Introduction
Nephrocalcinosis continues to be one of the most important production disorders in the production of Atlantic salmon and rainbow trout (Moldal et al 2025). Nephrocalcinosis is multifactorial, and amongst other causes associated with factors such as high fish densities and suboptimal water quality. A classical long-term experiment by Smart et al (1979) showed strong correlation between high CO2 in water and nephrocalcinosis in rainbow trout. However, these results have not been confirmed in later studies with rainbow trout (60-900 g) in RAS (Good et al., 2010). Mota et al. (2019 exposed A. salmon to six dissolved CO2 concentration ranging from 5-40 mg CO2 l-1over a 12-week period. Except for skin they found no major effect of health and welfare, including nephrocalcinosis. The causes of nephrocalcinosis in A. salmon seem to be more complex than in rainbow trout. The aim of this study was to confirm that CO2 as the only environmental variable leads to the development of nephrocalcinosis in rainbow trout.
Material and Methods
All female rainbow trout juveniles were exposed to 5 different levels of added CO2 (nominal 0, 10, 15, 20 and 25 mg CO2 l-1) in the inlet water for 7 weeks. Actual CO2 concentrations were 7.3, 11.9, 16.7, 22.1 and 26.3 mg CO2 l-1. 200 juvenile rainbow trout (15 g) were distributed to each of 15 tanks; volume 500 l. Temperature 12°C. Two holding tanks were used to obtain the different CO2 concentrations. Only O2 was added to one tank, in the other one O2, CO2 and sodium bicarbonate as buffer. An Oxyguard Pacific unit was used to monitor and regulate CO2 and pH in this tank. The setpoints in the holding tank for CO2 dosing was 25 mg CO2 l-1 and pH 6.9, and dosing of CO2 and buffer was regulated automatically. In the tanks, different CO2 levels were adjusted by proportional mixing of water from both holding tanks (0 and 25 mg CO2 l-1). Buffer addition allowed us to keep tank pH equal between treatments, independent of CO2 treatments. CO2-addition was stopped after 8 weeks. Sampling of fish for weight, kidney and skin histology and high-resolution radiography was done before start of the experiment, week 5 and week 8. Radiography and histology were used for documentation of nephrocalcinosis. Radiograph images of kidneys were scored on a range of 0-4.
Results
There was a strong correlation between added CO2 and prevalence of nephrocalcinosis; zero in the control group (no CO2 added) increasing to 41,7% in the 25% mg CO2 l-1group. The severity of nephrocalcinosis increased with increasing addition of CO2. The growth of the fish was also correlated with CO2 levels. At end of the experiment the mean weight of the control group was 89.2 g and 72.6 for highest CO2 group. There was little visible effect on skin quality at 50 g size, but clear changes were observed at the end of the experiment, suggesting long-term effects. The findings on skin histology indicate increased stress, weaker skin structure and thinner epidermis in the group that received 25 mg CO2 l-1compared to the control group without the addition of CO2. Results from the kidney histology evaluation will be presented later.
Conclusions
The experimental setup offered very good control of the water quality parameters O2, CO2 and pH. We found a very clear correlation between high CO2 and prevalence of nephrocalcinosis in rainbow trout. High resolution radiography seems to be an excellent tool to diagnose active nephrocalcinosis. Both growth and skin were negatively affected by increasing CO2 levels. The skin is an important barrier against infectious agents in fish, and suboptimal skin quality will increase the risk of wounds and infections. The results from this experiment suggest that the relationship between high CO2 in the water and the development of nephrocalcinosis is much stronger in rainbow trout than in salmon, which appears to have a more complex causal mechanism.
Acknowledgment
This work is funded by FHF - Norwegian Seafood Research Fund ("Tidlig utvikling hos regnbue��rret" project number: 901899).
References
Smart, G.R., Knox, D., Harrison, J.G., Ralph, J.A., Richards, R.H., Cowey, C.B (1979). Nephrocalcinosis in rainbow trout Salmo girdneri Richardson; the effect of exposure to elevated CO2 concentrations. J. Fish. Diseases, 279-289.
Good, C., Davidson, J., Welsh, C., Snekvik, K., Summerfelt, S., 2010. The effects of carbon dioxide on performance and histopathology of rainbow trout Oncorhynchus mykiss in water recirculation aquaculture system Aquac. Eng. 42, 51-56.
Moldal, T., Wiik-Nielsen, J., Oliveira, W.H.S., Svendsen, J.C., Somerset, I. Fiskehelserapporten 2025, Veterin��rinstituttets rapportserie nr. 5a/2026. In Norwegian.
Mota, V.C., Nilsen, T.O., Gerwins, J., Gallo, M., Ytteborg, E., B��verfjord, G., Kolarevic, J., Summerfelt, S.T., Terjesen, B.F., 2019. Aquaculture 498, 578-586.
Smart, G.R., Knox, D., Harrison, J.G., Ralph, J.A., Richards, R.H., Cowey, C.B (1979). Nephrocalcinosis in rainbow trout Salmo girdneri Richardson; the effect of exposure to elevated CO2 concentrations. J. Fish. Diseases, 279-289.