SEASONAL, GEOGRAHIC AND SIZE-DEPENDENT DRIVERS OF FILLET VITAMIN D3 IN NORWEGIAN ATLANTIC SALMON Salmo salar

Introduction

Vitamin D is crucial for fish health(Lock et al., 2009) and the nutritional value of salmon fillets. While dietary supply has long been considered the main source for teleosts (Lock et al., 2009; Soto-Dávila et al., 2020;, recent experimental and field evidence indicates cutaneous UV‑B driven synthesis contributes substantially in Atlantic salmon (Fossen et al., 2025; Husebø et al., 2025). This study examined whether season, latitude and fish size explain variation in fillet vitamin D3 consentration across Norwegian commercial production of Atlantic salmon from 2005–2025.

Material and methods

The study comprises 1,473 samples of farmed Atlantic salmon (Salmo salar) collected year‑round along the Norwegian coast between 2005 and 2025 by the Norwegian Food Safety Authority and the Institute of Marine Research (IMR). From each farm, five to ten market‑size fish were taken, transported frozen (−20 °C) to IMR and processed according to standardized protocols. From 2011 onwards, muscle was sampled using the Norwegian Quality Cut (NQC; NS 9401), while samples from 2005, 2006 and 2010 used a whole‑side fillet protocol; samples through 2022 were generally individual fish, with some pooling by area and from 2023–2025 three NQC cuts were pooled prior to analysis. Individual samples were homogenized, freeze‑dried, re‑homogenized to a fine powder and used for most analyses, whereas wet homogenates (prior to freeze‑drying) were reserved for lipid and vitamin determinations. Wet material was stored at −80 °C until analysis, and freeze‑dried powders were kept dry at room temperature pending further assays(Reksten et al., 2022).

Results

Three consistent patterns emerged from the data. First, a pronounced seasonality: fillet D3 concentrations show an annual cycle, rising from spring and peaking in late summer/early autumn and falling to their lowest levels during mid‑winter (p < 0.01). Second, a negative weight effect: D3 declines with increasing fish weight, smaller fish contained higher µg/100 g D3 than larger fish (p < 0.01). Third, a modest but statistically significant positive latitude association: fillet D3 increases slightly with higher latitude, maybe an effect that likely reflects a combination of slower growth and longer residence times in northern, colder waters and maybe site‑specific optical conditions that favour deeper UV‑B penetration (p < 0.01).

Smaller fish have a larger skin surface area relative to mass and therefore greater potential for UV‑B driven synthesis. Because vitamin D metabolites in salmon turn over rapidly, these patterns imply practical management options: targeted winter supplementation, size‑class specific feeding regimes with higher vitamin D input for larger fish (and dosing in the weeks before harvest), and production‑system decisions (open surface cages receive substantial seasonal photogenic input, while submerged, covered or land‑based systems will require greater or continuous dietary supply).

Figure 1: Vitamine D3 content increases for each latitude one moves north (p < 0.01).

References

Fossen,I., Backström, T., Holte,T. and Klakegg, Ø. "UV-B Light Stimulates the Production of Vitamin D3 in Atlantic Salmon," Aquaculture 611 (2025): 743058, https://doi.org/10.1016/j.aquaculture.2025.743058.

Husebø CA, Berge K, Keitel-Gröner F, Hoel E, Rennemo J, Sandstad M, Bjerkestrand KM, Lorentzen L, Welde E, Morken T, Stubhaug I, Mullins J, Bjørgen H, Bossum T, Brede C, Knudsen DL. Field Evidence of Endogenous Vitamin D Synthesis in Atlantic Salmon Induced by Natural Sunlight. Aquac Nutr. 2025 Dec 20;2025:3823472. doi: 10.1155/anu/3823472. PMID: 41424696; PMCID: PMC12717467.

Lock, E.J., Waagbø, R., Wendelaar Bonga, S., Flik, G., 2009. The significance of vitamin D for fish: a review. Aquaculture Nutrition 16(1), 100–116. https://doi.org/https://doi.org/10.1111/j.1365-2095.2009.00722.x

Moxness Reksten A, Ho QT, Nøstbakken OJ, Wik Markhus M, Kjellevold M, Bøkevoll A, Hannisdal R, Frøyland L, Madsen L, Dahl L. Temporal variations in the nutrient content of Norwegian farmed Atlantic salmon (Salmo salar), 2005-2020. Food Chem. 2022 Mar 30;373(Pt B):131445. doi: 10.1016/j.foodchem.2021.131445. Epub 2021 Oct 21. PMID: 34731805.

Soto-Dávila, M., Valderrama, K., Inkpen, S.M., Hall, J.R., Rise, M.L., Santander, J., 2020. Effects of Vitamin D2 (Ergocalciferol) and D3 (Cholecalciferol) on Atlantic Salmon (Salmo salar) Primary Macrophage Immune Response to Aeromonas salmonicida subsp. salmonicida Infection. Frontiers in Immunology 10. https://doi.org/10.3389/fimmu.2019.0301