Metals
In a healthy aquatic environment, metal concentrations remain low enough to avoid toxic effects on fish. Many metals are present in open seawater and at toxic levels can significantly affect fish welfare, for example, by damaging gills. Metals are generally more damaging and toxic to fish in freshwater conditions than in seawater conditions, as the higher salinity can act as a protective mechanism.
Thresholds are provided for cadmium and copper only, as these have the strongest evidence. Cadmium accumulates in tissues (particularly liver, kidney, and gills) and causes chronic toxicity, including immunosuppression, growth impairment, and organ damage, even at low concentrations. Copper is acutely toxic to fish, causing gill damage, osmoregulatory failure, and mortality at relatively low concentrations, and its toxicity is strongly influenced by water hardness and dissolved organic matter. Although thresholds are not provided due to limited available scientific evidence, it is also recommended that producers measure other metals (including iron, aluminum, and zinc) to alert them to potential welfare problems.
Cadmium
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
|
≤ 1 µg/L
|
> 1 to ≤ 5 µg/L
|
> 5 to ≤ 10 µg/L
|
> 10 µg/L
|
Copper
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
|
≤ 1 µg/L
|
> 1 to ≤ 5 µg/L
|
> 5 to ≤ 20 µg/L
|
> 20 µg/L
|
Overall: Overall score is determined by the most severe subindicator
Protocol application
Field test kits are available, but are unlikely to be accurate at low concentrations for thresholds in the µg/L range. For more accurate measurements at low concentrations, send samples to labs and use acid-washed bottles. Samples should be stored refrigerated until analysis.
It is recommended to measure metals in high-risk periods (e.g., spring runoff or storm season). Outside of these periods, monitor metals annually, as they rarely vary. If the site is located near potential metal sources, monitor quarterly or monthly, depending on proximity to industrial areas.
Measuring metals 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) and at multiple horizontal locations (e.g., at the center and near the net edges). It is recommended to monitor near pollution sources and any painted structures.
Scoring notes
- Cadmium accumulates in fish tissues. Repeated exposure, even if brief, contributes to a chronic toxic burden.
- Antifouling paint can act as a hidden source of copper.
- For copper, the thresholds should be interpreted conservatively when pH < 7.0, as acidic conditions increase copper toxicity. If pH is high (e.g., > 8.5), then copper toxicity reduces.
- When copper approaches level 2 thresholds, farm managers should investigate the causes of copper, as pH shifts can rapidly escalate toxicity.
- When fish are exposed to multiple problematic metals (e.g., cadmium and copper, or other metals), welfare risk is elevated beyond the sum of individual metal effects. Increase monitoring frequency and urgently investigate sources. Multiple metals at concerning levels indicate a systemic water-quality problem requiring immediate intervention.
- Validation: The suggested thresholds do not account for other parameters, such as water quality (particularly pH), which can significantly influence toxicity of metals. Apply these thresholds with caution. The thresholds have been largely informed by results in freshwater (and adjusted down). Nowak and Duda (1996) investigated rainbow trout in brackish waters, however reported measurements in total copper rather than dissolved copper. These results have been treated as referring to the maximum possible dissolved copper. The thresholds are therefore conservative.
Key evidence
| Source | Summary of source | Key result |
|---|---|---|
| Haux and Larsson, 1984 | Experimental study investigating the effects of and recovery from long-term sublethal cadmium exposure in 135 rainbow trout (100g) in brackish water conditions (around 7 ppt) over 87 weeks. Fish were exposed to either 0, 10, or 100 µg/L of cadmium for 30 weeks, and then recovered in clean water for 57 weeks. Fish were sampled from each treatment group at weeks 18, 30, 55, and 87, with 9–12 fish being sampled per occasion. |
The liver and kidneys in the exposure groups continuously accumulated cadmium. Exposure groups experienced anemia (with the higher exposure group being slightly worse), hypocalcemia (only the higher exposure group), hypermagnesemia (only the higher exposure group), blood glucose elevation (higher in the higher exposure group) and muscle glycogen depletion (with the higher exposure group being more severely depleted). Anemia and ion balances normalized within 25 weeks of recovery, however blood glucose elevation and muscle glycogen depletion were persistently present more than 1 year after exposure ceased. |
| Majewski and Giles, 1981 | Experiment investigating the effects of cadmium exposure on 96 adult (375 g) rainbow trout in freshwater laboratory conditions over 129 days. Two different treatments were used: 3.6 µg/L and 6.4 µg/L. |
At the higher concentration (6.4 µg/L), fish exhibited significant increases in ventilation rate, hematocrit, and hemoglobin levels over time, suggesting elevated physiological stress and possible compensation for impaired oxygen transport or uptake. Additionally, erythrocyte ATP levels decreased significantly at the higher concentration, indicating potential disruption to cellular energy metabolism. Many of the measured parameters worsened progressively over the course of the trial, suggesting a compounding effect on welfare with chronic cadmium exposure. Overall, the lower dose (3.6 µg/L) appeared to cause mild-to-moderate acute physiological stress, while the higher dose (6.4 µg/L) led to more severe and chronic welfare impairments. |
| Heydarnejad et al., 2013 | Experimental study investigating the effect of cadmium exposure over 30 days on 60 juvenile rainbow trout (19.5 g) in laboratory conditions. Three different treatments were conducted: control (0.07µg/L), low 1 µg/L, and high 3µg/L. | No recorded welfare issues in the control group. AST, ALT, and ALP tended to increase over time with higher exposure, indicating higher stress. Weight gain, specific growth rate, and condition factor reduced with increasing exposure to cadmium. Feed conversion ratio increased with increasing cadmium, indicating poorer efficiency. |
| Dethloff et al., 2001 | Experimental study investigating the effects of dissolved copper in wild conditions on ~45 ongrowing rainbow trout in freshwater creeks. The dissolved copper concentrations were 0.56, 1.71, 3.82, and 10.06 µg/L. | The two lowest copper exposures had limited impacts on welfare. Increased copper exposure negatively affected key blood and immune parameters, including reductions in muscle protein and leucocrit, as well as hepatic copper accumulation. |
| Sommers et al., 2016 | Experimental study investigating the effects of salinity on olfactory toxicity and behavioral responses of 39 coho salmon (30.8g in freshwater, 56.7g in seawater) and 54 Chinook salmon (23.5g) in laboratory settings. Olfactory function was measured using electro-olfactograms after an acute 30-minute exposure to 50µg/L of dissolved copper. This was conducted on coho salmon in freshwater, at 10 ppt, and in seawater (28–30 ppt), and Chinook salmon in seawater (28–30 ppt). Avoidance behavior was assessed in Chinook salmon in both freshwater and seawater over 60 minutes using a multi-chambered tank, where dissolved copper was added to the segment over time. A physical structure was also added in the exposed segment vs. a control, to test whether the fish’s attraction to the structure would be overridden by the presence of copper. In the freshwater experiment, the background copper levels were 2.6–3.0µg/L, with the exposure segment reaching a concentration of 7.9µg/L after 30 minutes. In seawater, the background level of copper was 0.4µg/L initially, and reached 16.9µg/L in the treated segment after 30 minutes. |
The acute 30-minute exposure to 50µg/L of copper significantly reduced electro-olfactogram responses in freshwater, but not at 10 ppt or seawater, indicating salinity protects against olfactory toxicity. Fish avoided the copper exposed segment in freshwater and seawater. In the non-exposed groups, Chinook salmon preferred the submerged structure in freshwater but not in seawater. However, in the exposed freshwater group, the presence of copper negated the preference for the structure. |
| Nowak and Duda, 1996 | Experimental study investigating the effects of chronic exposure to copper in 200 rainbow trout (65–375g) in laboratory conditions. Two experiments were conducted. The first experiment was conducted at 19.6 ppt with 15 fish per tank with 8 tanks (including two controls) over 6 weeks, exposing fish to concentrations of 30, 70, and 200µg/L of nominal copper. This corresponded to 10.75–22.00µg/L, 12.6–17.12µg/L, and 41.55–46.77µg/L of total copper respectively. The second experiment was conducted at 14.6 ppt (though this was variable, ranging from 0–23 ppt) with 10 fish per tank with 8 tanks over 7 weeks, also exposing fish to 30, 70, and 200µg/L of nominal copper. This corresponded to 16.00–17.50µg/L, 33.25–43.50µg/L, and 44.50–70.75µg/L of total copper, respectively. |
In the first experiment, except for fish exposed to the highest concentrations of copper, mortality was 0%. However, high mortality was associated with total copper of 56–60µg/L (reflecting a spike in copper levels), with mortality rates of 20 and 27% respectively. In experiment two, at a total copper concentration of 17.5µg/L, survival was 100%, but then at 16µg/L, survival was 80% (however, this may have been caused by salinity stress). There were very high mortality rates above 33.25µg/L, ranging from 30–80%. Growth was consistently reduced at total copper concentrations of 22.00µg/L or higher. Sodium and potassium levels were elevated in fish exposed to the highest copper concentrations, and gill histology showed increased mucous cells. |
Supporting evidence
- Dethloff, G. M., H. C. Bailey, and K. J. Maier. “Effects of Dissolved Copper on Select Hematological, Biochemical, and Immunological Parameters of Wild Rainbow Trout (Oncorhynchus mykiss).” Archives of Environmental Contamination and Toxicology 40, no. 3 (March 2001): 371–80. https://doi.org/10.1007/s002440010185.
- Förlin, L., C. Haux, L. Karlsson-Norrgren, P. Runn, and Å. Larsson. “Biotransformation Enzyme Activities and Histopathology in Rainbow Trout, Salmo Gairdneri, Treated with Cadmium.” Aquatic Toxicology 8, no. 1 (April 1986): 51–64. https://doi.org/10.1016/0166-445x(86)90072-x.
- Gündoğdu, Ayşe, and Muammer Erdem. “The Accumulation of the Heavy Metals (Copper and Zinc) in the Tissues of Rainbow Trout (Oncorhynchus mykiss, Walbaum 1792).” Journal of FisheriesSciences.com 2, no. 1 (2008): 41–50. https://doi.org/10.3153/jfscom.2008004.
- Hansen, James A., Paul G. Welsh, Josh Lipton, Dave Cacela, and Anne D. Dailey. “Relative Sensitivity of Bull Trout (Salvelinus Confluentus) and Rainbow Trout (Oncorhynchus mykiss) to Acute Exposures of Cadmium and Zinc.” Environmental Toxicology and Chemistry 21, no. 1 (January 1, 2002): 67–75. https://doi.org/10.1002/etc.5620210110.
- Haux, Carl, and Åke Larsson. “Long-Term Sublethal Physiological Effects on Rainbow Trout, Salmo Gairdneri, during Exposure to Cadmium and after Subsequent Recovery.” Aquatic Toxicology 5, no. 2 (May 1984): 129–42. https://doi.org/10.1016/0166-445x(84)90004-3.
- Heydarnejad, M. Saeed, Mozhdeh Khosravian-Hemamai, and Amin Nematollahi. “Effects of Cadmium at Sub-Lethal Concentration on Growth and Biochemical Parameters in Rainbow Trout (Oncorhynchus mykiss).” Irish Veterinary Journal 66, no. 1 (June 20, 2013): 11. https://doi.org/10.1186/2046-0481-66-11.
- Hollis, L., C. Hogstrand, and C. M. Wood. “Tissue-Specific Cadmium Accumulation, Metallothionein Induction, and Tissue Zinc and Copper Levels during Chronic Sublethal Cadmium Exposure in Juvenile Rainbow Trout.” Archives of Environmental Contamination and Toxicology 41, no. 4 (November 2001): 468–74. https://doi.org/10.1007/s002440010273.
- Hollis, Lydia, James C. McGeer, D. Gordon McDonald, and Chris M. Wood. “Effects of Long Term Sublethal Cd Exposure in Rainbow Trout during Soft Water Exposure: Implications for Biotic Ligand Modelling.” Aquatic Toxicology 51, no. 1 (November 2000): 93–105. https://doi.org/10.1016/s0166-445x(00)00099-0.
- Hwang, Un-Gi, Nao Kagawa, and Yasuo Mugiya. “Aluminium and Cadmium Inhibit Vitellogenin and Its MRNA Induction by Estradiol-17 β in the Primary Culture of Hepatocytes in the Rainbow Trout Oncorhynchus mykiss.” General and Comparative Endocrinology 119, no. 1 (July 1, 2000): 69–76. https://doi.org/10.1006/gcen.2000.7494.
- Karlsson-Norrgren, L., P. Runn, C. Haux, and L. Forlin. “Cadmium-Induced Changes in Gill Morphology of Zebrafish, Brachydanio Rerio (Hamilton-Buchanan), and Rainbow Trout, Salmo Gairdneri Richardson.” Journal of Fish Biology 27, no. 1 (July 1985): 81–95. https://doi.org/10.1111/j.1095-8649.1985.tb04011.x.
- Klinck, Joel S., and Chris M. Wood. “Gastro-Intestinal Transport of Calcium and Cadmium in Fresh Water and Seawater Acclimated Trout (Oncorhynchus mykiss).” Comparative Biochemistry and Physiology. Part C, Toxicology & Pharmacology/Comparative Biochemistry and Physiology. Toxicology & Pharmacology 157, no. 2 (March 1, 2013): 236–50. https://doi.org/10.1016/j.cbpc.2012.11.006.
- Majewski, H., and M. A. Giles. “Cardiovascular-Respiratory Responses of Rainbow Trout (Salmo Gairdneri) during Chronic Exposure to Sublethal Concentrations of Cadmium.” Water Research 15, no. 10 (1981): 1211–17. https://doi.org/10.1016/0043-1354(81)90096-8.
- Mansouri, Borhan, Seyed Ali Johari, Nammam Ali Azadi, and Mehrdad Sarkheil. “Effects of Waterborne ZnO Nanoparticles and Zn2+ Ions on the Gills of Rainbow Trout (Oncorhynchus mykiss): Bioaccumulation, Histopathological and Ultrastructural Changes.” Turkish Journal of Fisheries and Aquatic Sciences 18, no. 5 (2018): 739–46. https://doi.org/10.4194/1303-2712-v18_5_09.
- McGeer, James C., Cheryl Szebedinszky, D. Gordon McDonald, and Chris M. Wood. “Effects of Chronic Sublethal Exposure to Waterborne Cu, Cd or Zn in Rainbow Trout. 1: Iono-Regulatory Disturbance and Metabolic Costs.” Aquatic Toxicology 50, no. 3 (September 2000): 231–43. https://doi.org/10.1016/s0166-445x(99)00105-8.
- Nasri, Fereshteh, M. Saeed Heydarnejad, and Amin Nematollahi. “Toxicidad de Zinc Por Exposición Subletal En Trucha Arco Iris (Oncorhynchus mykiss).” CES Medicina Veterinaria Y Zootecnia 15, no. 1 (May 1, 2020): 9–21. https://doi.org/10.21615/cesmvz.15.1.1.
- 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. Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
- Nowak, Barbara, and Susan Duda. “Effects of Exposure to Sub-Lethal Levels of Copper on Growth and Health of Sea Farmed Rainbow Trout.” Tasmania, Australia: Supervising Scientist, 1996. https://hdl.handle.net/102.100.100/497699.
- Olsson, Per-Erik, Åke Larsson, and Carl Haux. “Influence of Seasonal Changes in Water Temperature on Cadmium Inducibility of Hepatic and Renal Metallothionein in Rainbow Trout.” Marine Environmental Research 42, no. 1-4 (June 1996): 41–44. https://doi.org/10.1016/0141-1136(95)00072-0.
- Olsson, Per-Erik, Åke Larsson, Amund Maage, Carl Haux, Keith Bonham, Muhammad Zafarullah, and Lashitew Gedamu. “Induction of Metallothionein Synthesis in Rainbow Trout, Salmo Gairdneri, during Long-Term Exposure to Waterborne Cadmium.” Fish Physiology and Biochemistry 6, no. 4 (July 1, 1989): 221–29. https://doi.org/10.1007/bf01875025.
- Reichert, William L., David A. Federighi, and Donald C. Malins. “Uptake and Metabolism of Lead and Cadmium in Coho Salmon (Oncorhynchus Kisutch).” Comparative Biochemistry and Physiology Part C: Comparative Pharmacology 63, no. 2 (January 1979): 229–34. https://doi.org/10.1016/0306-4492(79)90066-2.
- Scott, Graham R., Katherine A. Sloman, Claude Rouleau, and Chris M. Wood. “Cadmium Disrupts Behavioural and Physiological Responses to Alarm Substance in Juvenile Rainbow Trout (Oncorhynchus mykiss).” Journal of Experimental Biology 206, no. 11 (June 1, 2003): 1779–90. https://doi.org/10.1242/jeb.00353.
- Shaw, Benjamin J., Genan Al-Bairuty, and Richard D. Handy. “Effects of Waterborne Copper Nanoparticles and Copper Sulphate on Rainbow Trout, (Oncorhynchus mykiss): Physiology and Accumulation.” Aquatic Toxicology 116-117 (July 15, 2012): 90–101. https://doi.org/10.1016/j.aquatox.2012.02.032.
- Sloman, Katherine A., Graham R. Scott, Zhongyu Diao, Claude Rouleau, Chris M. Wood, and D. Gord McDonald. “Cadmium Affects the Social Behaviour of Rainbow Trout, Oncorhynchus mykiss.” Aquatic Toxicology (Amsterdam, Netherlands) 65, no. 2 (October 29, 2003): 171–85. https://doi.org/10.1016/s0166-445x(03)00122-x.
- Sommers, Frank, Emma Mudrock, Jana Labenia, and David Baldwin. “Effects of Salinity on Olfactory Toxicity and Behavioral Responses of Juvenile Salmonids from Copper.” Aquatic Toxicology 175 (April 4, 2016): 260–68. https://doi.org/10.1016/j.aquatox.2016.04.001.
- Taysi, Mehmet Reşit, Muammer Kirici, Mahinur Kirici, Bünyamin Söğüt, Mehmet Akif Bozdayi, and Seyithan Taysi. “Gökkuşağı Alabalıklarında (Oncorhynchus mykiss) Kadmiyum Toksisitesi: Kalp ve Kas Üzerine Bir Araştırma.” Türk Tarım ve Doğa Bilimleri Dergisi 7, no. 4 (October 20, 2020): 983–87. https://doi.org/10.30910/turkjans.802746.
- Thuvander, A. “Cadmium Exposure of Rainbow Trout, Salmo Gairdneri Richardson: Effects on Immune Functions.” Journal of Fish Biology 35, no. 4 (October 1989): 521–29. https://doi.org/10.1111/j.1095-8649.1989.tb03003.x.
- Varanasi, Usha, and Dennis Markey. “Uptake and Release of Lead and Cadmium in Skin and Mucus of Coho Salmon (Oncorhynchus Kisutch).” Comparative Biochemistry and Physiology Part c Comparative Pharmacology 60, no. 2 (January 1, 1978): 187–91. https://doi.org/10.1016/0306-4492(78)90092-8.