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Is osmoregulation the cause of nephrocalcinosis in juvenile salmon?

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By Milthon Lujan

Nephrocalcinosis is a challenge in the intensive production of Atlantic salmon smolts, and the Norwegian Veterinary Institute reports that nephrocalcinosis is a growing welfare problem in the Norwegian salmon industry.

To contribute to the common effort of understanding the etiology of nephrocalcinosis in Atlantic salmon, a team of researchers from Aqua Kompetanse AS, Norwegian University of Science and Technology, and Pharmaq Analytiq AS conducted a nephrocalcinosis survey in 11 hatcheries and analyzed a wide range of water quality parameters and food composition.

Based on the survey results, the scientists monitored the development of nephrocalcinosis in one of the hatcheries that had a high prevalence.

Nephrocalcinosis

Nephrocalcinosis is characterized by calcium deposits in the renal tubules and collecting ducts. In Atlantic salmon, the mineral deposits mainly consist of amorphous carbonated apatite (amCAP).

Currently, there is no consensus on the etiology of nephrocalcinosis. Various studies conducted on farmed Atlantic salmon have reported that both acute and long-term exposure to high levels of CO2 in the water can cause nephrocalcinosis. However, there are also studies indicating the opposite.

Prevalence of nephrocalcinosis

The scientists conducted a large-scale survey in which they collected data on as many environmental parameters as possible, including water chemistry and food composition, to correlate them with the prevalence and severity of nephrocalcinosis.

“Nephrocalcinosis was observed in all fish groups in the survey, but the prevalence varied among the hatcheries. More than half of the individuals with nephrocalcinosis showed changes in kidney tissues,” they report.

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Among the 37 parameters, the researchers describe, only two significantly influence the prevalence of nephrocalcinosis.

According to the regression model, the supplementation of seawater in the production water and the concentration of sulfates were the two investigated parameters that explain the prevalence of most cases of nephrocalcinosis.

The scientists report that the concentrations of brackish water (0.5 – 15 ppt) in smolt production increase the risk of developing nephrocalcinosis. “We observed that the production water in hatcheries with a prevalence of nephrocalcinosis of over 25% had salinity ranging from 0.5 to 15 ppt,” they emphasized

Characteristics of nephrocalcinosis

“In our long-term survey, fish with nephrocalcinosis primarily showed minor changes in the kidney (mild nephrocalcinosis), with little or no damage to renal tissue. Plasma calcium increased prior to the increase in nephrocalcinosis prevalence and returned to homeostasis,” they reported.

They emphasize that if the fish had not been transferred to the sea, the progression of the disease could have resulted in renal damage and, in turn, an increase in plasma calcium due to the lack of divalent ion excretion.

Conclusion

“Overall, the results of this study point to a causal relationship between osmotic stress induced by changes in salinity and/or a challenging level of salinity combined with the fish’s developmental stage and prior history, and the development of nephrocalcinosis,” the scientists report.

However, the researchers highlight that fluctuating salinity is not the only factor that can cause osmotic stress in salmon. “In our survey, we observed a hatchery with a nephrocalcinosis prevalence of over 25% that does not add seawater to its operational water.”

The scientists recommend that future studies on the etiology of nephrocalcinosis should include the experimental induction of osmoregulatory stress through the manipulation of water quality in Atlantic salmon juveniles.

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The study was funded by the Norwegian Research Council and the Norwegian Seafood Research Fund (FHF) as part of the NEFROSMOLT project.

Contact
C. Klykken
Aqua Kompetanse AS,
Flatanger, Norway.
Email: christine@aqua-kompetanse.no

Reference (open access)
Klykken, C., Khan, E., Karlsen, C., Reed, A. K., Attramadal, K. J. K., Olsen, R. E., & Boissonnot, L. (2023). Nephrocalcinosis in juvenile farmed Atlantic Salmon (Salmo salar) may be linked to osmoregulatory stress. Journal of Fish Diseases, 00, 1– 14. https://doi.org/10.1111/jfd.13815

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