A four-week trial was conducted to compare the effects of a conventional flow-through system diet (FTS) and an experimental RAS diet (ERAS) on fish performance, water quality and general system implication in a replicated recirculation aquaculture system (RAS). Six identical RAS, each with a total system water volume of 1500 L and cylindrical rearing tanks of 1000 L were stocked with Atlantic salmon (Salmo salar) post-smolts with an average weight of 199.7 ± 28 g, to an average stocking density of 30 kg/m³ and reaching approximately 49 kg/m³ at the trial end. The ERAS diet were composed to inhabit typical RAS feed characteristics compared with the FTS diet, such as a higher fecal stability and reduced protein levels (−12%), but at the same time increased fat content (+8%) to secure similar gross energy levels (22–23 MJ kg⁻¹) between the two diets. Water quality parameters were measured individually. The total accumulation of minerals and metals was analyzed in water from different parts of the system at the start and end of the trial period for both diets. No differences in growth, condition factor, feed conversion rate (FCR), or survival of fish fed the two dietary adaptations were observed. The system using the ERAS diet showed significantly higher pH (+1.2%) and alkalinity (+17%) and lower total ammonia nitrogen (TAN) (−18%) and NO₂⁻ (−46%) compared with the FTS diet. The count of drum filter activations was also significantly lower (−13%) with the ERAS diet. Compared with the FTS diet, the ERAS diet had a lower probability (−4%) of generating particles smaller than 50 μm, and that the RAS was also more effective in removing particles from the drum filter, prompting a lower daily activation of the filter of 22.1 ± 3.0 counts compared with 25.5 ± 3.5 for the FTS diet. Mineral analysis showed a significantly lower accumulation of total phosphorus (TP) (−90%) and dissolved phosphorus (DP) (−92%) in the RAS units using the ERAS diet compared with those using the FTS diet. Compared with a traditional flow-through diet, these results highlight the benefits of using an RAS-adapted diet that matches the energy requirement of flow-through diets regarding water quality, system performance, satisfactory growth, and condition.

Metabolic responses to different dietary levels of vitamin D ₃ and available phosphorus (avP) was investigated in cultured Atlantic salmon ( Salmo salar ). The study was carried out in triplicate with a 2‐level full factorial design with a center point and three additional points within the design space. Over a 17‐ week period at the initial weight of 453 ± 9 g, salmon reared indoor on land, were fed diets containing different levels of vitamin D ₃ and avP. In summary,growth performance, measured as specific growth rate (SGR), improved when either vitamin D ₃ or avP was increased independently, but declined when both were elevated excessively. Bone mineralization was maintained at intermediate vitamin D ₃ inclusion levels, though high vitamin D ₃ reduced bone ash. A diet containing 0.63 mg/kg vitamin D ₃ and 0.70% avP supported growth and mineral retention. A significant reduction in fecal soluble phosphorus when vitamin D ₃ was increased in the diet, suggests that management of vitamin D ₃ can contribute to improved waste control and reduced environmental load for phosphorus. Finally, the study highlighted that today’s commercial dietary inclusion of vitamin D ₃ can result in lower vitamin D ₃ accumulation in muscle of land‐based farmed Atlantic salmon compared to wild Atlantic salmon.

This review provides a comprehensive synthesis of phosphorus (P) dynamics relevant to salmonid aquaculture, with a particular focus on Atlantic salmon. It explores the global P cycle, the chemical forms of P in aquatic systems, and the implications of P sourcing, processing, and availability in aquafeeds. The review distinguishes between digestibility and availability of P, summarizes requirement studies, and evaluates the contribution of marine, animal, vegetable, and inorganic sources to dietary P. It also examines how aquaculture system design, particularly recirculating aquaculture systems (RASs), influences P accumulation and emissions. By integrating nutritional, physiological, and environmental perspectives, this review offers a uniquely holistic view of P efficiency and sustainability in salmonid aquaculture.

Robust, healthy, and fast-growing smolt is of high importance for fish farmers as a way of reducing the mortality and production time of Atlantic salmon (Salmo salar) in open sea pens. Lowering the water temperature in flow-through systems (FTSs) compared to recirculating aquaculture systems (RASs) has shown promising results for the growth and health of fish post-transfer to sea; unfortunately, limited information is available on the same parameters in replicated RAS setups. Hence, the current study aimed to compare the performance of Atlantic salmon reared at 12 and 14 °C over a 9-week RAS period and a subsequent 10-week post-transfer period, while also investigating the accumulation pattern of minerals in RASs. The results showed a 100% survival and comparable condition factors and cardiosomatic index (CSI) across both temperatures. During the RAS period, the thermal growth coefficient (TGC) was higher at 12 °C, but body weight gain and feed consumption were lower. No differences in mineral retention or fecal stability were observed. However, the production water accumulated more dissolved phosphorus (DP) and total iron (Fe) at 12 °C. Post-transfer, the TGC remained higher for fish with a rearing history of 12 °C. This fish also had higher body weight gain and feed consumption while exhibiting a lower hepatosomatic index (HSI) and viscerosomatic index (VSI), indicating improved muscle growth. Overall, lower water temperature reduced growth and increased DP and Fe in RASs. However, it improved post-transfer weight gain of muscle tissue, highlighting its benefits for seawater performance.

A total of 1,473 samples of farmed Atlantic salmon (Salmo salar) collected along the Norwegian coast between 2005 and 2025 were analysed to quantify patterns and drivers of cholecalciferol (D3) concentrations in fillets. The D3 concentration in Norwegian farmed salmon showed a pronounced seasonal cycle, with the lowest concentrations in late winter/early spring and the highest concentrations in autumn. Vitamin D3 concentrations declined with increasing fish weight, with model estimates indicating a reduction of 0.7 µg/100 g fillet per additional kg (with a steeper decline in fish < 2 kg in weight). Latitude was positively associated with fillet D3 concentrations (+0.18 µg/100 g fillet per degree). Incorporating the surface area (SA) to weight (W) proxy (SA = 13.9·W^0.61) improved the model-fit, consistent with the greater integumentary photoproduction in fish with higher surface-area-to-mass ratios. Interannual variations may also have been partly responsible for these differences due to the year-to-year differences in ambient UV exposure. Together, these results imply that sunlight-driven photoproduction is an important determinant of D3 concentrations in Norwegian farmed salmon. The relative importance of different factors, including feed versus sunlight, must be assessed through more controlled and targeted experiments.