Nitrate on Endocrine Function in Atlantic Salmon, Salmo salar.” Aquaculture 436 (January 20, 2015): 8–12. https://doi.org/10.1016/j.aquaculture.2014.10.041.
- 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.
- Papadopoulos, Dimitrios K., Athanasios Lattos, Ioanna Chatzigeorgiou, Aphrodite Tsaballa, Georgios K. Ntinas, and Ioannis A Giantsis. “The Influence of Water Nitrate Concentration Combined with Elevated Temperature on Rainbow Trout Oncorhynchus mykiss in an Experimental Aquaponic Setup.” Fishes 9, no. 2 (February 13, 2024): 74–74. https://doi.org/10.3390/fishes9020074.