Nofima research confirms link between carbon dioxide levels and kidney stones in rainbow trout.
Scientists at Nofima have documented a clear connection between elevated carbon dioxide (CO₂) levels in freshwater tanks and the development of nephrocalcinosis—commonly known as kidney stones—in juvenile rainbow trout.
The findings come from controlled experiments at Nofima’s research station in Sunndalsøra, funded by the Norwegian Seafood Research Fund (FHF). Fish exposed to 13 mg CO₂/litre showed kidney stones in 5% of cases, while 26 mg CO₂/litre led to more than 40% incidence. The study confirms longstanding industry assumptions, but now with quantifiable data.
“Our results show that CO₂ values of 13 mg/l and above increase the risk of nephrocalcinosis in rainbow trout,” said Ingrid Lein, project scientist at Nofima. “The extent and severity of kidney stones increase steadily as CO₂ concentration rises.”
The trials are part of the FHF-funded “Early Development in Rainbow Trout” project, which runs through autumn 2026 and aims to provide best practice recommendations for juvenile trout production. The fish are now being monitored at the Institute of Marine Research in Austevoll through sea-phase grow-out.
Veterinarian Kirsti Hjelde, also of Nofima, highlighted that affected fish may appear healthy externally but suffer severe internal damage. “It is uncertain how painful nephrocalcinosis is, but the fish may look perfectly healthy on the outside while having destroyed kidneys,” Hjelde said. New X-ray methods developed by Nofima now allow non-lethal diagnosis of kidney stones.
The findings are expected to inform future production standards for rainbow trout, and could have implications for salmon farming practices as well.