Continuous light induces bone resorption and affects vertebral morphology in Atlantic salmon (Salmo salar L.) fed a phosphorous deficient diet

This study investigated the combined effects of photoperiod and dietary P level on bone osteoclast and osteoblast activity, and morphology, as well as plasma vitamin D status in Atlantic salmon post-smolts. The fish were reared under continuous light (LL) or 12 h light/ dark (LD) per day, and fed diets with insufficient phosphorous (P) (4 g kg(-1) available dietary P, LP) or sufficient P (8 g kg(-1) available dietary P, HP) for 79 days in seawater. LL significantly increased plasma 25(OH)D-3 level and bone tartrate resistant acid phosphatase (TRACP) activity, and decreased bone collagen XI alpha 1 chain (col11a1) mRNA transcription. The LP diet significantly increased plasma 1,25(OH)(2)D-3 level and bone alkaline phosphatase activity, but decreased bone TRACP activity and matrix metalloproteinase 13 (mmp13) mRNA transcription, and vertebral mineral content, stiffness and length/dorso-ventral diameter (l/d) ratio. A significant interaction between light and P was only observed on the l/d ratio, and the LP-LL group was the only group that developed vertebrae with a compressed morphology (significantly lowest l/d ratio). In practical terms, these results show that Atlantic salmon post-smolts under continuous light may need P supplemented diets to support normal bone development.