Nephrocalsinosis and mineral deposition in Atlantic salmon (Salmo salar L.) during smoltification under different commercial rearing conditions

Nephrocalcinosis (NC), characterized by excessive mineral deposition in the kidneys, represents an increasing challenge in commercial smolt production of Atlantic salmon. The aim of this study was to survey NC and mineral status in whole fish and kidneys in Atlantic salmon smolts from commercial smolt facilities, aiming to identify potential triggers of NC development. Samples were collected from eight fish groups across six commercial smolt facilities, sharing the same genetic origin and feed supplier. Three flow-through systems and three recirculating aquaculture systems (RAS) were included, and the facilities applied different smoltification protocols. NC was found to be a commonly occurring production disorder during smoltification, affecting 41% of the sampled fish, with a higher prevalence (52.5%) and more severe cases in fish from RAS facilities. However, variation between facilities was large, clearly showing that RAS does not necessarily equal high NC prevalence. More frequent and severe NC was observed in fish exposed to continuous light compared to winter signal within the same facility, but the latter group also had lower CO 2 levels presumably due to reduced feeding. No differences were seen between fish given salt feed and standard feed in the same facility. Among other parameters, increased salinity early in smoltification appeared to contribute to NC. There was a strong association between the analyzed content of calcium (Ca), phosphorus (P), magnesium (Mg), and manganese (Mn) in the kidneys and increasing severity of NC. Most microminerals accumulated in the water in RAS facilities, reflected in higher levels in whole fish and kidneys, with selenium as an exception. Neither whole body minerals nor any plasma parameters were related to NC. Our findings highlight the potential for optimizing production technologies and smoltification protocols to reduce NC. • Nephrocalcinosis was common across production systems and smoltification regimes. • Within the same facility, fish exposed to LL had higher prevalence and severity of NC than fish exposed to a winter signal. • No effect was seen of salt feed compared to normal feed. • Results underline the complex and multifactorial nature of the disorder.