Field Evidence of Endogenous Vitamin D Synthesis in Atlantic Salmon Induced by Natural Sunlight

Fish are believed not to synthesize vitamin D through UV exposure but to meet their requirements from dietary sources. The high vitamin D levels found in many fish species are thought to originate from UV-induced synthesis in plankton, with vitamin D subsequently accumulating through the aquatic food chain. Atlantic salmon is a rich dietary source of vitamin D, but limited data exist on circulating vitamin D levels. To address this, plasma levels of the three main vitamin D metabolites were measured in Atlantic salmon from Norwegian fish farms using mass spectrometry. Here, we show that salmon reared in open sea pens have significantly higher vitamin D levels than those raised indoors. Monitoring an outdoor farm over 18 months revealed a distinct seasonal pattern between vitamin D levels and day length. In a follow-up experiment, 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 feed, yet the outdoor group exhibited a fivefold increase in whole-body vitamin D content. These surprising findings provide field evidence of endogenous vitamin D synthesis in fish induced by natural sunlight. This discovery could have important implications for aquaculture, emphasizing the risk of suboptimal vitamin D levels in farmed fish when sunlight exposure is limited.