Where do fish get their vitamin D from? This question resurfaced after several research consistently revealed lower vitamin D content in farmed salmon compared to its wild-caught counterparts. For years, it was widely believed that salmon exclusively obtain their vitamin D from the food web. This assumption stems from UVB rays being absorbed at a depth of 20 meters in the sea, rendering sunlight an unreliable source for vitamin D synthesis, considering salmon's tendency to swim at deeper depths. Therefore, in our endeavor to elucidate the sources of vitamin D in salmon, we aimed to investigate the individual effects of both feed and UVB exposure on salmon vitamin D levels. First, to unravel the impact of feed on salmon's vitamin D levels, we conducted a study within the controlled land-based production conditions offered by Atlantic Sapphire®, which offer ideal environments for precisely examining how diet influences the vitamin D content of salmon. The adult salmon at Atlantic Sapphire are fed with a commercial salmon feed provided by bioMar A/S, a standard fare for farmed salmon. This feed's vitamin D content has been analyzed. Second, we assessed the vitamin D content in 10 distinct fish species, including Atlantic herring, Atlantic salmon, Atlantic Cod, Small-spotted catshark, Whiting, Tub gurnard, Common dab, Haddock, European plaice, and Saithe. Their skin was exposed to UVB radiation using a prototype LED UVB lamp equipped with 60 diodes emitting at 295 nm, with an intensity of 2mW. The exposure conditions were investigated in the wavelength range 280–315 nm for achieving optimal vitamin D bio-fortification. Despite the uniformity in feed across all farmed salmon, indoor-raised salmon display notably lower levels of vitamin D (0.48±0.21 μg/100 g) compared to sea cage-raised (2.3 to 10 μg/100g) and wild salmon (8 to 55 μg/100g). This discrepancy hints at a potential link between limited sun exposure in onshore tanks and decreased vitamin D content, underscoring the need for additional research. In experiments with UVB light, where fish skin was exposed to a dose of 3,000 J/m2, only tub gurnard didn’t show an increase in the concentration of vitamin D in the skin. Salmon stood out significantly in the UVB test, where an increase in the concentration of vitamin D from 120 ng/g to 3.1103 ng/g was seen. Based on the results, it is concluded that Salmon can form vitamin D when exposed to UVB light.