Introduction
Vitamin D3 is crucial for mineral homeostasis, skeletal health, and immune function in vertebrates. Historically, fish were thought to rely solely on dietary sources for vitamin D rather than synthesizing it via UV exposure. However, there is limited information about typical plasma vitamin D levels in farmed salmon under various production and seasonal conditions. The transition from marine to plant-based raw materials raises further concerns about vitamin D intake in farmed fish. To address these gaps and document dietary supplementation in commercial settings, several studies have been conducted and will be presented.
Methods
Trial 1: Blood sampled from two commercially farmed fish groups in open pens in seawater from June 2023 to Dec 2024 in Rogaland, Southern Norway*
Trial 2: Whole-body analyses of a fish group of juvenile salmon kept in freshwater indoors and outdoors for 52 days*
Trial 3: Muscle (NQC) sampled from fish fed a standard diet with 4-5000 IU/kg vitamin D3 or an experimental diet with 35-45000 IU/kg vitamin D3. Fish (1-4 kg) were grown at a commercial seawater farm in Rogaland, Southern Norway**
Results and Conclusion
The first trial combined observational data from commercial farming sites with a controlled dietary intervention to characterize the concentrations and variability of cholecalciferol (vitamin D3), 25hydroxycholecalciferol [25(OH)D3], and 1,25dihydroxycholecalciferol [1,25(OH)2D3] in plasma of Atlantic salmon (Salmo salar). Substantial variation in vitamin D metabolite levels was observed both within populations and between fish groups. Monitoring an outdoor farm over 18 months revealed a distinct seasonal pattern between vitamin D levels and day length, showing significantly higher levels during the summer months compared to winter months.
In a follow-up trial (Trial 2), indoor-raised fish were divided into two groups, one remained indoors, while the other was transferred outdoors for 52 days. Both groups received the same commercial diet, yet the outdoor group exhibited a fivefold increase in whole-body vitamin D content.
In a third trial, metabolic responses to increased dietary vitamin D were assessed and salmon were fed either a standard diet (~4000 IU/kg vitamin D3) or a diet with a tenfold higher inclusion (~40 000 IU/kg vitamin D3) for 32 weeks under commercial conditions. Fish tolerated the high dietary vitamin D without adverse effects on growth or mortality. Elevated dietary inclusion increased concentrations of vitamin D3 in plasma, liver, and muscle, while hydroxylated metabolites showed more moderate responses and seasonal variation.
In conclusion, our findings demonstrate that vitamin D status is strongly influenced by season and provide evidence for endogenous vitamin D synthesis in Atlantic salmon induced by natural sunlight. Furthermore, winter dietary fortification can achieve tissue vitamin D concentrations comparable to those observed during summer. Nutritional quality of salmon fillet can be improved as elevated dietary vitamin D enhanced tissue deposition. Dietary fortification of vitamin D is particularly important for fish raised indoors and during seasons with limited sunlight exposure outdoors.
Acknowledgment
A substantial part of the samplings was carried out under R&D licenses RS0001 and RS0002, granted by the Norwegian Directorate of Fisheries to Skretting AS. Special thanks to FishLab AS, Stavanger, Norway for developing and performing analysis of vitamin D metabolites (LC-MS). This research was partly funded by Norwegian Research Council project # 328674.
References
* Husebø et al. (2025). Field Evidence of Endogenous Vitamin D Synthesis in Atlantic Salmon Induced by Natural Sunlight. Aquaculture Nutrition, 2025(1), 3823472. https://doi.org/10.1155/anu/3823472
** Keitel-Gröner et al. (2026). Elevated dietary vitamin D supplementation during winter can mimic vitamin D levels induced by summer daylight exposure in Atlantic salmon. Manuscript under review in Aquaculture Nutrition.