Dietary Vitamin D in Salmonids: Effects on Growth, Welfare and Product Quality 

Vitamin D is essential for fish and a major driver of the nutritional value of salmon and trout for human consumers. As salmonid diets have shifted from marine to plant-based formulations, dietary vitamin D levels have declined, and fish reared in land-based or shielded sea systems with limited sunlight access show further reductions. This study, led by Anne Kristine Hansen, examines how dietary vitamin D level and form affect growth, welfare, and product quality across different production systems in rainbow trout and Atlantic salmon. 

Two trials were conducted. Rainbow trout (initial weight 206 g) were reared for nine weeks in a recirculating aquaculture system at the BioMar ATC in Hirtshals, Denmark, with diets containing increasing levels of synthetic vitamin D3 from 8,000 to 85,000 IU/kg. Atlantic salmon (initial weight 1,393 g) were grown for nine months in semi-commercial sea cages at LetSea, Norway, with five diets formulated on a basal D3 level of 137.5 µg/kg: a low D3 diet (+0 µg/kg), a high D3 diet (+1,000 µg/kg), two diets with active metabolites (+200 and +600 µg/kg), and one diet with intermediate metabolite (+1.25 µg/kg). The seawater trial included three handling events to mimic mechanical sea lice operations. 

Results 

Growth Performance 

In rainbow trout, dietary vitamin D3 showed a positive dose-response relationship with fish growth. In Atlantic salmon, supplementation with higher levels of the active metabolite was associated with a negative impact on growth, while D3 supplementation alone did not affect growth performance. 

Fillet Vitamin D3 Deposition 

Fillet D3 content increased with dietary D3 levels in both species. In trout, the effect was strong and dose-dependent across the full range tested. In salmon, fillet D3 increased with higher D3 inclusion, combining dietary D3 with active and intermediete metabolite for A. salmon favoured NQC D3 accumulation at low D3, especially the active metabolite at high levels and from December to March.  

Pigmentation 

A positive correlation was observed between dietary D3 levels and fillet redness in rainbow trout. In Atlantic salmon, no effect on pigmentation was observed with increasing D3 levels, but a negative impact was noted in groups receiving higher active metabolite supplementation. 

Welfare and Harvest Quality 

A range of welfare and harvest quality parameters were monitored across both trials. No clear negative effects were observed at the dietary levels tested, supporting the safety of higher D3 inclusion within current EU regulatory limits. 

The findings show that fillet vitamin D3 levels can be increased through feed formulation in both rainbow trout and Atlantic salmon, supporting strategies that improve the nutritional value of farmed fish for consumers. The performance and pigmentation response, however, depends on species, fish size, and production system. Supplementation with active metabolites in particular requires careful management in seawater cages, where higher levels affected both growth and pigmentation. As land-based and shielded production systems expand, vitamin D becomes a more relevant nutrient to manage actively in aquafeeds, both for fish welfare and for the nutritional quality of the final product. 

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