The anadromous Atlantic salmon undergo preparatory physiological transformations before seawater entry, referred to as smoltification. Little is known about the photoperiod-influence and genome regulatory processes driving smoltification such as the large scale changes in lipid metabolism and energy homeostasis in the developing smolt liver. To shed light on this, we performed a smoltification trial using contrasting photoperiod regimes and generate ATAC-seq data from livers throughout smoltification and after seawater transfer to assess the differences in chromatin accessibility. In this experiment Atlantic salmon were reared for a total of 46 weeks from the parr stage, through smoltification, and seawater transfer. After 21 week from first feeding, the group was given artificial winter photoperiod (8 hours light, 16 hours dark) for 8 weeks to induce smoltification before returning to constant light. Liver tissue was sampled from individuals first at week 1, 21 weeks after first feeding, then again at week 10, after the winter period, at week 19, after the expected smoltification time, and lastly at week 25, after transfer to seawater.