Nephrocalcinosis: look beyond carbon dioxide

Nephrocalcinosis is the deposition of mineral material in renal tubules and collecting ducts. In intensively reared salmonids, it may remain subclinical, accompany poor performance or only become visible at post-mortem examination. A 2026 survey across six commercial smolt facilities delivers a useful operational message: the disorder was common, but no single parameter explained it. Investigation therefore needs to connect water chemistry, smolt production practices, nutrition and renal pathology.

Common does not mean uniform

The commercial survey compared eight Atlantic salmon groups from the same genetic origin and feed supplier. Three were held in flow-through facilities and three in recirculating aquaculture systems. Nephrocalcinosis affected 41% of sampled fish. Prevalence reached 52.5% in RAS groups, where lesions were also more severe, but variation among facilities was substantial.

This does not show that recirculation automatically causes nephrocalcinosis. RAS is a broad engineering category covering very different loading rates, hydraulics, alkalinity, degassing capacity and mineral accumulation. The result points to combinations of exposure rather than to a system label.

Within one facility, fish exposed to continuous light had more frequent and severe lesions than fish receiving a winter photoperiod signal. Yet the winter-signal group also ate less and experienced lower carbon dioxide concentrations. Continuous light may have a direct effect, but it may also alter activity, feed input, metabolic carbon dioxide production and mineral balance. The study cannot separate those pathways.

Carbon dioxide is evidence, not a verdict

Dissolved carbon dioxide remains a major risk factor in intensive systems. Chronic hypercapnia alters acid–base regulation and can contribute to mineral precipitation in the kidney. The long-term trial by Good and colleagues illustrates why one universal threshold is unsafe. Atlantic salmon post-smolts were exposed for 384 days to approximately 20 mg/L or 8 mg/L CO₂ in replicated freshwater RAS. Overall growth and feed conversion did not differ under the tested conditions, where dissolved oxygen was kept at saturation and alkalinity averaged about 237 mg/L as CaCO₃.

The trial does not certify 20 mg/L as harmless in every farm. It shows that response depends on life stage, temperature, pH, alkalinity, oxygenation, exposure duration and acclimation. A carbon dioxide result must be interpreted alongside system operation and fish condition.

A midday spot sample may miss post-feeding peaks, flow changes or loss of degassing efficiency. A useful monitoring plan characterises daily cycles, hydraulic zones, feeding load and technical events. It also checks the analytical method. Indirect CO₂ calculation from pH and alkalinity can become unreliable when water chemistry departs from the model assumptions.

Link water data to lesions and minerals

In the 2026 survey, lesion severity was strongly associated with kidney concentrations of calcium, phosphorus, magnesium and manganese. Several trace minerals accumulated in RAS water, and this pattern was reflected in whole fish and kidneys. In contrast, whole-body mineral concentrations and plasma variables did not predict nephrocalcinosis.

Normal blood results therefore cannot rule out the condition. Gross examination may reveal pale or irregular kidneys and deposits in the urinary tract, but histology is needed to locate and grade lesions. It also helps distinguish mineral deposition from other causes of poor condition or sporadic mortality.

Water samples should be collected in the same period as fish examinations. Carbon dioxide, pH, alkalinity, hardness, calcium, magnesium, salinity, temperature and oxygen provide complementary information. Feed history, mineral supplements, salinity changes and lighting programmes complete the exposure profile. Comparing affected and less-affected tanks within one facility is often more informative than comparing one tank against a generic limit.

Investigate before changing the system

The first step is to confirm the lesion in a representative sample spanning several tanks and performance levels. The second is to map plausible exposures: CO₂ concentration and dynamics, degasser performance, transient overloading, mixing, photoperiod transitions, salinity and diet.

Changing several variables at once prevents learning and may destabilise the system. Corrective action should be progressive, documented and paired with predefined outcomes: repeated water measurements, feed intake, growth, mortality, post-mortem findings and scheduled histological grading. Fish already carrying severe deposits are poor short-term indicators of improvement; prevalence and grade in subsequent groups provide a more useful trend.

The commercial comparison found no effect of salt feed versus standard feed in the available groups. This neither excludes every nutritional contribution nor supports empirical mineral restriction or supplementation. It underlines the need to test decisions in the farm context and to avoid turning an association into a causal claim.

A practical monitoring sequence

Start by standardising where and when CO₂, oxygen and pH are measured. Include the tank inlet and outlet, periods before and after feeding, and any location where flow is weak. Record calibration, units and the method used so that apparent trends are not instrument artefacts.

Next, align veterinary sampling with production records. Select clinically normal and poorly performing fish from matched tanks. Divide tissues appropriately between histology and any additional diagnostic work. Record renal lesion grades against tank, cohort, light programme, feed and water profile.

Finally, choose one or two plausible interventions and define the expected timescale. Improving gas transfer, adjusting loading or reviewing a transition programme should not be judged after a single day. Conversely, an acute rise in mortality or water-quality failure requires immediate stabilisation before the longer investigation continues.

Conclusion

Nephrocalcinosis is a multifactorial production signal. Carbon dioxide deserves close monitoring, but its meaning depends on pH, alkalinity, oxygen, life stage and husbandry. Recent commercial data also direct attention to photoperiod, salinity and mineral accumulation. A sound investigation combines repeated water measurements, post-mortem examination, histology and within-farm comparisons before major changes are made.

Vetofish can help farms design sampling, interpret renal lesions with water-quality data, rank competing hypotheses and follow a corrective trial while preserving veterinary and production traceability.